Mm to Inches Calculator

Q: How many mm are in 1 inch?

There are exactly 25.4 millimeters in 1 inch. This number is exact, not rounded. It is the basis for every conversion this calculator performs.

Q: What fraction resolution should I use?

It depends on your work. For rough carpentry, 1/16 is usually fine. For finish work or machining, use 1/32 or 1/64 . The finer the fraction, the closer the result will be to the exact decimal value.

Mm To Inches Calculator

Updated Jun 27, 2026 By Jehan Wadia

Conversion Results

Decimal Inches

—

Enter a value above

Fractional Inches

—

Nearest fraction

Feet + Inches

—

Whole feet & remaining inches

Visual Inch Ruler

Ruler showing 0 inches.

Step-by-Step Solution

Reference Conversion Table

Common millimeter to inch conversion reference values
Millimeters (mm)	Decimal Inches (in)	Fractional Inches	Feet + Inches
0.01	0.000394 in	—	0 ft 0.000394 in
0.1	0.003937 in	—	0 ft 0.003937 in
1	0.03937 in	3/64 in	0 ft 0.03937 in
2	0.07874 in	5/64 in	0 ft 0.07874 in
3	0.11811 in	7/64 in	0 ft 0.11811 in
5	0.19685 in	13/64 in	0 ft 0.19685 in
10	0.39370 in	25/64 in	0 ft 0.39370 in
20	0.78740 in	51/64 in	0 ft 0.78740 in
25.4	1.00000 in	1 in	0 ft 1 in
50	1.96850 in	1 63/64 in	0 ft 1.96850 in
100	3.93701 in	3 15/16 in	0 ft 3.93701 in
200	7.87402 in	7 7/8 in	0 ft 7.87402 in
300	11.81102 in	11 13/16 in	0 ft 11.81102 in
304.8	12.00000 in	12 in	1 ft 0 in
500	19.68504 in	19 11/16 in	1 ft 7.68504 in
1000	39.37008 in	39 3/8 in	3 ft 3.37008 in

Introduction

This mm to inches calculator converts millimeters to inches instantly. Type in a value in millimeters, and the tool gives you the result in decimal inches, fractional inches, and feet plus inches. It works the other way too — enter inches or feet to get millimeters back.

One inch equals exactly 25.4 millimeters. To convert mm to inches, you divide by 25.4. This calculator does that math for you and shows each step so you can follow along. You can also pick how many decimal places you want and choose a fraction size down to 1/64 of an inch.

Builders, machinists, and engineers often need to switch between metric and imperial measurements. A bolt spec might be in millimeters, but your tape measure reads in inches. This tool makes that conversion fast and accurate, with a visual ruler so you can see the measurement at a glance. If you need to convert centimeters instead, try our cm to inches calculator.

How to Use Our Mm to Inches Calculator

Type a measurement into any input field below and the calculator will instantly convert it. You will get results in decimal inches, fractional inches, and feet plus inches.

Millimeters (mm): Enter the number of millimeters you want to convert to inches. This is the main input field.

Decimal Inches (in): Enter a value in inches to convert it back to millimeters. All other fields will update to match. If you need to convert inches to centimeters, use our inches to cm calculator.

Feet (ft): Enter the whole feet part of a measurement. Use this with the remaining inches field to convert feet and inches to millimeters. For more complex feet and inches math, see our feet and inches calculator.

Inches (remaining): Enter the leftover inches after whole feet. This works together with the feet field above.

Decimal Precision: Pick how many decimal places you want shown in your results. The default is 4 places.

Fraction Resolution: Pick the smallest fraction of an inch to round to, such as 1/16 or 1/64. The default is 1/64. If you need help working with fractions in general, our fraction calculator can help.

Press the Convert button to run the conversion, or simply type in any field to see live results. Press Reset to clear all fields and start over.

How to Convert Millimeters to Inches

Millimeters (mm) and inches (in) are both units used to measure length. Millimeters are part of the metric system, used in most countries around the world. Inches are part of the imperial system, used mainly in the United States. In construction, you will often need to switch between these two units because tools, materials, and plans may use either one. For converting larger metric measurements, our meters to feet calculator is also useful.

The key number to remember is 25.4. There are exactly 25.4 millimeters in one inch. To convert any millimeter value to inches, you simply divide by 25.4. For example, 100 mm ÷ 25.4 = 3.937 inches. To go the other way and convert inches back to millimeters, you multiply by 25.4.

Why This Conversion Matters in Construction

Many building materials like plywood, pipes, bolts, and fasteners are sold in metric sizes in some regions and imperial sizes in others. If you order a part measured in millimeters but your project plan uses inches, you need an accurate conversion to make sure everything fits. Even a small error can cause parts to not line up, which wastes time and money on a job site. When estimating material quantities, tools like our concrete calculator or board foot calculator can help once you have the right measurements in place.

Decimal Inches vs. Fractional Inches

When you divide millimeters by 25.4, you get a decimal number like 3.937 inches. However, most tape measures and rulers in the U.S. show fractions like 1/16 or 1/32 of an inch instead of decimals. That is why this calculator gives you both the decimal inch value and the closest fractional inch. Knowing the fraction makes it much easier to mark and cut materials with a standard tape measure. You can also use our decimal to fraction calculator to convert any decimal number into a fraction.

Feet and Inches

For longer measurements, it helps to break the result into feet and inches. There are 12 inches in one foot. So if a millimeter value converts to more than 12 inches, the calculator splits it into whole feet plus the remaining inches. This is how most builders in the U.S. read and call out measurements on a job site. If you need to calculate total length in linear feet for a project, our linear feet calculator or square footage calculator can help you plan materials accurately.

Formulas used

Millimeters to Decimal Inches

\text{inches} = \frac{\text{mm}}{25.4}

Whole Feet from Total Inches

\text{feet} = \left\lfloor \frac{\text{inches}}{12} \right\rfloor

Remaining Inches after Whole Feet

\text{remaining} = \text{inches} - (\text{feet} \times 12)

Nearest Fractional Inch

\text{fraction} = \frac{\operatorname{round}(\text{inches} \times N)}{N}

Frequently asked questions

How many mm are in 1 inch?

There are exactly 25.4 millimeters in 1 inch. This number is exact, not rounded. It is the basis for every conversion this calculator performs.

How do I convert mm to inches by hand?

Divide the millimeter value by 25.4. For example, 50 mm ÷ 25.4 = 1.9685 inches. That's it — one simple division is all you need.

How do I convert inches back to mm?

Multiply the inch value by 25.4. For example, 3 inches × 25.4 = 76.2 mm. You can also type a value into the Decimal Inches field in the calculator and it will convert to mm automatically.

What fraction resolution should I use?

It depends on your work. For rough carpentry, 1/16 is usually fine. For finish work or machining, use 1/32 or 1/64. The finer the fraction, the closer the result will be to the exact decimal value.

Why does the fractional result show an approximate symbol (≈)?

Most mm values do not convert to an exact fraction of an inch. The calculator rounds to the nearest fraction based on the resolution you pick. The ≈ symbol tells you the fraction is very close but not perfectly exact.

What does decimal precision do?

It controls how many digits appear after the decimal point in your results. A higher number gives a more precise answer. For most construction work, 4 decimal places is plenty.

Can I enter a value with decimals in the mm field?

Yes. You can type any decimal number, like 12.7 or 0.5. The calculator accepts whole numbers and decimals in every input field.

What happens if my value is larger than 12 inches on the ruler?

The visual ruler only shows up to 12 inches. If your value is larger, the red marker pins at the 12-inch mark and a notice appears telling you the actual measurement. Your numeric results are still fully accurate.

Can I type feet and inches to get millimeters?

Yes. Enter the whole feet in the Feet field and the leftover inches in the Inches field. The calculator will convert the total length to millimeters and update all result fields.

Is 10 mm the same as 1 cm?

Yes. There are 10 millimeters in 1 centimeter. So 10 mm = 1 cm = 0.3937 inches.

How many mm is 1/4 inch?

One-quarter inch equals 6.35 mm. You get this by multiplying 0.25 × 25.4.

How many mm is 1/2 inch?

One-half inch equals 12.7 mm. You get this by multiplying 0.5 × 25.4.

How accurate is this calculator?

The calculator uses the exact conversion factor of 25.4 mm per inch. Results are accurate to the number of decimal places you choose, up to 10 digits. It is precise enough for construction, machining, and engineering work.

Can I copy the results?

Yes. Each result card has a small copy button. Click it to copy that value to your clipboard. You can also copy the step-by-step solution using the Copy button in its header.

Do I need to press the Convert button every time?

No. The calculator updates live as you type in any field. The Convert button is there if you prefer to click it, but it is not required.

What are common mm to inch conversions for bolts and fasteners?

Here are a few common ones: 3 mm ≈ 1/8 in, 5 mm ≈ 3/16 in, 6 mm ≈ 1/4 in, 8 mm ≈ 5/16 in, 10 mm ≈ 3/8 in, 12 mm ≈ 1/2 in, and 16 mm ≈ 5/8 in.

Can I enter negative numbers?

No. If you type a negative number, the calculator resets that field to zero and shows an error message. Length values must be zero or greater.