How to Tell If a Screw Bought in the US Fits Your Project

You’ve bought a screw from a US website, or a relative living there gave it to you, and now you’re wondering: does that thread work for your project where everything is in millimeters? The short answer is that they’re almost never interchangeable without adjustments, even though sometimes they’ll “fit” by force. Here’s the exact process to check before you ruin a part or a nut.

Why an American screw doesn’t always work in a metric project

The United States uses the imperial system (inches) for standard hardware, while Europe and most of the world works in millimeters. It’s not just a matter of units: the thread pitch, the thread angle, and the nominal diameter are designed to different specifications. A 1/4-inch screw isn’t exactly equivalent to any standard metric screw, even though its diameter is close to 6.35 mm.

This means that even if you manage to thread it in by hand for a few turns, the actual fit will be flawed: the thread won’t fully match, it will wear out faster, and it can loosen on its own from vibration.

The three details you need from the American screw

Before comparing anything, write down these three details about the screw in your hand:

• Nominal diameter in inches (for example: 1/4″, 5/16″, 3/8″)
• Number of threads per inch, or TPI (for example: 20, 24, 28)
• Thread type: UNC (coarse) or UNF (fine)

This information is usually stamped on the packaging, listed on the online store’s product page, or on the screw itself if the diameter is large enough. If you bought a loose screw and don’t have this data, you’ll need to measure it yourself, and there’s a simple method for that further down.

Screw bought in the US: does it fit my project?

To know if a screw bought in the US fits your project, compare these three points against the metric part you need to replace or fit:

• Diameter: convert the inch measurement to millimeters and compare it with the metric size you need (for example, M6, M8, M10)
• Thread pitch: the pitch in mm of the metric screw must match, not just the diameter
• Nut or tapped hole type: if you’re threading into an existing metric nut, the American screw will rarely fit well

A very common case: a 1/4″-20 screw (1/4 inch, 20 threads per inch) has a diameter of 6.35 mm. It looks a lot like an M6 (6 mm), but it isn’t the same: the M6’s thread pitch is 1 mm, while the 1/4″-20 has a pitch equivalent to 1.27 mm. They can fit by force, but the result is a weak, loose connection.

How to convert the inch measurement to millimeters

The first practical step is to convert the screw’s inch measurement into millimeters so you have a number you can compare with your project. The formula is simple: multiply the fraction of an inch by 25.4. For example, 3/8″ x 25.4 = 9.525 mm.

If you don’t want to do the math by hand every time, the quickest option is to use an inches to millimeters calculator. Enter the fraction or decimal in inches and get the exact millimeter equivalent instantly, with no rounding error.

Keep in mind that the diameter in millimeters is only half of the comparison. You also need the thread pitch, which isn’t converted with the same formula: it’s measured in threads per inch (TPI) in the imperial system and in millimeters per turn in the metric system.

American vs metric screw compatibility: when NOT to force the fit

Compatibility between American and metric screws fails more often than it seems at first glance. Some signs that you shouldn’t force the assembly:

• The screw goes in with uneven resistance, not a smooth, steady turn
• You notice the thread “skipping” or making noise as it turns
• The screw head doesn’t sit flat on the surface
• You need to force the first few turns with a wrench instead of your fingers

Forcing an incompatible screw damages the threading of the original metric part, and after that, not even the correct screw will fit properly. In structural fastening parts (furniture that holds weight, supports, machinery) this isn’t just a cosmetic annoyance: it’s a real risk of failure.

US vs metric screw: cases where substitution does work

Not everything is incompatible. There are cases of US vs compatible metric screws where substitution works fine:

• When the screw passes through an unthreaded through-hole (the pitch doesn’t need to match, only the diameter needs to be smaller than the hole)
• When you’ll use a matching American nut with the American screw, both bought in the same system
• When the project allows some tolerance, like temporary or low-stress fastenings

If your project is a DIY piece of furniture with through-holes and you’ll use a nut and screw bought together in the same US order, there’s no conflict: the whole system is consistent in inches from start to finish.

Difference between UNC, UNF, and metric threads

Within the American system itself there are two thread families: UNC (coarse thread, fewer threads per inch) and UNF (fine thread, more threads per inch). Neither matches the standard pitch of ISO metric threads, which use a different thread angle and a different pitch progression depending on diameter.

To fully understand this difference and avoid confusing an M8 with a 5/16″-18, it’s worth reviewing the difference between UNC/UNF imperial threads and metric threads, which explains how they’re numbered and what makes them incompatible between systems.

Tools you need to check the screw at home

If the screw doesn’t come with a spec sheet, measure it yourself with these basic tools:

• Calipers, to measure the actual outer diameter of the thread
• A thread pitch gauge, to count the threads per inch
• A reference metric nut (M6, M8, M10) to test whether it fits or not

If you’re going to buy an additional nut or washer to complete the assembly, check the conversion table for nuts and washers between systems first to make sure you buy the right part instead of another incompatible one.

What to do if the American screw doesn’t fit

If, after checking diameter and pitch, you confirm there’s no compatibility, you have three practical options:

• Buy the closest equivalent metric screw at a local hardware store
• Use a threaded insert adapter, if the project allows it (not very advisable on thin parts)
• Also replace the nut or threaded part with its American equivalent, if the rest of the assembly allows it

The safest option is almost always the first one: buy the correct metric part. Mixing systems only works well when you control both ends of the connection (screw and nut, or screw and female thread) in the same measurement system.

Frequently asked questions

Is a 1/4-inch screw the same as an M6?

No. A 1/4-inch screw measures 6.35 mm in diameter, slightly more than an M6 (6 mm), and its thread pitch is also different. They may look interchangeable at first glance, but they aren’t.

Can I use a metric nut with an American screw?

Generally no, because the thread pitch doesn’t match between systems even though the diameter may look similar. It may go in a few turns by force, but the connection will end up loose and the threading will get damaged over time.

How do I know if my screw bought in the US fits my project without measuring tools?

Without tools, the most reliable check is trying to thread it in by hand into the original metric nut or part. If it goes in smoothly, with even resistance, and the head sits flat, there’s reasonable compatibility. If it makes noise, jams, or you need to force it with a wrench, it isn’t compatible.

What happens if I force an American screw into a metric thread?

You permanently damage the threading of the original metric part. From that point on, not even the correct screw will fit firmly, and you’ll have to repair or replace the entire threaded part.

Are there adapters to join American and metric hardware?

Yes, threaded insert adapters exist, but they’re rarely used in home projects because they add an extra weak point to the connection. It’s better to replace the screw or nut with its correct equivalent in the system you need.