How to use this calculator
Select your input format: fractions (like 3/8"), decimal inches (like 0.375"), or millimeters. Then type your measurement. The converter shows results in all three formats instantly as you type.
For fractional inputs, you can type simple fractions like "3/8" or mixed numbers like "5 1/2". Use the quick select buttons for common tape measure fractions you see marked on a standard tape.
This tool is especially useful when working with building materials, hardware specifications, or any project where you need to convert between imperial and metric measurements. The reference table below lists all sixteenth inch increments with their decimal and millimeter equivalents.
Reading a tape measure
A standard tape measure divides each inch into 16 equal parts. The longest marks after the inch marks represent 1/2 inch. The next longest represent 1/4 inch, then 1/8 inch, and finally the shortest marks represent 1/16 inch.
The trick to reading a tape measure is counting the smallest marks from the nearest inch line. If you count 5 of the smallest (1/16") marks past the 3-inch line, your measurement is 3 5/16". This converter eliminates the guesswork by giving you the decimal and metric equivalents instantly.
Common tape measure conversions
| Fraction | Decimal (in) | Millimeters |
|---|---|---|
| 1/16" | 0.0625" | 1.588 mm |
| 1/8" | 0.125" | 3.175 mm |
| 3/16" | 0.1875" | 4.763 mm |
| 1/4" | 0.25" | 6.35 mm |
| 5/16" | 0.3125" | 7.938 mm |
| 3/8" | 0.375" | 9.525 mm |
| 7/16" | 0.4375" | 11.113 mm |
| 1/2" | 0.5" | 12.7 mm |
| 9/16" | 0.5625" | 14.288 mm |
| 5/8" | 0.625" | 15.875 mm |
| 11/16" | 0.6875" | 17.463 mm |
| 3/4" | 0.75" | 19.05 mm |
| 13/16" | 0.8125" | 20.638 mm |
| 7/8" | 0.875" | 22.225 mm |
| 15/16" | 0.9375" | 23.813 mm |
| 1" | 1.0" | 25.4 mm |
Frequently asked questions
What is 3/8 on a tape measure in decimal?
3/8 of an inch equals 0.375 inches, or 9.525 millimeters. On a tape measure, the 3/8" mark is the third-longest line between inch marks.
How do I convert inches to millimeters?
Multiply the inch value by 25.4. For example, 1/2 inch = 0.5 × 25.4 = 12.7 mm. This converter does this calculation automatically for any measurement you enter.
What are the marks on a tape measure?
The longest marks are full inches. Between them, marks decrease in size for 1/2, 1/4, 1/8, and 1/16 inch increments. The smallest marks on a standard tape measure represent 1/16 of an inch (about 1.6 mm).
How accurate is this converter?
This converter is mathematically precise. It uses the exact conversion factor of 1 inch = 25.4 millimeters, which is the internationally defined standard.