Measurements
Tape Measure Math Calculator
Add, subtract, multiply, and divide the fractional measurements you actually read off a tape — like 5 3/8" − 2 1/4"137 mm − 57 mm. The answer renders as an exact decimal plus a clean cut size to the nearest 1/16" or 1/32"millimeter, with a visual tape measure that shows exactly where your result lands.
Tape Measure Math
Fractions, mixed numbers, and decimals all work.
Fractions, mixed numbers, and decimals all work.
Operation
Ruler Precision
Result
Exact answer, clean fraction, and a visual on the tape.
5 3/8 − 2 1/4 = 3 1/8"
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How tape measure math works
The trick to doing fraction math in the shop is to stop thinking in fractions for the math and start thinking in decimals. Every measurement is converted to a plain number of inches, the operation is performed, and the result is rounded back to the nearest tape mark for your cut.The trick to doing math in the shop is to use exact decimal millimeters. Every measurement is calculated precisely, the operation is performed, and the result is rounded to a clean metric cut size for your tape.
1. Convert to decimals
A mixed number like 5 3/8 becomes 5.375 inches. The calculator accepts mixed numbers, construction-hyphen forms like 14-3/8, plain fractions, and decimals.
5 3/8 = 5 + 3 ÷ 8 = 5.375"
2. Do the math, then round
Add, subtract, multiply, or divide the decimals, then snap the answer to the nearest 1/16" or 1/32" — whatever your tape shows. The result is reduced to lowest terms automatically, so you'll never see 14 6/16" when you mean 14 3/8".
5.375 − 2.25 = 3.125 → 3 1/8"
1. Work in millimeters
Metric shop math is straightforward because there are no fractions to convert. Just enter your exact millimeter measurements directly into the calculator. It accepts standard whole numbers and decimals.
137 mm and 57 mm
2. Do the math, then cut
Add, subtract, multiply, or divide your measurements. The calculator performs the math exactly, then snaps the final answer to the nearest clean millimeter mark on your tape so you can cut without guessing.
137 − 57 = 80 mm
Common Fractional Equivalents
Handy when you're converting a call-out to a decimal for a CNC or a calculator. These reduce to lowest terms automatically.
| Fraction | Decimal | Millimeters |
|---|---|---|
| 1/16" | 0.0625 | 1.59 mm |
| 1/8" | 0.125 | 3.18 mm |
| 3/16" | 0.1875 | 4.76 mm |
| 1/4" | 0.25 | 6.35 mm |
| 3/8" | 0.375 | 9.53 mm |
| 1/2" | 0.5 | 12.7 mm |
| 5/8" | 0.625 | 15.88 mm |
| 3/4" | 0.75 | 19.05 mm |
| 7/8" | 0.875 | 22.23 mm |
| 1" | 1 | 25.4 mm |
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Frequently Asked Questions
How do you add and subtract fractions on a tape measure?
Convert every measurement to a decimal first (for example 5 3/8" becomes 5.375"), do the math, then convert the result back to the nearest fraction. This calculator does the conversion for you and renders the answer as a clean cut size to the nearest 1/16" or 1/32".
How do you add and subtract measurements on a tape measure?
Make sure your measurements are in millimeters, perform the math, and read the clean result. This calculator does the math for you and renders the exact cut size to the nearest millimeter.
What does it mean to multiply two measurements?
Multiplying a length by a scale factor is common in the shop — for example enlarging a pattern by 3/4 or figuring what two-and-a-half board lengths add up to. The calculator treats the product as a new length and rounds it to a clean fraction so you can set your fence and cut.
How do I read a result that shows 1/16" precision?
The nearest 1/16" is the closest of the sixteen tick marks between every inch on a standard tape measure. For finer work, switch the ruler precision to 1/32" — the tape measure graphic updates to show the smaller ticks and drops an amber marker right on the exact position of your answer.
How do I read a result in millimeters?
A millimeter is the smallest tick mark on a standard metric tape measure. The calculator renders your exact decimal answer and then snaps it to the nearest clean millimeter so you can mark your cut accurately without guessing.
Can I type measurements in different formats?
Yes. The calculator accepts mixed numbers like 5 3/8137, the construction-hyphen form like 14-3/8137, plain fractions, and decimals. Use the InchesMillimeters toggle in the calculator header to switch how you type and read the measurements.
Why does the divide result look different?
Division of two measurements is a ratio or a count — for example, how many 3/4"19 mm sections fit in a 6"152 mm length. It is a number of times, not a length, so it is shown as a decimal rather than placed on the tape measure graphic.
More shop calculators
Looking for something else in the shop? Browse the full MyShopCalc calculator directory for cabinet doors, drawer slides, deck spacing, and more.