Convert Millimeters to Inches
This tool converts millimeters (mm) to inches. Enter a value in either field and get the result as a decimal, fraction, and the nearest standard fractional inch used in woodworking, machining, and hardware sizing.
Formula
inches = mm ÷ 25.4
One inch is defined as exactly 25.4 millimeters. This relationship was standardized internationally in 1959. To convert millimeters to inches, divide by 25.4. To convert inches to millimeters, multiply by 25.4.
Common Millimeters to Inches Conversions
Reference table for frequently needed mm to inch conversions. Values rounded to 4 decimal places.
| Millimeters | Decimal Inches | Fractional Inch (approx.) | Common Use |
|---|---|---|---|
| 1 mm | 0.0394" | ≈ 1/32" (close) | Paper thickness |
| 2 mm | 0.0787" | ≈ 5/64" | Pencil lead, thin wire |
| 3 mm | 0.1181" | ≈ 1/8" (close) | 3D printer nozzle, small screws |
| 4 mm | 0.1575" | ≈ 5/32" | Allen keys, drill bits |
| 5 mm | 0.1969" | ≈ 13/64" | Hex bolts, Allen keys |
| 6 mm | 0.2362" | ≈ 15/64" | Standard bolt size |
| 6.35 mm | 0.2500" | = ¼" (exact) | Quarter-inch reference |
| 8 mm | 0.3150" | ≈ 5/16" | Wrench sizes, bolts |
| 10 mm | 0.3937" | ≈ 25/64" | Socket wrench, bolts |
| 12 mm | 0.4724" | ≈ 15/32" | Plywood thickness |
| 12.7 mm | 0.5000" | = ½" (exact) | Half-inch reference |
| 15 mm | 0.5906" | ≈ 19/32" | Pipe fittings |
| 18 mm | 0.7087" | ≈ 45/64" | Plywood, MDF panels |
| 19 mm | 0.7480" | ≈ ¾" (close) | Socket wrench, lumber |
| 19.05 mm | 0.7500" | = ¾" (exact) | Three-quarter inch |
| 20 mm | 0.7874" | ≈ 25/32" | Bolts, pipe |
| 25 mm | 0.9843" | ≈ 63/64" | Close to 1 inch |
| 25.4 mm | 1.0000" | = 1" (exact) | Exact inch definition |
| 30 mm | 1.1811" | ≈ 1 3/16" | Watch case diameter |
| 40 mm | 1.5748" | ≈ 1 37/64" | Watch case, camera lens |
| 50 mm | 1.9685" | ≈ 1 31/32" | Standard lens focal length |
| 75 mm | 2.9528" | ≈ 2 61/64" | Close to 3 inches |
| 100 mm | 3.9370" | ≈ 3 15/16" | Close to 4 inches |
| 150 mm | 5.9055" | ≈ 5 29/32" | Close to 6 inches |
| 200 mm | 7.8740" | ≈ 7 7/8" | Close to 8 inches |
| 300 mm | 11.8110" | ≈ 11 13/16" | Close to 1 foot |
When You Need mm to Inches Conversion
Millimeters and inches serve the same purpose — measuring length — but belong to different systems. Millimeters are part of the metric system used in most countries. Inches are part of the imperial system used mainly in the United States, United Kingdom, and Canada.
You typically need this conversion when:
- Buying hardware or tools: Metric bolts, screws, and drill bits need to match imperial-sized holes (or vice versa).
- 3D printing: Nozzle diameters (0.4 mm, 0.6 mm) and filament sizes (1.75 mm, 2.85 mm) are metric, but some design files use inches.
- Woodworking: Plywood and lumber in metric-standard countries have thicknesses like 12 mm and 18 mm that need to match imperial sizing (½", ¾").
- Machining and engineering: Technical drawings may use either system depending on origin country.
- Electronics: Component dimensions, PCB measurements, and enclosure sizes often mix metric and imperial specs.
- Jewelry and watches: Ring sizes, chain widths, and watch case diameters are commonly specified in mm.
Understanding Fractional Inches
Imperial measurements use fractions rather than decimals in many practical applications. Standard fractional increments are based on powers of 2: halves, quarters, eighths, sixteenths, thirty-seconds, and sixty-fourths of an inch.
This means metric values rarely convert to exact fractions. For example, 10 mm = 0.3937", which falls between 25/64" (0.3906") and 13/32" (0.4063"). The nearest standard fraction depends on the precision you need:
- To 1/16": rough carpentry, general DIY
- To 1/32": fine woodworking, hardware sizing
- To 1/64": machining, precision work
This converter shows the nearest fraction at 1/64" precision, plus the exact error so you know how close the approximation is.
Worked Examples
Example 1: Converting a Bolt Size
A metric bolt is labeled M8 (8 mm diameter). What is 8 mm in inches?
8 ÷ 25.4 = 0.3150"
Nearest fraction: 5/16" (0.3125")
Difference: 0.0025" (0.064 mm) — close enough for most applications
Example 2: Plywood Thickness
A sheet is labeled 18 mm. What imperial thickness is this closest to?
18 ÷ 25.4 = 0.7087"
Nearest standard lumber size: ¾" (19.05 mm)
Note: 18 mm plywood is not the same as ¾" — the 1.05 mm difference matters for joinery
Example 3: 3D Printer Nozzle
A 0.4 mm nozzle — how does that compare in inches?
0.4 ÷ 25.4 = 0.01575"
Nearest fraction: 1/64" (0.01563")
In practice, 3D printing dimensions stay in metric
Frequently Asked Questions
How do you convert millimeters to inches?
Divide the millimeter value by 25.4. One inch is defined as exactly 25.4 millimeters. For example, 10 mm ÷ 25.4 = 0.3937 inches.
How many inches is 25 mm?
25 mm is approximately 0.9843 inches, which is very close to — but not exactly — 1 inch. You need 25.4 mm for exactly 1 inch.
What is 10 mm in inches?
10 mm equals approximately 0.3937 inches, which is close to 25/64 of an inch (0.3906"). In practical terms, 10 mm falls between 3/8" and 13/32".
How do I convert mm to fractional inches?
First divide by 25.4 to get decimal inches. Then find the nearest standard fraction. For example, 6.35 mm ÷ 25.4 = 0.25 inches = ¼ inch exactly. Most mm values won't land on an exact fraction — use the converter above to find the nearest one and see the error.
Why is 1 inch equal to 25.4 mm?
The international yard and pound agreement of 1959 defined 1 inch as exactly 25.4 millimeters. This standardized the relationship between the imperial and metric systems across the US, UK, Canada, Australia, New Zealand, and South Africa.
What is the difference between mm and inches?
Millimeters are metric units equal to 1/1000 of a meter, used in most countries worldwide. Inches are imperial units used mainly in the United States, United Kingdom, and Canada. One inch equals 25.4 mm, making millimeters the smaller unit.
Is 5 mm the same as 3/16 inch?
Close but not exact. 5 mm = 0.1969", while 3/16" = 0.1875". The difference is about 0.24 mm. For most practical purposes, 5 mm sits between 3/16" and 13/64" (0.2031").
Does this converter store my data?
No. All conversions run entirely in your browser using JavaScript. No data is sent to any server.
Related Tools
- Inches to Centimeters Converter — convert inches to cm
- Centimeters to Millimeters Converter — convert cm to mm
- Meters to Feet Converter — convert between metric and imperial length
- Length Unit Converter — convert between all common length units
- Decimal to Fraction Converter — convert decimal inches to fractions
Privacy & Limitations
- All calculations run entirely in your browser -- nothing is sent to any server.
- Results are estimates and may vary based on actual conditions.
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This tool FAQ
How do you convert millimeters to inches?
Divide the millimeter value by 25.4. One inch is defined as exactly 25.4 millimeters. For example, 10 mm ÷ 25.4 = 0.3937 inches.
How many inches is 25 mm?
25 mm is approximately 0.9843 inches, which is very close to 1 inch. The exact fraction is 25/25.4.
What is 10 mm in inches?
10 mm equals approximately 0.3937 inches, which is close to 25/64 of an inch.
How do I convert mm to fractional inches?
Divide by 25.4 to get decimal inches, then find the nearest standard fraction. For example, 6.35 mm ÷ 25.4 = 0.25 inches = 1/4 inch exactly.
Why is 1 inch equal to 25.4 mm?
The international yard and pound agreement of 1959 defined 1 inch as exactly 25.4 millimeters. This standardized the relationship between the imperial and metric systems.
What is the difference between mm and inches?
Millimeters are metric units (1/1000 of a meter) used in most countries worldwide. Inches are imperial units used mainly in the United States, United Kingdom, and Canada. One inch equals 25.4 mm.
Is 5 mm the same as 3/16 inch?
Very close but not exact. 5 mm = 0.1969 inches, while 3/16 inch = 0.1875 inches. The difference is about 0.24 mm. For most practical purposes, 5 mm is between 3/16 and 13/64 of an inch.
Does this converter store my data?
No. All conversions run entirely in your browser using JavaScript. No data is sent to any server.