MM to inches converter and quick-reference chart
Last reviewed on 2026-08-28.
To convert mm to inches, divide by 25.4 — 1 inch = 25.4 mm, exactly. Use the converter below for any value (for example 1.5875 mm = 1/16", 4.7625 mm = 3/16", 7.9375 mm = 5/16"), or scroll down for the mm to inch chart, the fraction table, and a visual comparison of how big a millimeter is next to an inch.
Edit either field. The nearest 1/64" fraction is shown here.
For small, everyday measurements — screw sizes, beads, wire gauges, ring sizes, small electronic components — the millimeter is the workhorse unit in the metric system and the natural counterpart to the fractional inch in the imperial system. Converting between the two is defined exactly, and for quick work, a handful of reference values cover most situations.
The exact definition
Since 1959, the inch has been defined as exactly 25.4 millimeters. Every other factor — millimeters per inch, inches per millimeter, centimeters per inch — derives from this single relationship.
Quick facts
1 in = 25.4 mm (exact)
1 mm = 0.03937007874… in
10 mm = 1 cm = 0.3937 in
1/16 in ≈ 1.5875 mm
1/8 in = 3.175 mm
1/4 in = 6.35 mm
Formulas
To convert millimeters to inches, divide by 25.4:
inches = mm ÷ 25.4
To convert inches to millimeters, multiply by 25.4:
mm = inches × 25.4
Millimeter to inch table
| mm | Inches (decimal) | Nearest 1/16" |
|---|---|---|
| 1 mm | 0.0394" | 1/16" |
| 2 mm | 0.0787" | 1/16" |
| 3 mm | 0.1181" | 2/16" (1/8") |
| 4 mm | 0.1575" | 3/16" |
| 5 mm | 0.1969" | 3/16" |
| 6 mm | 0.2362" | 4/16" (1/4") |
| 7 mm | 0.2756" | 4/16" (1/4") |
| 8 mm | 0.3150" | 5/16" |
| 9 mm | 0.3543" | 6/16" (3/8") |
| 10 mm | 0.3937" | 6/16" (3/8") |
| 12 mm | 0.4724" | 8/16" (1/2") minus a bit |
| 15 mm | 0.5906" | 9/16"–10/16" |
| 20 mm | 0.7874" | 13/16" |
| 25 mm | 0.9843" | 16/16" (≈1") minus 0.4 mm |
| 25.4 mm | 1.0000" | Exactly 1" |
| 30 mm | 1.1811" | 1 3/16" |
| 40 mm | 1.5748" | 1 9/16" |
| 50 mm | 1.9685" | 1 15/16" |
| 75 mm | 2.9528" | 2 15/16" |
| 100 mm | 3.9370" | 3 15/16" |
Common fractional inch → millimeter
| Fraction | Decimal inches | mm | Typical use |
|---|---|---|---|
| 1/64" | 0.0156" | 0.397 mm | Small fastener pitch, fine woodworking |
| 1/32" | 0.0313" | 0.794 mm | |
| 1/16" | 0.0625" | 1.5875 mm | Finest common imperial tick |
| 3/32" | 0.0938" | 2.381 mm | |
| 1/8" | 0.1250" | 3.175 mm | Audio jack, small dowel |
| 3/16" | 0.1875" | 4.7625 mm | |
| 1/4" | 0.2500" | 6.35 mm | Guitar cable, common bolt size |
| 5/16" | 0.3125" | 7.9375 mm | |
| 3/8" | 0.3750" | 9.525 mm | Standard socket wrench drive |
| 7/16" | 0.4375" | 11.1125 mm | |
| 1/2" | 0.5000" | 12.7 mm | Plumbing, common bolt size |
| 5/8" | 0.6250" | 15.875 mm | |
| 3/4" | 0.7500" | 19.05 mm | Garden hose, common bolt size |
| 7/8" | 0.8750" | 22.225 mm | |
| 1" | 1.0000" | 25.4 mm | Reference |
How big is a millimeter compared to an inch?
A millimeter is small: about the thickness of a credit card, or the width of the finest tick mark on a metric ruler. It takes 25.4 of them to make one inch, so 1 mm is roughly 1/25 of an inch — a little smaller than the 1/16" tick on an imperial ruler (1/16" = 1.5875 mm). Some comparisons people ask about most often:
| Metric size | In inches | Compared with a fractional inch |
|---|---|---|
| 1 mm | 0.039" | A bit under 1/16" (1/16" = 1.59 mm) |
| 4 mm | 0.157" | Just under 3/16" (3/16" = 4.76 mm); about 5/32" |
| 5 mm | 0.197" | Smaller than 1/4" (1/4" = 6.35 mm) — about 1.3 mm less |
| 6 mm | 0.236" | Slightly under 1/4" |
| 14 mm | 0.551" | Closer to 1/2" (12.7 mm) than to 3/4" (19.05 mm); about 9/16" |
| 18 mm | 0.709" | A little under 3/4" (19.05 mm) — about 1 mm short |
| 25 mm | 0.984" | Just under 1" (0.4 mm short) |
| 30.2 mm | 1.189" | Not 1 3/8" (34.9 mm) — closer to 1 3/16" (30.16 mm) |
| 33 mm | 1.299" | About 1 5/16"; 1 1/2" would be 38.1 mm |
To see any of these at actual size, open the online ruler, calibrate it, and drag the red line to the value in one unit before switching to the other.
Mental shortcuts
- Every 25 mm is just under one inch (it's short by 0.4 mm).
- Every 3 mm is roughly 1⁄8" (short by 0.175 mm).
- Every 6 mm is roughly 1⁄4" (short by 0.35 mm).
- For "close enough" work, treat 1 mm as 1⁄25" and 10 mm as 4⁄10".
Where millimeters are the right unit
Millimeters are the default unit in most technical drawings worldwide, in nearly all scientific and medical work, and in consumer goods in the metric world: paper sizes (A4 is 210 × 297 mm), plumbing diameters, hardware specifications, jewelry (ring diameters, wire gauges), photography (lens focal lengths — "50 mm" — and filter thread diameters), firearms (caliber), and camera/smartphone sensor sizes.
Rounding rules of thumb
- For day-to-day measuring, round to the nearest 1 mm or 1/32".
- For machined parts, follow the dimension's implied tolerance — typically 0.1 mm or 0.005" unless otherwise specified.
- For medical, legal, or safety contexts, don't round at all beyond the precision the source value actually carries, and verify with a calibrated physical tool.
Related references
- Inches ↔ centimeters — larger-scale reference table.
- Calibration guide — how to make the on-screen ruler accurate enough to trust.
- Measurement accuracy — when an on-screen ruler is fine, and when it isn't.
- How to read a ruler in inches — turning decimal inches into ruler fractions.
- The ruler tool.