How to Find a Screw’s Equivalent from Inches to Millimeters
A screw marked as 1/4″ doesn’t measure 1/4 of anything useful until you convert it to millimeters. If you need to buy a replacement, fit an imported part, or simply understand what screw you’re holding, here’s the exact conversion, no fuss.
The basic formula: from inches to millimeters
One inch equals exactly 25.4 mm. That figure is fixed, not an approximation. To convert any screw measured in inches, multiply the inch value by 25.4 and you get the diameter in millimeters.
For example: a 3/8″ screw equals 0.375 × 25.4 = 9.525 mm. A 1/2″ screw is 0.5 × 25.4 = 12.7 mm.
How much is 1/4 inch in mm on a screw
This is the most searched question, and the answer is straightforward: 1/4 inch (0.25″) equals 6.35 mm. It’s a very common size in household hardware, especially in furniture and appliances made in countries that use the imperial system.
Quick equivalence table: screw inches to mm
- 1/8″ = 3.175 mm
- 3/16″ = 4.762 mm
- 1/4″ = 6.35 mm
- 5/16″ = 7.937 mm
- 3/8″ = 9.525 mm
- 7/16″ = 11.112 mm
- 1/2″ = 12.7 mm
- 5/8″ = 15.875 mm
- 3/4″ = 19.05 mm
Save this list or check it whenever you have a screw in front of you and don’t know which wrench or drill bit to use.
How to measure the actual diameter of a screw
A screw’s diameter is measured on the threaded part, from one edge to the other (outer diameter), not on the head. Use a caliper if you want precision. If the screw already has a marked size (for example, on the original packaging), that figure in inches is the one you should convert using the formula above.
If you don’t have a caliper handy, the fastest way to solve this is to use an inches to millimeters calculator: you enter the measurement in inches and get the exact equivalent in millimeters instantly, with no margin for manual calculation error.
Converting screw size to millimeters: fractions vs decimals
Screw sizes in inches are usually expressed as fractions (1/4″, 5/16″, 3/8″), not decimals. To convert, first turn the fraction into a decimal by dividing numerator by denominator, then multiply by 25.4.
Example with 5/16″: 5 ÷ 16 = 0.3125. Then 0.3125 × 25.4 = 7.937 mm. This intermediate step is what confuses people most who aren’t used to working with fractions.
Why screws exist in both inches and millimeters
The imperial system (inches) dominates in the United States, while the metric system (millimeters) is the standard in most of the rest of the world. This means furniture, machinery, or appliances made in different countries can use incompatible hardware, even if it looks similar visually.
If you want to understand this difference in more depth, check out this guide on hardware and fasteners in inches vs. the metric system.
Common mistakes when converting screws
What to do if you can’t find the exact size in mm
If after converting you get a value like 6.35 mm and your local hardware store only sells 6 mm or 6.5 mm, just any size won’t do: the difference, though it may seem minimal, can affect thread fit. The safest option is to look for specific inch-sized hardware or adapters, rather than forcing an approximate metric size.
To dig deeper into the thread differences between both systems, you might be interested in this article on UNC/UNF imperial threads vs. metric threads.
Frequently asked questions
How much is 1/4 inch in mm on a screw?
1/4 inch equals exactly 6.35 mm. It’s a very common size in household hardware and furniture made in countries with the imperial system.
How is the screw inches to mm equivalence calculated?
The inch value (in decimal) is multiplied by 25.4, the fixed equivalent of one inch in millimeters. If the measurement is a fraction, first convert it to decimal by dividing numerator by denominator.
Can I convert a screw size to millimeters without a caliper?
Yes, if you know the size in inches marked on the packaging or on the original screw, you don’t need to measure physically: just apply the conversion formula or use an online calculator to get the exact value in millimeters.
Is a 6 mm screw the same as a 1/4 inch screw?
They are not identical. 1/4 inch equals 6.35 mm, not exactly 6 mm. The 0.35 mm difference may seem small, but it affects thread fit and can cause looseness or difficulty screwing in correctly.
Why doesn’t my inch screw fit a similar metric nut?
Even if the diameter is similar after conversion, the thread pitch (the distance between threads) is usually different between imperial and metric systems. That’s why, even though they may seem to fit at first glance, the thread often won’t turn correctly or gets damaged if forced.