Inches to Millimeters Calculator

Measurement Converter

Enter decimals, fractions, mixed numbers, or scientific notation.

From Inches (in)

To Millimeters (mm)

Measurement Examples: 5.5, 3/8, 2 1/2, 1e-3

in

Spaces are allowed in mixed fractions.

Decimal precision

Rounding mode

Fraction denominator

Workshop, tolerance, and comparison options

Material allowance Input unit

Kerf deduction Input unit

Quantity multiplier

Positive tolerance Output unit

Negative tolerance Output unit

Target value Output unit

Remove trailing zeros Show scientific notation

Batch Conversion Table

Convert many measurements, sort them, copy the table, or download reports.

Formula Used

Millimeters = Inches × 25.4

One international inch equals exactly 25.4 millimeters. This conversion factor is exact, not approximate.

Example: 5.5 × 25.4 = 139.7 millimeters.

Exact relationship

Every calculated digit originates from the exact 25.4 factor.

Displayed rounding

Precision changes the displayed value, not the underlying conversion.

Fraction rounding

Fractional inches use your selected denominator for practical measurements.

Example Data Table

Example	Inches	Millimeters	Calculation
Thin sheet	0.04	1.016	0.04 × 25.4
Quarter inch	0.25	6.35	0.25 × 25.4
Half inch	0.5	12.7	0.5 × 25.4
One inch	1	25.4	1 × 25.4
Five and one-half inches	5.5	139.7	5.5 × 25.4
One foot	12	304.8	12 × 25.4
One yard	36	914.4	36 × 25.4

Common Fractional Inches

Use this machining and construction reference for common fractions.

Fraction	Decimal inches	Millimeters	Micrometers
1/64 in	0.015625	0.396875 mm	396.875 µm
1/32 in	0.03125	0.79375 mm	793.75 µm
3/64 in	0.046875	1.190625 mm	1,190.625 µm
1/16 in	0.0625	1.5875 mm	1,587.5 µm
5/64 in	0.078125	1.984375 mm	1,984.375 µm
3/32 in	0.09375	2.38125 mm	2,381.25 µm
7/64 in	0.109375	2.778125 mm	2,778.125 µm
1/8 in	0.125	3.175 mm	3,175 µm
9/64 in	0.140625	3.571875 mm	3,571.875 µm
5/32 in	0.15625	3.96875 mm	3,968.75 µm
11/64 in	0.171875	4.365625 mm	4,365.625 µm
3/16 in	0.1875	4.7625 mm	4,762.5 µm
13/64 in	0.203125	5.159375 mm	5,159.375 µm
7/32 in	0.21875	5.55625 mm	5,556.25 µm
15/64 in	0.234375	5.953125 mm	5,953.125 µm
1/4 in	0.25	6.35 mm	6,350 µm
5/16 in	0.3125	7.9375 mm	7,937.5 µm
3/8 in	0.375	9.525 mm	9,525 µm
7/16 in	0.4375	11.1125 mm	11,112.5 µm
1/2 in	0.5	12.7 mm	12,700 µm
9/16 in	0.5625	14.2875 mm	14,287.5 µm
5/8 in	0.625	15.875 mm	15,875 µm
11/16 in	0.6875	17.4625 mm	17,462.5 µm
3/4 in	0.75	19.05 mm	19,050 µm
13/16 in	0.8125	20.6375 mm	20,637.5 µm
7/8 in	0.875	22.225 mm	22,225 µm
15/16 in	0.9375	23.8125 mm	23,812.5 µm
1 in	1	25.4 mm	25,400 µm

Inches to Millimeters Reference Chart

Inches	Millimeters	Centimeters	Meters	Feet
0.001	0.0254	0.00254	0.0000254	0.00008333
0.01	0.254	0.0254	0.000254	0.00083333
0.03125	0.79375	0.079375	0.00079375	0.00260417
0.0625	1.5875	0.15875	0.0015875	0.00520833
0.125	3.175	0.3175	0.003175	0.01041667
0.1875	4.7625	0.47625	0.0047625	0.015625
0.25	6.35	0.635	0.00635	0.02083333
0.3125	7.9375	0.79375	0.0079375	0.02604167
0.375	9.525	0.9525	0.009525	0.03125
0.4375	11.1125	1.11125	0.0111125	0.03645833
0.5	12.7	1.27	0.0127	0.04166667
0.625	15.875	1.5875	0.015875	0.05208333
0.75	19.05	1.905	0.01905	0.0625
0.875	22.225	2.2225	0.022225	0.07291667
1	25.4	2.54	0.0254	0.08333333
1.5	38.1	3.81	0.0381	0.125
2	50.8	5.08	0.0508	0.16666667
2.5	63.5	6.35	0.0635	0.20833333
3	76.2	7.62	0.0762	0.25
4	101.6	10.16	0.1016	0.33333333
5	127	12.7	0.127	0.41666667
6	152.4	15.24	0.1524	0.5
8	203.2	20.32	0.2032	0.66666667
10	254	25.4	0.254	0.83333333
12	304.8	30.48	0.3048	1
18	457.2	45.72	0.4572	1.5
24	609.6	60.96	0.6096	2
30	762	76.2	0.762	2.5
36	914.4	91.44	0.9144	3
48	1,219.2	121.92	1.2192	4
60	1,524	152.4	1.524	5
72	1,828.8	182.88	1.8288	6
84	2,133.6	213.36	2.1336	7
96	2,438.4	243.84	2.4384	8
120	3,048	304.8	3.048	10
144	3,657.6	365.76	3.6576	12

Calculation History

History stays in this browser until you clear it.

How to Use This Calculator

Choose the required conversion direction.
Enter a decimal, fraction, or mixed number.
Select precision and a rounding method.
Open advanced options when adjustments are needed.
Press the calculation button to view results.
Copy, print, save, or export your conversion.

Practical note: Measure twice before cutting costly material. Tool calibration and manufacturing tolerances may affect real dimensions.

Detailed Conversion Guide

Understanding inches

An inch is an imperial and United States customary length unit. Twelve inches form one foot. Thirty-six inches form one yard. Inches remain common in construction, woodworking, displays, hardware, and many consumer products.

Understanding millimeters

A millimeter is one-thousandth of a meter. Ten millimeters form one centimeter. Millimeters are useful for compact, precise measurements. Engineering drawings, manufacturing specifications, medical devices, and international standards commonly use millimeters.

The exact conversion

The international inch is defined as exactly 25.4 millimeters. Therefore, multiplying inches by 25.4 produces millimeters. Dividing millimeters by 25.4 produces inches. The factor itself requires no approximation.

Decimal input

Type decimal values directly. For example, enter 2.75 for two and three-quarter inches. The calculator preserves the exact numeric conversion before applying your chosen display precision.

Fraction input

Fractions are common on rulers and workshop plans. Enter values like 3/8 or 7/16. Mixed values also work. Enter 2 1/2 for two and one-half inches.

Rounding choices

Standard rounding suits most everyday conversions. Upward rounding prevents undersizing in selected applications. Downward rounding prevents exceeding a strict maximum. Truncation removes extra digits without conventional rounding.

Machining and tolerances

Manufactured parts may require tolerance bands. Positive and negative tolerances describe acceptable dimensional variation. This calculator displays a convenient output range, but project drawings remain authoritative.

Kerf and allowance

Kerf represents material removed by a cutting tool. Allowance adds extra material for trimming, finishing, or fitting. Enter these adjustments in the original input unit before conversion.

Batch conversion

Batch mode converts many values simultaneously. Add rows, paste values, sort results, and export a structured report. This is useful for cut lists, product dimensions, and drawing schedules.

Avoiding mistakes

Do not confuse millimeters with centimeters. One inch equals 25.4 millimeters, but only 2.54 centimeters. Confirm the selected direction before copying results. Preserve sufficient precision for technical work.

Choosing precision

Two decimals usually suit general measurements. Three or four decimals suit detailed comparison. Engineering requirements may demand more digits. Excessive displayed precision does not improve the original measurement quality.

Reliable workflow

Start with a verified source measurement. Select the proper direction. Keep the exact result during intermediate steps. Apply rounding only at the final stage. Document tolerances when dimensions influence fit or safety.

Frequently Asked Questions

How many millimeters are in one inch?

One international inch equals exactly 25.4 millimeters.

How do I convert inches to millimeters?

Multiply the inch measurement by 25.4. For example, four inches equal 101.6 millimeters.

Can I enter fractional inches?

Yes. Enter simple fractions like 3/8. Mixed numbers like 2 1/2 also work.

Is 25.4 an exact conversion factor?

Yes. The international inch is legally defined as exactly 25.4 millimeters.

How do I convert millimeters back to inches?

Divide the millimeter measurement by 25.4. Use the swap button for reverse conversion.

Why does my result show many decimals?

Some reverse conversions repeat indefinitely. Choose a smaller precision for a cleaner practical result.

What fraction denominator should I select?

Use sixteenths for general work. Use thirty-seconds or sixty-fourths for finer measurement needs.

Can I download several conversions?

Yes. Batch mode creates CSV and PDF reports from all valid rows.