Convert Inches to Millimeters Instantly
Use this Inches to Millimeters converter to change inches (in) into millimeters (mm) with the exact international conversion factor. Enter whole numbers, decimals, or common fractional inch measurements, reverse millimeters back to inches, and compare related metric and imperial length units in one responsive calculator.
Choose one of four conversion modes for decimal inches, reverse conversion, fractional inch measurements, or multi-unit length conversion.
Enter any non-negative inch measurement to calculate millimeters plus related metric and imperial equivalents.
Decimals such as 0.5, 1.25 and 3.75 are supported.
Reverse the conversion by dividing millimeters by 25.4.
25.4 mm is exactly 1 inch.
Convert measurements written as a whole number plus a common fraction, such as 2 3/8 inches.
Start with inches, millimeters, centimeters, meters or feet and view all common equivalents together.
The Inches to Millimeters conversion is one of the most common imperial-to-metric length calculations. The rule is simple because the relationship is exact: one international inch is defined as exactly 25.4 millimeters. That means there is no approximation in the conversion factor itself. Any rounding happens only when you choose how many decimal places to display in the final answer.
To convert inches to millimeters, multiply the number of inches by 25.4. For example, 6 inches × 25.4 = 152.4 millimeters. If you are working with construction drawings, machinery dimensions, fastener sizes, screen dimensions, woodworking plans, pipe components, automotive parts, product specifications or engineering measurements, this exact factor gives a consistent bridge between inch-based and metric dimensions.
Because 1 inch is exactly 25.4 mm, this formula works for whole numbers, decimals and fractional inches after the fraction is converted to decimal form.
If your starting measurement is already metric, use the reverse converter above or visit the related Millimeters to Inches converter. For another everyday imperial conversion, the Inches to Feet calculator is useful when dimensions become too large to read conveniently in inches alone.
The international inch is exactly 25.4 millimeters, so the base calculation does not rely on an estimated factor.
Convert common workshop and construction fractions such as 1/8, 3/16, 1/4, 3/8 and 1/2 inch.
Switch millimeters back to inches without manually rearranging the formula.
Calculator cards, FAQs and conversion tables adapt to small screens, while wide tables stay scrollable.
The formula mm = in × 25.4 is based on the international yard and pound agreement that standardized the inch in metric terms. In practical use, you do not need to recreate that history each time you calculate; you only need the fixed factor 25.4. The factor is exact, which is especially helpful when converting drawings or specifications where several converted values need to stay internally consistent.
Therefore, 10 inches equals exactly 254 millimeters.
Decimal inch measurements are handled in exactly the same way. Multiply the decimal by 25.4 and keep as many decimal places as the precision of your original measurement justifies.
First convert the fraction to a decimal: 1 ÷ 2 = 0.5. Then multiply 0.5 × 25.4 = 12.7 mm. The fractional converter above performs these two steps automatically.
This quick-reference table covers common whole-inch, decimal-inch and fractional-inch measurements. On a phone or narrow tablet, the table remains inside the responsive .ctc-table-wrap container so it can scroll horizontally without breaking the page layout.
| Inches | Millimeters | Centimeters | Typical Reference |
|---|---|---|---|
| 1/16 in | 1.5875 mm | 0.15875 cm | Fine fractional measurement |
| 1/8 in | 3.175 mm | 0.3175 cm | Common small fraction |
| 1/4 in | 6.35 mm | 0.635 cm | Quarter inch |
| 3/8 in | 9.525 mm | 0.9525 cm | Common hardware size |
| 1/2 in | 12.7 mm | 1.27 cm | Half inch |
| 5/8 in | 15.875 mm | 1.5875 cm | Fractional size |
| 3/4 in | 19.05 mm | 1.905 cm | Three-quarter inch |
| 1 in | 25.4 mm | 2.54 cm | Exact base relationship |
| 2 in | 50.8 mm | 5.08 cm | Two inches |
| 3 in | 76.2 mm | 7.62 cm | Three inches |
| 4 in | 101.6 mm | 10.16 cm | Four inches |
| 6 in | 152.4 mm | 15.24 cm | Half a foot |
| 12 in | 304.8 mm | 30.48 cm | One foot |
| 24 in | 609.6 mm | 60.96 cm | Two feet |
| 36 in | 914.4 mm | 91.44 cm | One yard |
| 48 in | 1219.2 mm | 121.92 cm | Four feet |
Fractional inches appear frequently on tape measures, drill-bit charts, bolt dimensions, lumber plans and mechanical drawings. The safest method is to convert the fraction to a decimal inch value before applying the 25.4 factor. For a mixed measurement such as 2 3/8 inches, divide 3 by 8 to get 0.375, add the two whole inches to get 2.375 inches, then multiply by 25.4.
Example: 2 3/8 in = (2 + 3 ÷ 8) × 25.4 = 60.325 mm.
Common fraction equivalents include 1/8 in = 3.175 mm, 1/4 in = 6.35 mm, 3/8 in = 9.525 mm, 1/2 in = 12.7 mm, 5/8 in = 15.875 mm, 3/4 in = 19.05 mm and 7/8 in = 22.225 mm. If a specification uses sixty-fourths of an inch, keep the full fractional value during the calculation and round only at the end.
When a part, drawing or product specification is given in millimeters but your measuring tools are marked in inches, reverse the relationship by dividing by 25.4.
For example, 100 mm ÷ 25.4 = approximately 3.9370079 inches.
Remember that many metric dimensions will not land on a simple imperial fraction. A result such as 3.937 inches may be closer to a familiar fraction on a tape measure, but converting the decimal to a fraction introduces another rounding step. For machining, fabrication or precision fitting, work with the decimal result or the original millimeter specification whenever possible.
The same physical length can be labeled in inches or millimeters. Every 1-inch interval adds exactly 25.4 mm.
Plans, imported tools, fasteners and building products may mix inch and metric dimensions. Converting inches to millimeters helps compare drill diameters, board thicknesses, clearances and hardware sizes. For larger length changes, you may also use Yards to Meters or Centimeters to Feet.
Mechanical drawings and component catalogs often publish both systems. Exact unit conversion is useful for dimension review, but converted numbers should not be treated as permission to substitute a different part or tolerance without checking the specification.
Socket sizes, bolt diameters and tubing dimensions are commonly described in either millimeters or fractional inches. A mathematical conversion helps compare nominal sizes, but near-equivalent sizes are not always mechanically interchangeable.
Screen sizes are commonly marketed in inches while product drawings may use millimeters. The same formula converts diagonal dimensions, widths, heights and thicknesses as long as you are converting a linear measurement rather than area or volume.
Understanding nearby units helps you check whether a result is reasonable. Since 10 millimeters make 1 centimeter, converting inches to centimeters can be done by multiplying inches by 2.54. Since 1 meter contains 1,000 millimeters, divide the millimeter result by 1,000 to obtain meters. In the imperial system, 12 inches make 1 foot.
For dedicated related calculations, see Inches to Centimeters, Centimeters to Meters, and Yards to Inches. Linking related units makes it easier to move through a full measurement workflow without repeatedly entering formulas by hand.
The inch-to-millimeter factor is exact, but the measurement you start with may not be. If a board is measured as 7.2 inches with a ruler that only supports tenths of an inch, displaying 182.880000 mm does not make the original measurement more precise. A practical result might be 182.9 mm or 183 mm depending on your required tolerance.
For general reference, keep two or three decimal places when the converted number is not naturally exact. For machining, scientific work or technical documentation, follow the tolerance and significant-digit rules of the original specification. For rough construction estimates, a whole millimeter may be sufficient. Never change the exact conversion factor itself simply to make the arithmetic easier.
The calculator uses the internationally standardized relationship of 1 inch = 25.4 millimeters. For authoritative unit definitions and SI guidance, consult recognized metrology sources rather than informal conversion charts.
Quick answers to common inch-to-mm conversion questions.