How to read a tape measure
Each inch on a standard US tape is divided into 16 parts. The lines get shorter as the fractions get smaller, so you can find any mark by its length:
| Line | Marks | In sixteenths |
|---|---|---|
| Longest (numbered) | Whole inches | 16/16 |
| Second longest | 1/2" | 8/16 |
| Third longest | 1/4", 3/4" | 4/16, 12/16 |
| Fourth longest | 1/8", 3/8", 5/8", 7/8" | 2/16, 6/16, 10/16, 14/16 |
| Shortest | Odd sixteenths: 1/16", 3/16" … 15/16" | 1/16, 3/16 … 15/16 |
To read a measurement, find the last whole inch, count the sixteenths after it, and reduce the fraction. 9 small lines past 7" is 7 9/16". 12 lines past 3" is 3 12/16", which reduces to 3 3/4".
Adding and subtracting fractions of an inch
- Write both fractions in sixteenths (or the smallest unit either one uses).
- Add or subtract the sixteenths. When subtracting, borrow 1 inch (16/16) if you need to.
- Add or subtract the whole inches.
- Reduce the fraction: divide top and bottom by 2 until the top is odd.
Example: 7 3/8" − 2 11/16". 3/8 is 6/16, which is less than 11/16, so borrow an inch: 6 22/16" − 2 11/16" = 4 11/16".
Fraction to decimal and mm
| Fraction | Decimal | mm |
|---|---|---|
| 0 1/16" | 0.0625" | 1.59 mm |
| 0 1/8" | 0.125" | 3.18 mm |
| 0 3/16" | 0.1875" | 4.76 mm |
| 0 1/4" | 0.25" | 6.35 mm |
| 0 5/16" | 0.3125" | 7.94 mm |
| 0 3/8" | 0.375" | 9.53 mm |
| 0 7/16" | 0.4375" | 11.11 mm |
| 0 1/2" | 0.5" | 12.70 mm |
| 0 9/16" | 0.5625" | 14.29 mm |
| 0 5/8" | 0.625" | 15.88 mm |
| 0 11/16" | 0.6875" | 17.46 mm |
| 0 3/4" | 0.75" | 19.05 mm |
| 0 13/16" | 0.8125" | 20.64 mm |
| 0 7/8" | 0.875" | 22.23 mm |
| 0 15/16" | 0.9375" | 23.81 mm |
For lengths with feet, use the feet and inches calculator, which carries every 12 inches into a foot and also multiplies and divides.
Tape measure questions
How do you read the small lines on a tape measure?
Between each inch, the longest line is 1/2", the next longest are 1/4" and 3/4", then the eighths, and the shortest lines are sixteenths. Count sixteenths from the last inch mark and reduce the fraction: 6 small lines past 4" is 4 6/16", which is 4 3/8".
How do you add fractions of an inch?
Give them the same denominator, add the tops, and simplify. 3/8" + 5/16" = 6/16" + 5/16" = 11/16". If the result is more than 1, the whole part becomes inches: 3/4" + 1/2" = 1 1/4".
What are the red numbers on a tape measure?
On most US tape measures, red numbers mark every 16 inches, the standard spacing for wall studs and floor joists. Small black diamonds (often at 19.2" intervals) mark spacing for engineered joists.
What is a sixteenth of an inch in mm?
1/16" is 1.5875 mm, so about 1.6 mm. 1/8" is 3.175 mm and 1/4" is 6.35 mm.