How to use the fraction tape measure calculator
- Enter each length as you read it off the tape: 5 3/8, 2' 3 1/2", 7/16, 5.375, 35 mm or 3.5 cm.
- Choose Add or Subtract for each length. Add more rows as needed.
- Optionally multiply or divide the result, for example by 4 boards or into 3 equal parts.
- Choose the nearest mark on your tape, from 1/2 to 1/64 inch, and read the answer in inches, feet and inches, decimals and millimeters.
Formula
Each length is converted to an exact fraction of an inch, with feet x 12 and millimeters / 25.4
Total = lengths added or subtracted, then multiplied or divided exactly
Marks = total x denominator, rounded to the nearest whole number (halves away from zero)
Answer = marks / denominator, reduced
No rounding happens until the last step, so the answer is as close to the exact total as the tape mark allows.
5 3/8 plus 2 7/8
5 3/8 plus 2 7/8 is 8 1/4 inches, which is 8.25 inches or 209.55 millimeters. On a different job, 2' 3 1/2" plus 1' 10 3/4" is 27.5 + 22.75 = 50.25 inches, shown as 50 1/4 inches or 4' 2 1/4".
A decimal of 1.3 inches rounds to 1 5/16 at the nearest 1/16, which is 0.0125 inch more than the exact value, and to 1 1/4 at the nearest 1/4. Dividing 10 1/2 inches into 3 equal parts gives 3 1/2 inches.
Why exact fractions matter
Adding fractions by hand invites mistakes with common denominators, and converting to decimals introduces rounding that builds up over many cuts. This tool keeps the arithmetic exact and rounds once at the end to the mark you can really read. It also reports how far the rounded answer is from the exact one.
It differs from the general fraction calculator in working with units, feet and inches, metric and tape marks. Check a conversion with the unit converter or hold a ruler against the screen with the online ruler.
Writing lengths
A mixed number can be written with a space or a hyphen, as in 5 3/8 or 5-3/8. Feet use a single quote or ft and inches a double quote or in. Millimeters, centimeters and meters can be used for any length. A result below zero is shown with a minus sign.
Cutting adds the saw's kerf and the width of your pencil mark, which this tool does not know. For lumber volume, the board foot calculator is a next step.
Assumptions and limits
- The nearest mark is your choice, and halves round away from zero.
- Millimeters and centimeters are converted with 25.4 millimeters per inch, which is exact.
- Kerf, tolerances and expansion are not modeled.
- Decimals may have up to 12 digits after the point.
Frequently asked questions
How do I add fractions of an inch?
Convert each to a common denominator and add the numerators, or enter them here and let the tool do it. 5 3/8 plus 2 7/8 is 8 1/4.
How do I subtract lengths on a tape measure?
Choose Subtract for the second length. The tool subtracts exactly and shows a minus sign if the answer is below zero.
How do I convert millimeters to inches?
Divide by 25.4. The tool does this exactly for you, so 25.4 mm is exactly 1 inch.
What does rounding to the nearest 1/16 mean?
The answer is moved to the closest mark on a tape that has 16 marks per inch, and the difference from the exact value is shown.