To read a tape measure, find the last whole-inch number the edge of the object passes, then count the small lines beyond it. On a standard US tape every small line is 1/16 inch, so a mark five lines past the 4 reads 4 5/16 inches. The line length tells you which fraction it is: the longest is 1/2, the next 1/4, then 1/8, and the shortest 1/16.
How are the markings on a tape measure laid out?
Between two numbered inch marks, a tape is divided into halves, quarters, eighths and sixteenths. Each finer division uses a shorter line, so the eye can tell them apart without counting every one.
| Line length | Fraction of an inch | Lines per inch |
|---|---|---|
| Longest (numbered) | 1 inch | 1 |
| Long | 1/2 | 2 |
| Medium | 1/4 | 4 |
| Short | 1/8 | 8 |
| Shortest | 1/16 | 16 |
Everything between two numbers is a fraction of one inch.
How do you read a measurement step by step?
- Find the whole inches. Look for the last number at or before the edge being measured.
- Count the small lines past it. Count every line, whatever its length, until you reach the edge. That count is the number of sixteenths.
- Put the count over 16. Eleven lines past the 6 is 11/16.
- Reduce the fraction. Divide top and bottom by the same number until nothing more divides evenly: 8/16 = 1/2, 4/16 = 1/4, 12/16 = 3/4, 6/16 = 3/8.
- Write the whole inches and the fraction together. 6 + 11/16 is 6 11/16 inches.
Only an odd count of sixteenths cannot be reduced: 1, 3, 5, 7, 9, 11, 13 and 15 sixteenths stay as they are, because each is odd.
Tape measure fraction chart
| Lines past the inch | Fraction | Reduced | Decimal inches | Millimeters |
|---|---|---|---|---|
| 1 | 1/16 | 1/16 | 0.0625 | 1.59 |
| 2 | 2/16 | 1/8 | 0.125 | 3.18 |
| 3 | 3/16 | 3/16 | 0.1875 | 4.76 |
| 4 | 4/16 | 1/4 | 0.25 | 6.35 |
| 5 | 5/16 | 5/16 | 0.3125 | 7.94 |
| 6 | 6/16 | 3/8 | 0.375 | 9.53 |
| 7 | 7/16 | 7/16 | 0.4375 | 11.11 |
| 8 | 8/16 | 1/2 | 0.5 | 12.7 |
| 9 | 9/16 | 9/16 | 0.5625 | 14.29 |
| 10 | 10/16 | 5/8 | 0.625 | 15.88 |
| 11 | 11/16 | 11/16 | 0.6875 | 17.46 |
| 12 | 12/16 | 3/4 | 0.75 | 19.05 |
| 13 | 13/16 | 13/16 | 0.8125 | 20.64 |
| 14 | 14/16 | 7/8 | 0.875 | 22.23 |
| 15 | 15/16 | 15/16 | 0.9375 | 23.81 |
The millimeter column uses the exact definition of the inch: NIST lists 1 in = 2.54 cm, which is 25.4 mm.
Worked examples
Example 1: a reading with a reducible fraction
An edge lands 6 lines after the 3-inch mark. That is 6/16, which reduces to 3/8. The reading is 3 3/8 inches. In decimals, 3 + 0.375 = 3.375 in, and 3.375 × 25.4 = 85.7 mm.
Example 2: an odd count
An edge lands 5 lines after the 12-inch mark: 12 5/16 inches. Five is odd, so the fraction stays as 5/16 = 0.3125; the decimal reading is 12.3125 in, or 312.7 mm.
Example 3: reading in feet and inches
A wall measures 8 feet 2 7/8 inches. In inches only: 8 × 12 = 96, plus 2.875 gives 98.875 in.
Example 4: adding two readings
Boards of 5 3/8 in and 2 7/8 in laid end to end: fractions first, 3/8 + 7/8 = 10/8 = 1 2/8 = 1 1/4. Whole numbers: 5 + 2 = 7. Total 8 1/4 inches. A decimal to fraction calculator helps when a result such as 8.25 has to be turned back into a tape reading.
How do you read the metric side?
Dual-scale tapes have a second edge in centimeters. The numbered lines are centimeters, and the small lines between them are millimeters, ten per centimeter (the SI base unit of length is the meter, and the millimeter is one thousandth of it). A mark 7 lines past the 23 reads 23.7 cm, or 237 mm. No fractions are involved: the reading is already decimal.
To switch between the two scales, use the exact factor of 25.4 mm per inch. A reading of 2 3/8 in is 2.375 × 25.4 = 60.3 mm, and 100 mm is 100 ÷ 25.4 = 3.94 in, a little under 3 15/16 in. The mm to inches and inches to mm converters do this for any value, and the inches to cm tool works in centimeters. For the wider picture of the two systems, see metric vs imperial.
Common mistakes
- Counting only the long lines. Count every line to get sixteenths, or the reading comes out too small.
- Forgetting to reduce. 8/16 is correct but 1/2 is how it is spoken and written on plans.
- Starting from the wrong point. Read from the zero end of the blade; if the tape is hooked on an edge, check that the end is seated before counting.
- Mixing scales. A reading such as "3 inches 4 mm" is meaningless; convert one side before adding.
- Rounding a fraction instead of reading it. 7/16 is not 3/8 or 1/2; the difference is 1.6 mm, enough to matter in woodwork or fittings.