📐 Fraction to MM Calculator
📊 Fraction to MM Reference Table
| Fraction | Decimal (in) | Millimeters |
|---|
Fraction to MM Calculator
You're staring at a tape measure that reads 5/16", and the part you need to order is specified in millimeters. Or maybe it's the other way around: a metric drawing gives you 12mm, and your ruler only shows inches and fractions. Either way, doing that math in your head is easy to get wrong. This fraction to mm calculator converts any inch fraction to millimeters (and back) in one step, with the exact decimal shown alongside it.
Try the Calculator
Enter a fraction like 3/4 or a mixed number like 1 1/2, and the calculator returns the millimeter equivalent instantly. Switch the mode to go the other direction, from millimeters back to the closest standard fraction.
(Calculator tool embedded above)
How to Use the Calculator
- Choose your conversion direction. Pick "Fraction to MM" if you're starting from an inch measurement, or "MM to Fraction" if you're starting from a metric one.
- Enter your fraction. Type it as a simple fraction (3/4), or as a whole number plus a fraction (2 3/8) if your measurement is more than one inch.
- Read the result. The calculator shows the millimeter value, usually rounded to two decimal places, which is precise enough for almost any woodworking, metalworking, or DIY project.
- Round if you need to. Most tape measures and rulers only mark increments of 0.5mm or 1mm, so round the result to match the precision of the tool you're actually using.
Why Fraction to MM Conversion Matters
The United States still measures most everyday things in inches and fractions of an inch. Most of the rest of the world uses millimeters. That split shows up constantly in real work: a piece of hardware made in Germany, a set of drill bits made in the US, a set of plans drawn up by an engineer trained on the metric system, or a part ordered from an overseas supplier.
Get the conversion wrong and you don't just get a number that looks a little off. A drill bit that's 0.5mm too small will bind and snap in a hole. A board cut 1mm short on every piece adds up to a visibly crooked frame by the time you assemble ten of them. Precision here isn't optional, it's the difference between a project that fits and one that doesn't.
The Math Behind the Conversion
One inch equals exactly 25.4 millimeters. That number is fixed, it's the internationally agreed definition of the inch, so every fraction to mm conversion comes back to this one constant.
To convert a fraction of an inch to millimeters:
- Convert the fraction to a decimal (divide the numerator by the denominator).
- Multiply that decimal by 25.4.
Worked Example: 5/16" to mm
5 ÷ 16 = 0.3125
0.3125 × 25.4 = 7.9375mm
So 5/16" equals approximately 7.94mm.
Worked Example: 1 1/4" to mm
Convert the mixed number to a decimal first: 1 + (1 ÷ 4) = 1.25
1.25 × 25.4 = 31.75mm
So 1 1/4" equals 31.75mm exactly.
Common Fraction to MM Conversions
Here's a quick-reference chart for the fractions that come up most often on a standard tape measure or ruler.
| Fraction (inches) | Decimal (inches) | Millimeters |
|---|---|---|
| 1/16" | 0.0625 | 1.59mm |
| 1/8" | 0.125 | 3.18mm |
| 3/16" | 0.1875 | 4.76mm |
| 1/4" | 0.25 | 6.35mm |
| 5/16" | 0.3125 | 7.94mm |
| 3/8" | 0.375 | 9.53mm |
| 7/16" | 0.4375 | 11.11mm |
| 1/2" | 0.5 | 12.7mm |
| 9/16" | 0.5625 | 14.29mm |
| 5/8" | 0.625 | 15.88mm |
| 11/16" | 0.6875 | 17.46mm |
| 3/4" | 0.75 | 19.05mm |
| 13/16" | 0.8125 | 20.64mm |
| 7/8" | 0.875 | 22.23mm |
| 15/16" | 0.9375 | 23.81mm |
| 1" | 1.0 | 25.4mm |
Bookmark this table if you're doing repeated conversions on a job site where signal or battery life makes pulling up a calculator less convenient than a printed chart.
Converting Millimeters Back to Fractions
Going the other direction is a little trickier, because millimeters don't divide evenly into the sixteenths and thirty-seconds most rulers use. Here's the process:
- Divide the millimeter value by 25.4 to get inches as a decimal.
- Find the closest standard fraction (sixteenths are the most common increment on US tape measures).
- Round to that fraction, understanding you're accepting a small margin of error, usually under half a millimeter.
For example, 10mm ÷ 25.4 = 0.3937". The closest sixteenth is 6/16", which simplifies to 3/8" (9.53mm). That's a difference of about 0.47mm, small enough to ignore for most home projects, but worth knowing if you're working to tighter tolerances.
When Precision Really Matters
For most carpentry, framing, and general DIY work, rounding to the nearest sixteenth of an inch or half-millimeter is fine. But a few situations call for exact figures instead of rounded ones:
- Machining and metalworking, where a part needs to fit inside a tolerance of a few hundredths of a millimeter.
- 3D printing and CAD work, where files are often designed in millimeters but printed or measured against imperial reference parts.
- Plumbing and fittings, where a mismatch between imperial and metric threads can mean a fitting that looks close but doesn't actually seal.
- International parts ordering, where a supplier's spec sheet is in millimeters and your existing hardware is in inches, or vice versa.
In any of these cases, use the exact decimal output from the calculator rather than rounding to the nearest marked line on your ruler.
Fractions vs. Decimal Inches vs. Millimeters
It helps to see all three systems side by side, since drawings and spec sheets don't always use the same one.
| System | Example | Best for |
|---|---|---|
| Fraction (inches) | 3/8" | US tape measures, lumber, hardware |
| Decimal (inches) | 0.375" | Machining, precision tools, calipers |
| Millimeters | 9.53mm | Metric hardware, most of the world, engineering drawings |
If you're working from a set of plans that mixes systems, converting everything to millimeters first, then working from that single unit, usually cuts down on rounding errors compared to bouncing back and forth between fractions and decimals.
Frequently Asked Questions
Get the Exact Number Every Time
Guessing at inch-to-mm conversions is how small errors turn into parts that don't fit and cuts that don't line up. Use the calculator above whenever you need an exact figure, and keep the chart handy for the conversions you'll run into most often.