Metric to SAE Bolt Conversion Chart: M6, M8, M10, M12

You have a bolt in your hand, a socket tray that mixes both standards, and one question: what is the closest SAE size to this metric bolt — and can I actually use it? This page answers both, size by size, with the exact difference in millimetres and thousandths of an inch.

The short version: no metric bolt has a true SAE twin. The closest pair in common use — M8 and 5/16" — differ by only 0.06 mm in major diameter, which is precisely what makes that swap the most damaging mistake on this page. Diameter is never the whole story; thread pitch decides whether a fastener threads or destroys.

Key Takeaway

Metric and SAE bolts were designed under two separate standards and never share a common size. The nearest neighbours are M6 to 1/4"-20 (0.35 mm apart), M8 to 5/16"-18 (0.06 mm apart), M10 to 3/8"-16 (0.48 mm apart), M12 to 1/2"-13 (0.70 mm apart), M14 to 9/16"-12 (0.29 mm apart), and M16 to 5/8"-11 (0.13 mm apart). None of these pairs are interchangeable, because the pitch also differs: an M8 advances 1.25 mm per turn while a 5/16"-18 advances 1.41 mm per turn. Over a 20 mm engagement that gap accumulates to roughly 2.6 mm of misalignment, which is enough to strip aluminium threads on the first pass. The closer two sizes look, the more expensive the mistake: a visibly wrong bolt gets rejected by hand, while a 0.06 mm mismatch threads far enough to ruin the hole before anything feels wrong.

Thread checker board with labelled metric and SAE threaded posts for bolt size identification — A threaded gauge board settles the metric-or-SAE question by fit, not by eye — the only reliable method when both standards are in the same tray.

Metric to SAE Bolt Size Conversion Chart (M3–M24)

Diameters below are nominal major diameters. SAE values are converted at 25.4 mm per inch and rounded to three decimals. "Δ" is the absolute difference between the two major diameters.

Metric	Major dia.	Coarse pitch	Closest SAE (UNC)	Major dia.	Pitch	Δ diameter	Swap?
M3	3.00 mm	0.50 mm	#4-40	2.845 mm	0.635 mm	0.16 mm	No
M4	4.00 mm	0.70 mm	#8-32	4.166 mm	0.794 mm	0.17 mm	No
M5	5.00 mm	0.80 mm	#10-24	4.826 mm	1.058 mm	0.17 mm	No
M6	6.00 mm	1.00 mm	1/4"-20	6.350 mm	1.270 mm	0.35 mm	No
M8	8.00 mm	1.25 mm	5/16"-18	7.938 mm	1.411 mm	0.06 mm	Never — closest and most destructive
M10	10.00 mm	1.50 mm	3/8"-16	9.525 mm	1.588 mm	0.48 mm	No
M12	12.00 mm	1.75 mm	1/2"-13	12.700 mm	1.954 mm	0.70 mm	No
M14	14.00 mm	2.00 mm	9/16"-12	14.288 mm	2.117 mm	0.29 mm	No
M16	16.00 mm	2.00 mm	5/8"-11	15.875 mm	2.309 mm	0.13 mm	No
M18	18.00 mm	2.50 mm	11/16"-11	17.463 mm	2.309 mm	0.54 mm	No (11/16" is rarely stocked)
M20	20.00 mm	2.50 mm	3/4"-10	19.050 mm	2.540 mm	0.95 mm	No
M22	22.00 mm	2.50 mm	7/8"-9	22.225 mm	2.822 mm	0.23 mm	No
M24	24.00 mm	3.00 mm	1"-8	25.400 mm	3.175 mm	1.40 mm	No (15/16"-9 is closer but uncommon)

Metric coarse pitches follow the ISO metric M profile, adopted in the United States as ASME B1.13M, Metric Screw Threads: M Profile. Inch-to-millimetre conversions use the exact 25.4 mm definition maintained by NIST's Office of Weights and Measures. A full M1–M100 pitch reference is published by Fuller Fasteners.

Quick Answers: Every Metric Size to Its Closest SAE

Each block below is a standalone answer. If you arrived here searching for one specific size, read only that block.

M5 to SAE

The closest SAE size to M5 is #10-24. M5 measures 5.00 mm across the major diameter; #10-24 measures 0.190" (4.826 mm). The gap is 0.17 mm, about seven thousandths of an inch. Pitch differs more sharply: M5 coarse advances 0.80 mm per turn against 1.058 mm for #10-24. A #10-24 screw will start in an M5 hole and bind within two or three turns.

M6 to SAE

The closest SAE size to M6 is 1/4"-20. M6 is 6.00 mm; 1/4" is 0.250" (6.350 mm), so the SAE bolt is 0.35 mm larger — roughly fourteen thousandths. This is the most common substitution attempt in furniture, bicycle and light machinery work, and it is the one people usually get away with in a clearance hole and never get away with in a tapped hole. Pitch is 1.00 mm versus 1.270 mm.

M8 to SAE

The closest SAE size to M8 is 5/16"-18, and it is the single most dangerous pairing in this chart. M8 is 8.00 mm; 5/16" is 0.3125" (7.938 mm). The difference is 0.06 mm — about two and a half thousandths of an inch, small enough that a 5/16" bolt will thread two or three turns into an M8 hole under hand pressure. It then jams, and by the time you feel resistance the aluminium threads are already cut. Pitch is 1.25 mm versus 1.411 mm.

M10 to SAE

The closest SAE size to M10 is 3/8"-16. M10 is 10.00 mm; 3/8" is 0.375" (9.525 mm), leaving the SAE bolt 0.48 mm undersize — around nineteen thousandths. That is large enough to feel as slop in a clearance hole and large enough that a 3/8" bolt will spin loosely in an M10 nut without ever catching. Pitch is 1.50 mm versus 1.588 mm, the closest pitch match in the whole table.

M12 to SAE

The closest SAE size to M12 is 1/2"-13. M12 is 12.00 mm; 1/2" is 0.500" (12.700 mm), so the SAE bolt is 0.70 mm oversize — nearly twenty-eight thousandths. A 1/2" bolt will not enter an M12 tapped hole at all, and in a clearance hole it usually will not pass through. Pitch is 1.75 mm versus 1.954 mm. This is one of the few pairs where the mismatch is obvious enough to protect you.

M14 to SAE

The closest SAE size to M14 is 9/16"-12. M14 is 14.00 mm; 9/16" is 0.5625" (14.288 mm), a 0.29 mm difference of about eleven thousandths. M14 is uncommon in general hardware but standard on many European automotive suspension and driveline fasteners, which is exactly where a 9/16" substitute is most likely to be reached for and most likely to cause a failure under load.

M16 to SAE

The closest SAE size to M16 is 5/8"-11. M16 is 16.00 mm; 5/8" is 0.625" (15.875 mm), a difference of just 0.13 mm — five thousandths. Along with M8, this is one of the two pairs close enough to start threading before binding. Pitch is 2.00 mm versus 2.309 mm, so the mismatch builds fast: after five turns the two are already more than 1.5 mm out of phase.

M20 to SAE

The closest SAE size to M20 is 3/4"-10. M20 is 20.00 mm; 3/4" is 0.750" (19.050 mm), leaving the SAE bolt 0.95 mm undersize — the largest gap among the common pairs, at roughly thirty-seven thousandths. At this size the difference is visible side by side. Pitch is 2.50 mm versus 2.540 mm, which is nearly identical and occasionally fools people into assuming the diameters must match too.

Metric and SAE bolts side by side showing how similar M8 and 5 16 inch diameters appear — M8 and 5/16" differ by 0.06 mm. At arm's length they are indistinguishable, which is why this pair damages more tapped holes than any obviously wrong bolt ever will.

SAE to Metric: Reverse Lookup

Working the other direction — you have an inch-series bolt and need the nearest metric equivalent:

SAE size	Major dia.	Closest metric	Major dia.	Δ diameter	Pitch: SAE vs metric
#8-32	4.166 mm	M4	4.00 mm	0.17 mm	0.794 vs 0.70 mm
#10-24	4.826 mm	M5	5.00 mm	0.17 mm	1.058 vs 0.80 mm
1/4"-20	6.350 mm	M6	6.00 mm	0.35 mm	1.270 vs 1.00 mm
5/16"-18	7.938 mm	M8	8.00 mm	0.06 mm	1.411 vs 1.25 mm
3/8"-16	9.525 mm	M10	10.00 mm	0.48 mm	1.588 vs 1.50 mm
7/16"-14	11.113 mm	M11 (rare) / M12	11.00 / 12.00 mm	0.11 / 0.89 mm	1.814 vs 1.50 / 1.75 mm
1/2"-13	12.700 mm	M12	12.00 mm	0.70 mm	1.954 vs 1.75 mm
9/16"-12	14.288 mm	M14	14.00 mm	0.29 mm	2.117 vs 2.00 mm
5/8"-11	15.875 mm	M16	16.00 mm	0.13 mm	2.309 vs 2.00 mm
3/4"-10	19.050 mm	M20	20.00 mm	0.95 mm	2.540 vs 2.50 mm
7/8"-9	22.225 mm	M22	22.00 mm	0.23 mm	2.822 vs 2.50 mm

Note the 7/16"-14 row. Its true nearest metric neighbour is M11 at 0.11 mm, but M11 is a non-preferred size that almost never appears in production hardware. In practice 7/16" sits in a gap between M10 and M12 with no usable metric counterpart — one of several reasons a mixed-standard assembly cannot be rationalised into a single socket set.

Thread Pitch: Where "Close Enough" Destroys Threads

Diameter tells you whether a bolt enters. Pitch tells you whether it keeps going. Metric pitch is stated as the distance between adjacent thread crests in millimetres; SAE pitch is stated as threads per inch (TPI). To convert, divide 25.4 by the TPI.

SAE TPI	Pitch in mm	Nearest metric pitch	Difference	Where you see it
40	0.635 mm	0.60 mm	0.035 mm	#4 screws, M3.5 coarse
32	0.794 mm	0.80 mm	0.006 mm	#8 screws, M5 coarse
24	1.058 mm	1.00 mm	0.058 mm	#10 screws, M6 coarse
20	1.270 mm	1.25 mm	0.020 mm	1/4" UNC, M8 coarse
18	1.411 mm	1.50 mm	0.089 mm	5/16" UNC, M10 coarse
16	1.588 mm	1.50 mm	0.088 mm	3/8" UNC, M10 coarse
13	1.954 mm	2.00 mm	0.046 mm	1/2" UNC, M14/M16 coarse
11	2.309 mm	2.50 mm	0.191 mm	5/8" UNC, M18/M20 coarse
10	2.540 mm	2.50 mm	0.040 mm	3/4" UNC, M20 coarse

Read this table carefully, because it is a trap. Several rows show pitch differences under 0.05 mm, which looks harmless. It is not, because pitch error is cumulative. A 0.02 mm per-thread difference is nothing on turn one and 0.20 mm after ten turns. The bolt does not fail where it started; it fails deep in the hole where you cannot see it and where the only remedy is a thread repair insert.

The specific case people search for is M8 × 1.25 versus a "standard" equivalent. There is none. The nearest inch pitch to 1.25 mm is 20 TPI at 1.270 mm, but 20 TPI belongs to 1/4" bolts, not 5/16". The 5/16" bolt that shares M8's diameter runs 18 TPI at 1.411 mm. So the size that matches on diameter mismatches on pitch, and the size that matches on pitch mismatches on diameter. That is the structural reason no clean M8 substitute exists.

Laser etched size markings on a thread checker showing metric and SAE designations — Pitch is invisible to the eye at these scales. Threading a fastener into a known-good gauge is the only field test that catches a pitch mismatch before the hole pays for it.

"8 mm to SAE" — Are You Asking About the Bolt or the Wrench?

This is the most frequently confused question in the whole metric-to-SAE topic, and the two answers are completely different.

If you mean the bolt thread: an 8 mm bolt is M8, and its nearest SAE thread is 5/16"-18 — 0.06 mm apart, not interchangeable.
If you mean the wrench or socket: an 8 mm wrench fits across the flats of the hex head, and 5/16" (7.938 mm) is only 0.06 mm smaller — one of the few genuinely usable cross-standard fits.

The same number can therefore be "never substitute" and "go ahead" depending on which part you are measuring. The head size of a bolt is not tied to its thread size in any fixed ratio, and it varies between standards and between manufacturers. An M8 bolt commonly takes a 13 mm wrench, but M8 flange bolts frequently take 12 mm, and some take 14 mm. Never infer thread size from wrench size, or the reverse.

Close view of metric and SAE threaded posts labelled by size on an identification board — Wrench size and thread size are independent properties. An M8 bolt may take a 12, 13 or 14 mm head depending on the flange design, so neither number predicts the other.

Wrench and Socket Size Conversion Chart (6–24 mm)

Across-the-flats sizes. "Δ" is how much larger (+) or smaller (−) the SAE socket is than the metric one. Anything beyond about ±0.15 mm will round a fastener under load.

Metric	Closest SAE	SAE in mm	Δ	Verdict
6 mm	1/4"	6.350	+0.350	Too loose
7 mm	9/32"	7.144	+0.144	Marginal
8 mm	5/16"	7.938	−0.062	Usable
9 mm	11/32"	8.731	−0.269	Too tight
10 mm	3/8"	9.525	−0.475	Will not seat
11 mm	7/16"	11.113	+0.113	Usable
12 mm	15/32"	11.906	−0.094	Usable (uncommon socket)
13 mm	1/2"	12.700	−0.300	Too tight
14 mm	9/16"	14.288	+0.288	Too loose
15 mm	19/32"	15.081	+0.081	Usable (uncommon socket)
16 mm	5/8"	15.875	−0.125	Usable
17 mm	11/16"	17.463	+0.463	Too loose
18 mm	23/32"	18.256	+0.256	Too loose
19 mm	3/4"	19.050	+0.050	Best match on the chart
20 mm	25/32"	19.844	−0.156	Marginal; 13/16" is +0.638, too loose
21 mm	13/16"	20.638	−0.362	Too tight
22 mm	7/8"	22.225	+0.225	Too loose
24 mm	15/16"	23.813	−0.187	Marginal

Only four pairs — 8 mm/5⁄16", 11 mm/7⁄16", 16 mm/5⁄8" and 19 mm/3⁄4" — sit inside ±0.15 mm using sockets you will actually own. The famous problem case is 10 mm: at −0.475 mm, a 3/8" socket is not a substitute in any circumstance, which is a large part of why the 10 mm socket is the one everyone loses and nobody can improvise around.

When Can You Safely Use a Close-Size Bolt?

Three conditions must all be true, and in practice they rarely are:

The hole is a clearance hole, not a tapped hole. If the bolt threads into the part, no substitution is safe at any tolerance.
The substitute is smaller, never larger. A smaller bolt in a clearance hole introduces slop; a larger one either will not pass or damages the bore going in.
The joint is not structural, not vibration-loaded, and not safety-critical. A cover panel qualifies. Suspension, brakes, engine mounts, load-bearing brackets and anything overhead do not.

If all three hold, the nut must still match the bolt, not the hole. A 1/4"-20 bolt requires a 1/4"-20 nut even when it is passing through an M6 clearance hole. Mixing an M6 nut onto a 1/4" bolt reproduces the same pitch-drift failure inside the nut instead of inside the part.

Bolt being threaded into a gauge post to confirm both diameter and pitch match — The pass criterion is the same everywhere: the fastener runs down by hand for its full length with no increase in resistance. Any tightening part-way in is a mismatch, not a tight fit.

How to Tell Metric from SAE in About Ten Seconds

Four field checks, in order of reliability:

Head markings. SAE hex bolts carry radial lines — three lines for Grade 5, six for Grade 8. Metric bolts carry a numeric property class such as 8.8, 10.9 or 12.9. A number means metric; lines mean inch. This alone resolves most cases.
Thread it into a known gauge. The only test that verifies diameter and pitch together. Detail in how to use a thread checker step by step.
Measure the major diameter with calipers. A reading near a whole millimetre (8.00, 10.00, 12.00) suggests metric. A reading near an awkward decimal (7.94, 9.53, 12.70) suggests inch. This narrows it down but never confirms pitch.
Check the assembly's origin. Useful context, not proof — many machines mix both standards deliberately. See how to match unknown thread sizes when the markings are worn off.

For a full identification walkthrough covering damaged heads and blind holes, see how to identify any bolt thread size. If you are deciding between a threaded gauge board and a pitch gauge, this comparison covers which tool suits which job.

Frequently Asked Questions

Can I replace an M8 bolt with a 5/16" bolt?

No. The diameters differ by only 0.06 mm, so a 5/16"-18 bolt will start into an M8 hole and feel correct for two or three turns. The pitches differ by 0.16 mm per thread, so it then binds and cuts new threads on top of the originals. In aluminium the hole is usually unrecoverable without a thread repair insert. This is the most damaging substitution on the chart precisely because it is the closest.

What is the metric equivalent of a 1/4-20 bolt?

The nearest metric size to 1/4"-20 is M6. A 1/4" bolt has a 6.350 mm major diameter against M6's 6.00 mm, a difference of 0.35 mm. Pitch is 1.270 mm for 1/4"-20 versus 1.00 mm for M6 coarse. They are not interchangeable in a tapped hole. In a clearance hole the situation is different: standard M6 clearance holes run 6.4 mm (close fit) to 6.6 mm (normal), so a 6.350 mm 1/4" bolt does physically pass — with as little as 0.05 mm of margin in a close-fit hole. Passing through is not the same as being the right fastener; the nut must still be 1/4"-20.

What is the metric equivalent of a 10-24 screw?

The nearest metric size to #10-24 is M5. A #10 screw measures 0.190" (4.826 mm) against M5's 5.00 mm, a difference of 0.17 mm. The pitch gap is larger in proportional terms: 1.058 mm for 24 TPI versus 0.80 mm for M5 coarse, roughly a 32 percent difference. At small diameters, thread engagement is shallow, so a pitch mismatch this large strips almost immediately.

Is there any bolt size that is identical in metric and SAE?

No thread size is identical. The closest thread pair is M8 and 5/16" at 0.06 mm apart, which is still a mismatch. On the wrench side, however, one pair is close enough to be functionally identical in practice: 19 mm and 3/4", which differ by 0.05 mm across the flats. That is a socket coincidence, not a thread standard overlap.

What does M8 × 1.25 convert to in standard sizes?

It does not convert. M8 × 1.25 means an 8.00 mm major diameter with 1.25 mm between threads, which works out to 20.3 threads per inch — a value that does not exist in the inch series. The closest inch pitch is 20 TPI, but 20 TPI is used on 1/4" bolts, not on the 5/16" bolt that shares M8's diameter. No inch fastener matches both properties at once.

Why do some machines have both metric and SAE fasteners?

Mixed assemblies are normal on equipment built from globally sourced subassemblies: an inch-standard frame with a metric engine, or a metric machine fitted with inch-series hydraulic and pipe fittings. It is not an error. It does mean you cannot assume a standard from one fastener and apply it across the machine. Pipe and hydraulic threads follow separate standards again, covered in using a thread checker for plumbing and hydraulic fittings.

What is the difference between UNC and UNF?

Both are inch-series threads with the same major diameters; only the pitch differs. UNC (coarse) runs fewer threads per inch and tolerates dirt, burrs and repeated assembly better. UNF (fine) runs more threads per inch, providing greater tensile area and better resistance to loosening under vibration. A 1/4"-20 is UNC; a 1/4"-28 is UNF. They share a diameter and will cross-thread each other, so a diameter check alone never tells you which one you are holding.

Workbench setup with a thread identification gauge alongside assorted metric and SAE hardware — Keeping one reference gauge at the bench removes the guesswork step entirely — you stop deciding whether a bolt "looks metric" and start confirming it.

The Bottom Line

Every pairing in these charts is a near-miss, not an equivalent. The practical rules are short:

Never substitute across standards in a tapped hole. Diameter proximity is what makes the damage possible, not what makes the swap safe.
Check pitch, not just diameter. M8 and 5/16" are 0.06 mm apart on diameter and 0.16 mm apart per thread. The second number is the one that ruins the part.
Treat wrench sizes as a separate question. 8 mm/5⁄16" and 19 mm/3⁄4" are fine on a hex head and meaningless as thread advice.
Confirm by fit, not by eye. Below about 0.5 mm, visual comparison is unreliable for everyone.

If you regularly work on mixed-standard equipment, a threaded reference gauge answers both the diameter and the pitch question in one motion. The AltitudeCraft Thread Checker carries labelled metric and SAE posts on a single board so an unknown fastener is identified by threading it, not by measuring and guessing. Related reading: thread checker FAQ on metric vs SAE and common mistakes, the complete thread checker guide, choosing a nut and bolt thread checker, and — for when the wrong bolt has already seized in place — how to remove stuck bolts.

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