Conversion Table for Nuts and Washers Between Systems

If you work in hardware, a workshop, or construction, sooner or later you’ll need to cross-reference inches and millimeters for nuts and washers. There’s no mental shortcut that works: you need a reliable nuts and washers inches mm conversion table, with exact figures, so you don’t hand over a piece that doesn’t fit. Here’s that table, explained straight to the point.

Why imperial and metric nuts and washers aren’t interchangeable

A metric M10 nut and an imperial 3/8″ nut look similar at first glance, but the actual diameter and thread pitch are different. Forcing an imperial-metric combination wears down the thread, loosens the joint under vibration, and in safety parts (automotive, structural) it can be outright dangerous. The key is identifying the original system before looking for a replacement.

Imperial-metric nut and washer conversion table: most common diameters

Here’s the approximate wrench width (distance across flats) equivalence for standard hex nuts:

• Metric nut M4 → 7 mm wrench ≈ 0.2756 in
• Metric nut M5 → 8 mm wrench ≈ 0.3150 in
• Metric nut M6 → 10 mm wrench ≈ 0.3937 in
• Metric nut M8 → 13 mm wrench ≈ 0.5118 in
• Metric nut M10 → 17 mm wrench ≈ 0.6693 in
• Metric nut M12 → 19 mm wrench ≈ 0.7480 in
• Imperial nut 1/4″ → 7/16″ wrench ≈ 11.11 mm
• Imperial nut 5/16″ → 1/2″ wrench ≈ 12.70 mm
• Imperial nut 3/8″ → 9/16″ wrench ≈ 14.29 mm
• Imperial nut 1/2″ → 3/4″ wrench ≈ 19.05 mm

Notice something important: an M6 (10 mm wrench) doesn’t match any round imperial size, which is why it’s worth always keeping both wrench sets on hand if you work with mixed machinery.

Nut inch-mm equivalence: how to read thread pitch

Outer diameter isn’t the only thing that matters. Thread pitch (distance between two consecutive crests) determines whether a screw goes in and turns smoothly in a nut. In the metric system it’s expressed in millimeters between threads (e.g., M10x1.5); in imperial, in threads per inch (TPI, e.g., 3/8-16 UNC). These figures don’t convert with a simple rule of three, because they follow different thread standards (ISO versus UNC/UNF).

To dig deeper into this technical difference, check out the difference between UNC/UNF imperial threads and metric threads, which explains why converting the diameter alone isn’t enough.

Imperial-metric washer size table: inner and outer diameter

Standard flat washers are sized by their inner diameter (the one that holds the screw) and their outer diameter. Here’s a practical reference:

• Washer M6 → inner 6.4 mm / outer 12.5 mm ≈ 0.2520 in / 0.4921 in
• Washer M8 → inner 8.4 mm / outer 17 mm ≈ 0.3307 in / 0.6693 in
• Washer M10 → inner 10.5 mm / outer 21 mm ≈ 0.4134 in / 0.8268 in
• Imperial washer 1/4″ → inner 6.73 mm / outer 15.88 mm
• Imperial washer 3/8″ → inner 10.31 mm / outer 23.81 mm
• Imperial washer 1/2″ → inner 13.49 mm / outer 31.75 mm

When the difference between one diameter and another is just a few tenths of a millimeter, the washer can end up loose or stuck. That’s where a precise conversion prevents returns and poorly assembled parts.

How to measure a nut or washer without calipers on hand

If you don’t have a caliper nearby, you can use a ruler graduated in mm, resting the nut flat-side to flat-side (not corner to corner, which gives a larger, false reading). For the washer, measure the central hole from inner edge to inner edge. With that figure in millimeters, all that’s left is converting it to inches to compare against American catalogs.

For that step, the fastest way is using the inches to millimeters calculator, which gives you the exact figure instantly without having to memorize conversion factors or work through decimals by hand.

Common mistakes when mixing nuts and washers from different systems

The most frequent mistakes at the hardware store counter or workshop:

• Selling an M8 washer for a 5/16″ imperial screw, thinking “close enough” is fine
• Confusing wrench width with the screw shaft diameter
• Using a UNC nut on a metric thread and forcing the fit, damaging both parts
• Not checking thread pitch when restocking, only the nominal diameter

A customer bringing in a machinery part imported from the US is a typical case where double-checking before dispatching the replacement part is worthwhile.

Tools that help verify the correct system

Besides the table, two physical tools help avoid mistakes: a digital caliper (with an mm/in switch) and a thread pitch gauge set to identify the exact pitch. If you also need to check the right wrench to tighten the nut, check out how many millimeters a 1/2 inch wrench measures before choosing a tool.

When to use the table and when to measure the part directly

The table works well for new catalog parts, where the manufacturer follows the standard. But for old, rusted, or non-standard manufactured parts, direct measurement with calipers always wins. Treat the table as a starting point, not a substitute for checking the actual physical part in front of you.

Frequently asked questions

Does an M10 nut fit a 3/8 inch screw?

Not safely. The diameter of an M10 is 10 mm (0.3937 in) and that of a 3/8″ screw is 9.525 mm. Although the nut might fit if forced, the thread pitch is different and the joint will end up loose or damage the thread over time.

How do I know if a washer is imperial or metric without a catalog?

Measure the inner diameter precisely in millimeters. If the result is a round number (6, 8, 10, 12 mm), it’s probably metric. If converting to inches gives a common fraction like 1/4″ (6.35 mm) or 3/8″ (9.525 mm), it’s imperial.

Why doesn’t wrench width match exactly between systems?

Because the metric (ISO) and imperial (ANSI/UNC) standards were developed separately with different size progressions. An M6 (10 mm wrench) has no exact equivalent in common inch fractions, only close approximations.

What margin of error is acceptable when converting these measurements?

For general fastening parts, a margin of 0.1 mm is usually tolerable. For structural or automotive safety parts, zero tolerance is recommended: always use the part from the original system, not one converted by approximation.

Do lock (grower) washers follow the same table as flat washers?

The inner diameter matches the flat washer table for the same nominal size, but the outer diameter and thickness vary by manufacturer, so it’s worth checking those two figures separately.