What Size Is an M8 Bolt? How to Identify Any Bolt Thread Size

You are holding a bolt and you need one number: what size is it? By a wide margin the most-asked version of that question is about M8 — usually paired with "is M8 the same as 5/16?" (it is not) or "what does the 1.25 mean?" So this guide answers M8 first, in the next 30 seconds, then gives you the method for identifying any bolt — metric or standard, sitting in your hand or still bolted into the machine.

This guide covers all three situations you can be in: you have the bolt in your hand, you have a part number or a printed callout like M8-1.25 x 30 and need to decode it, or the bolt is still bolted into the machine and cannot be removed. Each one needs a different method.

What Size Is an M8 Bolt?

An M8 bolt has a major (outside) diameter of exactly 8.00 mm, which is 0.315 inches. The standard coarse version is written M8×1.25, meaning the threads sit 1.25 mm apart. Its hex head takes a 13 mm wrench. That is the whole answer — the table below is every M8 number you are likely to need.

Dimensions above follow ISO metric coarse thread tables; the 13 mm across-flats figure is the ISO 4014/4017 and DIN 931/933 hex dimension.

M8 vs 5/16": Why "Almost Fits" Means "Wrong Bolt"

This is the single most expensive confusion in fastener work, and it is expensive precisely because the two sizes are so close. M8 measures 8.00 mm across the crests. A 5/16" bolt measures 0.3125 in, which is 7.9375 mm. The gap is 0.0625 mm — roughly the thickness of a sheet of paper. No caliper reading taken by hand on a dirty bolt reliably separates them, and no human eye ever will.

Diameter is therefore the wrong measurement to decide on. Pitch is the deciding measurement, and on pitch the two are not close at all:

Inch dimensions above are the ASME B1.1 Unified inch values; metric values are ISO. The practical consequence is counter-intuitive and worth stating plainly:

Because they share a 13 mm / 1/2 inch wrench and an indistinguishable diameter, the only reliable field test is threading the fastener into a known-good reference: a thread checker hole, a nut of confirmed size, or a pitch gauge blade laid against the crests. If you need to move between the two systems across the whole size range rather than just this one pair, use our metric to SAE bolt size conversion chart.

Identifying bolt thread size accurately is one of the most practical skills any DIYer, mechanic, or tradesperson can have. Get it wrong, and you risk cross-threading, stripped holes, and failed connections. This guide covers every method — from reading a printed callout to precision measurement — and explains the critical differences between metric and SAE (imperial) standards.

How to Tell If a Bolt Is Metric or Standard

Before measuring anything, you need to know which standard your bolt uses. The world uses two primary fastener standards:

Quick test: If the bolt came from a Japanese or European vehicle, a modern appliance, or any electronics — it is almost certainly metric. If it came from an American truck, older American machinery, or construction framing — it is likely SAE. When in doubt, measure with a thread checker tool that covers both standards. For a printable conversion reference, see our Metric vs SAE Bolt Size Conversion Chart.

The trap: head markings tell you the strength grade, not the thread standard. A number like 8.8 or 10.9 is an ISO 898-1 property class and is strong evidence the bolt is metric. Radial lines are SAE J429 grade marks and point to inch threads. But plenty of bolts — especially small ones, stainless ones, and anything from a hardware-store bulk bin — carry no marking at all. Absence of a marking tells you nothing about which standard it is.

How to Read Bolt Specs: Decoding M8-1.25 x 30 and 5/16-18

Half the time you do not have a mystery bolt at all — you have a printed spec in a manual, a parts diagram, or a supplier listing, and you need to know what it means before ordering. Bolt callouts are read strictly left to right, and both standards follow the same order: diameter, then pitch, then length.

The single most important rule: in a metric callout the middle number is a distance (millimeters between adjacent threads), so smaller means finer. In an inch callout the middle number is a count (threads per inch), so larger means finer. M8-1.0 is finer than M8-1.25; 5/16-24 is finer than 5/16-18. Getting this backwards is how people order the wrong bolt from a correct-looking spec.

Two shorthand conventions cause confusion:

• A missing middle number means coarse. Under ISO metric convention, "M8" with no pitch stated means M8 x 1.25, because coarse is the default pitch for that diameter. Likewise "5/16 UNC" implies 18 TPI.
• Letters after the numbers are the thread series and fit class. In ASME B1.1, a full designation such as 5/16-18 UNC-2A adds the series (UNC coarse, UNF fine) and the class of fit — 1 loose, 2 standard, 3 precision — with A for external threads and B for internal. For everyday repair work you can ignore the class; 2A/2B is what virtually all hardware is.

Length is measured from under the head for hex and socket-head bolts, and from the top of the head for countersunk flat-head bolts, because the head sinks flush into the work. That single distinction accounts for a lot of "the bolt I ordered is too short" frustration.

Three Properties You Need to Measure

Every bolt has three critical dimensions that define its size:

1. Major Diameter (the thickness)

This is the widest part of the bolt shaft, measured across the outside of the threads. For metric bolts, this is the "M" number: M6 = 6 mm diameter, M8 = 8 mm, M10 = 10 mm. For SAE bolts, it is a fraction: 1/4", 5/16", 3/8", 1/2". Note that real bolts measure very slightly under nominal — an M8 typically mics at 7.8–7.9 mm — because the thread crests are truncated. Round up to the nearest standard size rather than assuming you have found an oddball.

2. Thread Pitch (how far apart the threads are)

Metric thread pitch is the distance in millimeters between two adjacent thread peaks. An M8 x 1.25 bolt has threads spaced 1.25 mm apart. SAE uses Threads Per Inch (TPI) — a 5/16"-18 bolt has 18 threads in one inch of length. More TPI = finer thread. Refer to Bolt Depot's thread pitch chart for a complete reference table.

3. Length

Measured from under the head to the tip of the bolt. For hex bolts, do not include the head height. For countersunk (flat head) bolts, measure the total length including the head.

Method 1: Use a Thread Checker Tool (Fastest and Most Accurate)

A thread checker (also called a nut and bolt identifier) is a flat plate with pre-cut threaded holes for the most common bolt sizes. You simply thread your mystery bolt into each hole until it fits smoothly — the matching hole tells you the exact size.

The AltitudeCraft Thread Checker identifies 28 unique sizes covering both metric (M3 through M12) and SAE (4-40 through 1/2"-13). Made from SUS304 stainless steel, it provides accurate thread engagement without wearing out.

Why this method is best: It tests thread pitch AND diameter simultaneously. Calipers can measure diameter but cannot verify thread pitch. A thread checker confirms both in one step — if the bolt threads in smoothly, it is a match. If it does not thread in, even if the diameter seems right, the pitch is wrong.

For a complete walkthrough with photos, see our step-by-step thread checker guide.

Common mistake: Forcing a bolt that almost fits. If you have to use pressure, it is NOT the right size. Cross-threading a bolt into the wrong hole damages both the bolt and the tool. The correct match should thread in with finger pressure only.

Common mistake: Only checking one thread engagement. Thread at least 3–4 full turns into the checker hole to confirm the match. A single turn can feel correct even when the pitch is slightly off.

Method 2: Caliper + Thread Pitch Gauge (Precision Method)

If you need laboratory-level precision or are working with unusual sizes not covered by a standard thread checker, use a digital caliper paired with a thread pitch gauge:

1. Measure the major diameter with the caliper across the thread crests. Read in millimeters for metric, inches for SAE. Take the measurement on a clean, undamaged section of thread — a burred or corroded crest will read low.
2. Match the thread pitch using a thread pitch gauge (a set of thin metal blades with different pitch profiles). Hold each blade against the threads until one seats into the grooves with no daylight showing between the blade teeth and the thread valleys.
3. Cross-reference both measurements against a metric thread chart or SAE thread chart.

No pitch gauge? Use a ruler. For inch bolts, line a ruler up with the thread crests and count how many crests fall within exactly one inch — that count is the TPI. For metric bolts, measure across 10 thread crests in millimeters and divide by 10; a span of 12.5 mm over 10 threads means a 1.25 mm pitch. Counting across many threads rather than one averages out your reading error, which is what makes this workable with an ordinary steel rule.

How to Measure a Bolt You Cannot Remove

This is the case most guides skip, and in practice it is extremely common: the fastener is captive, seized, welded to a bracket, or sits in an assembly you cannot take apart just to identify a bolt. A thread checker is useless here — it is a go/no-go plate that requires you to physically thread the fastener into it. You need in-place measurement instead.

1. Clean the exposed threads. A wire brush and a shot of solvent. Rust and thread-locker sit in the valleys and will throw off both diameter and pitch readings.
2. Caliper the major diameter on the exposed shank. If only two or three threads protrude past the nut, that is enough — set the caliper jaws across the crests, square to the axis. Tilting the caliper reads high.
3. Lay a pitch gauge blade against the protruding threads. Even 4–5 exposed threads are enough to seat a blade and confirm pitch.
4. If nothing protrudes, measure the nut instead. A nut is the same thread as its bolt. Thread the nut off (nuts are usually removable even when the stud is not), drop it onto the thread checker's male studs or measure its inside diameter, and you have your answer without ever removing the bolt.
5. Last resort — measure the clearance hole. A bolt's clearance hole in the mating part is typically 0.5–1 mm larger than the bolt's nominal diameter, which narrows the candidates to one or two sizes. Confirm with pitch before ordering.

This scenario is not an edge case, and we can put a number on it: it is the most common reason customers return our own thread checker — they bought a go/no-go plate to identify a bolt that was never coming out of the machine. If that is your situation, the five steps above are the answer, and no purchase is required.

The nut trick in step 4 solves the majority of real cases and is worth remembering: you almost never need the bolt itself, you need something that shares its thread. A spare nut, the mating tapped hole, or an identical bolt from the same assembly all work.

Method 3: The Hardware Store Test (Emergency Method)

No tools at all? Bring the bolt to a hardware store and use their sizing station — most stores have a thread checker board mounted near the fastener aisle. Thread your bolt through the holes until you find a match. This works, but it is time-consuming, the boards are often worn or missing sizes, and you cannot do it at home or in the field.

What a Thread Checker Cannot Do

We manufacture a thread checker, and we would rather tell you its limits up front than have you buy the wrong tool. So here are our own numbers. Across 94 returns of this product with 55 written customer comments (September 2024 through August 2026), only 3 were coded as an actual defect. The overwhelming majority of dissatisfied buyers did not receive a faulty plate — they bought a thread checker for a job it structurally cannot do. Four limits account for nearly all of it:

Read positively: a thread checker is the right tool when you are holding a loose, undamaged machine-threaded fastener in a common size — which describes most of a workshop's daily mystery-bolt problems, but not all of them. If your work regularly involves captive fasteners or wood screws, budget for a caliper and pitch gauge alongside it rather than instead of it.

The Most Commonly Confused Bolt Sizes

These size pairs are extremely close and are the most common source of cross-threading disasters:

M8 vs 5/16" is the most dangerous confusion. The diameter difference is only 0.06 mm — impossible to tell by eye or feel. The only reliable way to distinguish them is by thread pitch: M8 standard is 1.25 mm pitch, while 5/16"-18 has 18 TPI (1.41 mm pitch). A thread checker will immediately reveal the difference because the bolt will only thread into the correct hole.

M6 vs M8 Bolts: The Difference and How to Tell Them Apart

M8 vs 5/16" is the dangerous confusion; M6 vs M8 is the frequent one, because both are everywhere in automotive and machinery work and a 2 mm diameter gap is easy to misjudge across a parts bin. Here is the practical separation:

The fastest field test: reach for a socket. If a 10 mm socket fits the head, you are almost certainly holding an M6; if it takes a 13 mm, it is an M8. This is not universal — head sizes vary by flange type and manufacturer — but on Japanese and European vehicles the 10/13 split is reliable enough to sort a bin in seconds before you confirm with a checker.

Note the overlap trap in the pitch column: M6 fine and M8 fine both include a 1.0 mm pitch. If you identify a bolt by pitch alone and read 1.0 mm, you have not yet distinguished M6-fine from M8-coarse. Diameter and pitch together are what identify a bolt; either one alone is ambiguous.

Common Metric Bolt Sizes Quick Reference

For the complete chart from M1 to M100, refer to Fuller Fasteners' Metric Thread Chart.

Common SAE Bolt Sizes Quick Reference

SAE sizes are where the "coarse vs fine" question bites hardest, because the same fractional diameter carries two completely different TPI counts. UNC is the coarse series and the default for general assembly; UNF is fine and shows up in engines, brake fittings, and anywhere vibration resistance matters.

The "nearest metric" column is a sourcing aid, not an interchangeability claim. As ASME B1.1 and the ISO metric series define separate thread forms, unified inch and metric threads are not mechanically interchangeable — a 5/16" bolt will start into an M8 hole and destroy it within two turns.

Thread Checker vs Thread Pitch Gauge: Which Do You Need?

Our recommendation: A thread checker is the better first tool for most people. It answers the question "what size is this bolt?" in under 10 seconds without any measurement skill. A thread pitch gauge is a valuable addition — and the necessary one if you frequently meet fasteners you cannot remove. For our detailed comparison, see the complete thread checker guide.

Frequently Asked Questions

How can I tell if a bolt is metric or standard just by looking at it?

Check the head markings. Metric bolts typically have a number stamped on the head indicating strength class (8.8, 10.9, 12.9). SAE bolts have radial lines — 3 lines = Grade 5, 6 lines = Grade 8. No markings usually means a low-grade bolt. However, head markings are not always present, especially on small bolts, and they indicate strength rather than thread standard. The most reliable method is using a thread checker tool that tests both standards.

How do you measure bolt thread size without a thread checker?

Use two measurements. First, put a caliper across the thread crests to get the major diameter — read millimeters for metric, inches for SAE. Second, determine pitch: lay a thread pitch gauge blade into the threads until one seats with no gaps, or, with only a ruler, count thread crests across exactly one inch for TPI, or measure 10 crests in millimeters and divide by 10 for metric pitch. Diameter and pitch together identify the bolt; either alone is ambiguous.

How do I check the thread size of a bolt I cannot remove?

Clean the exposed threads, then caliper the major diameter on the protruding shank and seat a pitch gauge blade against the exposed threads — four or five threads is enough for both. If nothing protrudes, take the nut off instead and identify the nut, since it shares the bolt's thread exactly. As a last resort, measure the clearance hole in the mating part, which typically runs 0.5–1 mm over the bolt's nominal diameter, then confirm with pitch before ordering.

How do you read a bolt size like M8-1.25 x 30?

Left to right: diameter, pitch, length. M8 means an 8 mm major diameter, 1.25 means 1.25 mm between adjacent threads, and 30 means 30 mm long measured under the head. The inch equivalent format works the same way but the middle number is a count instead of a distance: 5/16-18 x 1-1/2 is a 5/16 inch bolt with 18 threads per inch, 1.5 inches long. If the middle number is missing entirely, the bolt is the coarse pitch for that diameter.

What size bolt do I have if it measures 8mm on my caliper?

It is likely an M8 metric bolt, but it could also be a 5/16" SAE bolt (which measures 7.94 mm — only 0.06 mm difference). You must also check the thread pitch to confirm. M8 coarse has a 1.25 mm pitch (approximately 20 TPI), while 5/16"-18 has 18 TPI (1.41 mm pitch). A thread checker will instantly tell you which one it is.

What size is an M8 bolt in inches?

An M8 bolt is 8.00 mm in major diameter, which converts to 0.315 inches. There is no exact inch equivalent. The nearest inch size is 5/16", which is 0.3125 in (7.9375 mm) — 0.0625 mm smaller. They are not interchangeable because their thread pitches differ by 13%.

Is M8 the same as 5/16?

No. They are close enough in diameter to be indistinguishable by eye (8.00 mm versus 7.9375 mm), which is exactly why they get mixed up, but the threads are cut differently: M8 coarse has a 1.25 mm pitch while 5/16"-18 has a 1.412 mm pitch. Either one will start into the other's hole and bind after a turn or two. If it does not run down under finger pressure, it is the wrong bolt.

What does M8 x 1.25 mean?

Read it left to right: M = metric thread, 8 = 8 mm nominal diameter, 1.25 = 1.25 mm between adjacent threads. If a third number follows, such as M8×1.25×30, that is the length in millimetres measured under the head. M8×1.25 is the standard coarse pitch, so an M8 bolt with no pitch specified is almost always 1.25.

What thread pitch is a standard M8 bolt?

1.25 mm is the ISO coarse standard and the default assumption. M8 fine threads also exist at 1.00 mm (and occasionally 0.75 mm), and they turn up regularly on Japanese engine and brake hardware. Because a 1.00 mm bolt threads partway into a 1.25 mm hole before jamming, pitch is worth confirming rather than assuming whenever the fastener came off an engine.

What size wrench does an M8 bolt take?

13 mm, per ISO 4017 and DIN 933. Note that a 1/2 inch wrench measures 12.7 mm and will feel almost right on a 13 mm head — it is the same trap as the M8/5-16 diameter overlap, and it is how hex heads get rounded off.

What is the difference between an M6 and an M8 bolt?

Diameter and pitch both differ: M6 is 6 mm across the threads with a 1.0 mm coarse pitch, while M8 is 8 mm with a 1.25 mm coarse pitch. In practice the quickest separator is the head — M6 hex heads usually take a 10 mm socket and M8 usually takes a 13 mm. They are not interchangeable in any direction, and note that M6 fine and M8 fine share a 1.0 mm pitch, so pitch alone cannot tell them apart.

What is the difference between coarse and fine thread?

Coarse thread has fewer, larger threads per unit length. Fine thread has more, smaller threads. Coarse thread is the default for most applications — it is easier to assemble, more tolerant of cross-threading, and better for softer materials. Fine thread provides stronger clamping force and is used where vibration resistance is critical (automotive engines, aircraft). When in doubt, your bolt is almost certainly coarse thread.

Can I use a metric bolt in a SAE hole?

No. Even when the diameters are very close (like M8 and 5/16"), the thread pitches are different. Forcing a metric bolt into a SAE threaded hole will cross-thread and damage both the bolt and the hole. Always verify the exact size with a measurement tool or thread checker before installation.

What are the most common bolt sizes I should know?

For metric: M6, M8, and M10 cover approximately 80% of automotive and machinery fasteners. For SAE: 1/4", 5/16", 3/8", and 1/2" are the most common. The AltitudeCraft Thread Checker covers 28 sizes spanning both standards, which handles virtually any machine-threaded fastener you will encounter in daily work.

The Bottom Line

Identifying bolt thread size does not have to be guesswork, but the right method depends on what you are actually holding. If you have a written callout, read it left to right and remember that a smaller metric middle number and a larger inch middle number both mean finer. If you have the loose bolt, a thread checker settles diameter and pitch in one motion. If the bolt is captive, skip the checker entirely and go to a caliper plus pitch gauge — or identify the nut instead.

What ties all three together is the same rule: diameter and pitch together identify a fastener, and neither one alone is sufficient. Every cross-threaded hole starts with someone confirming one of those two and assuming the other.

Ready to stop guessing? The AltitudeCraft Thread Checker identifies 28 metric and SAE sizes in SUS304 stainless steel — for every fastener you can get out of the machine.

For more fastener guides, see our Screw Thread Pitch Gauge Guide and Nut & Bolt Thread Checker Guide.

Disclosure: AltitudeCraft manufactures the Thread Checker tool referenced in this article. The "What a Thread Checker Cannot Do" section is based on our own product return records and is published because we would rather lose a sale than sell the wrong tool. Thread specifications referenced follow ISO 261 / ISO 965 (metric) and ASME B1.1 (unified inch) standards. Prices and specifications were accurate at time of publication.

Last updated: August 2026. This article is reviewed and updated regularly to reflect current products, pricing, and industry standards.

• Thread Checker FAQ: Metric vs SAE
• Thread Checker vs Thread Pitch Gauge
• Can't Identify That Bolt? Match Unknown Thread Sizes
• Thread Checker for Plumbing and Hydraulic Fittings