Metric vs SAE Bolt Thread Sizes: Complete Conversion Chart (2026)

You need to replace a metric bolt with an SAE equivalent, or you are working on a machine that mixes both standards. Finding the closest metric-to-SAE bolt conversion can save you a trip to the hardware store — but only if you understand that "equivalent" does not mean "interchangeable." This guide provides the complete conversion chart, explains when you can safely substitute, and warns you about the sizes that will get you in trouble.

Metric to SAE Bolt Size Conversion Chart

This table shows the closest SAE equivalent for each common metric bolt size. The Diameter Difference column is critical — the smaller this number, the more likely someone will accidentally mix the two sizes.

Key takeaway: No metric and SAE bolt is truly interchangeable. Even when the diameters are close, the thread pitches are always different. Using the wrong bolt in a threaded hole will cross-thread and damage the receiving component. For reference charts, see Bolt Depot's US-Metric Conversion Table.

Thread Pitch Conversion: The Real Danger Zone

Diameter is only half the story. Thread pitch is where the real confusion happens. Even when two bolts have nearly identical diameters, their threads are spaced differently:

M10 x 1.5 vs 3/8"-16 is the sneakiest pair. The pitch difference is only 0.09mm — a bolt can start threading into the wrong hole and feel correct for 2-3 turns before binding. By then, you have already damaged the threads. This is why finger-threading (never using a wrench to start) and using a thread checker tool are critical habits.

Wrench Size Conversion Chart

When working with mixed metric/SAE fasteners, you also need to know the wrench size equivalents:

Common mistake: Using a 1/2" wrench on a 13mm hex bolt. The 0.30mm gap causes the wrench to round the bolt head corners over time, especially on corroded fasteners. Always use the correct metric wrench on metric bolts.

Common mistake: Assuming metric and SAE wrenches are interchangeable because they are "close enough." In an emergency, using a close-size wrench works once. For repeated use, it damages fasteners. Invest in both metric and SAE wrench sets if you work on mixed equipment.

When Can You Safely Use a Close-Size Bolt?

In unthreaded applications only. If a bolt passes through a clearance hole (a hole larger than the bolt, with a nut on the other side), you can use a metric bolt where an SAE was specified, as long as:

• The bolt fits through the clearance hole
• You use the matching nut (same standard as the bolt)
• The length and grade are appropriate for the load
• Thread Checker FAQ: Metric vs SAE
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In threaded applications (where the bolt screws directly into a tapped hole), you must always use the exact correct standard. No substitutions.

How to Quickly Identify Metric vs SAE in the Field

When you have a mixed collection of fasteners and need to sort them quickly:

1. Check head markings: Metric = number (8.8, 10.9, 12.9). SAE = radial lines (3, 5, or 6 lines).
2. Use a thread checker: Thread each bolt into the AltitudeCraft Thread Checker — it has both metric and SAE holes clearly labeled. This is the fastest method for batch sorting. See our step-by-step thread checker tutorial for detailed instructions.
3. Measure with calipers: Metric bolt diameters are clean round numbers (6.00, 8.00, 10.00mm). SAE diameters are messy in metric (6.35, 7.94, 9.53mm). If your caliper reads a clean number, it is likely metric.

For a deeper guide on identification methods, see our complete bolt thread identification guide.

Frequently Asked Questions

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

Not in a threaded hole. M8 has a 1.25mm thread pitch, while 5/16"-18 has a 1.41mm pitch. The diameters are nearly identical (0.06mm difference), which makes this the most dangerous substitution — the bolt can start threading and feel correct for 1-2 turns before the pitch mismatch causes cross-threading. In a clearance hole with a matching nut, the substitution works if the bolt clears the hole.

What is the difference between UNC and UNF threads in SAE?

UNC (Unified National Coarse) is the standard coarse thread — 5/16"-18 means 18 threads per inch. UNF (Unified National Fine) has more threads per inch — 5/16"-24 means 24 TPI. UNC is the default for most applications. UNF is used where vibration resistance or fine adjustment is needed (automotive brake components, precision instruments). When someone says "5/16 bolt" without specifying, they mean UNC. For full thread pitch data, see Fuller Fasteners' thread reference.

Why do some machines have both metric and SAE bolts?

Global manufacturing. A Japanese-designed machine assembled in the US may use metric bolts for the engine (Japanese engineering standards) and SAE bolts for the frame (American manufacturing standards). Off-road vehicles, agricultural equipment, and imported machinery commonly mix both standards.

Is there a bolt size that is the same in metric and SAE?

No bolt size is identical in both diameter and thread pitch across the two standards. The closest pair is M16 (16.00mm) and 5/8" (15.88mm) at only 0.12mm difference, but their thread pitches still differ (M16 x 2.0mm vs 5/8"-11 at 2.31mm pitch). They are never truly interchangeable in threaded applications.

What thread checker tool covers both metric and SAE?

The AltitudeCraft Thread Checker identifies 28 sizes covering both standards (M3-M12 metric + #4-40 through 1/2"-13 SAE) on a single SUS304 stainless steel plate. This eliminates the need for separate metric and SAE gauges.

Practical Scenarios: Which Standard Are You Dealing With?

Here are the most common real-world scenarios and which standard to expect:

When working on equipment from unknown origins, the safest approach is to assume nothing and verify every fastener. A single thread checker tool eliminates the guesswork from mixed-standard situations. It is particularly valuable for mechanics who work on diverse vehicle fleets where metric and SAE coexist on the same lift.

The Bottom Line

Metric and SAE bolts are never interchangeable in threaded applications, even when their diameters are within fractions of a millimeter. The conversion charts in this guide help you identify the closest equivalent for reference, but always verify the exact standard with a measurement tool or thread checker before installation.

For more fastener guides, see our How to Identify Any Bolt Thread Size, Thread Pitch Gauge Guide, and Nut & Bolt Thread Checker Guide.

Disclosure: AltitudeCraft manufactures the Thread Checker tool referenced in this article. Conversion data follows ISO 261 (metric) and ASME B1.1 (unified inch) thread standards. All diameter and pitch values are nominal specifications — actual manufacturing tolerances may vary. Prices were accurate at time of publication.

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