Calculate, add, subtract, and convert feet and inches with fractions, decimals, and metric units in seconds.
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Feet and Inches Formula
Every mode of this calculator works by first converting each measurement into a single number of total inches, doing the math, and then converting the answer back into feet and inches. The core conversions are:
T = (F * 12) + I + r
F = floor(T / 12) I = T - (F * 12)
Adding or subtracting two lengths:
T = T1 + T2 or T = T1 - T2
Multiplying or dividing a single length by a number n:
T = T1 * n or T = T1 / n
Converting a decimal or metric length into total inches with a unit factor k (inches per unit):
T = L * k
- T = total length expressed in inches
- F = the whole number of feet
- I = the whole number of inches (0 to 11)
- r = the fractional part of an inch, such as 0.5 for 1/2 inch
- T1, T2 = the first and second lengths converted to total inches
- n = the multiplier or divisor for scaling a length
- L = a length entered as a decimal or metric value
- k = inches per unit (for example 1 for inches, 12 for feet, 0.3937 for centimeters)
The "Add or subtract two lengths" mode converts both lengths to inches and combines them. The "Multiply or divide a length" mode scales one length by a number, which is useful for spacing or repeated cuts. The "Convert feet and inches" mode turns a feet and inches value into decimal feet, decimal inches, and metric units. The "Convert decimal or metric length" mode does the reverse, turning a single value into clean feet and inches rounded to your chosen fraction.
Inch Fraction and Unit Reference
The calculator rounds fractional inches to the nearest fraction you select. Use the table below to read the decimal value of each common sixteenth of an inch.
| Fraction | Decimal inch | Fraction | Decimal inch |
|---|---|---|---|
| 1/16 | 0.0625 | 9/16 | 0.5625 |
| 1/8 | 0.125 | 5/8 | 0.625 |
| 3/16 | 0.1875 | 11/16 | 0.6875 |
| 1/4 | 0.25 | 3/4 | 0.75 |
| 5/16 | 0.3125 | 13/16 | 0.8125 |
| 3/8 | 0.375 | 7/8 | 0.875 |
| 7/16 | 0.4375 | 15/16 | 0.9375 |
| 1/2 | 0.5 | 1 | 1.0 |
These factors let you convert any single length into total inches before working in feet and inches.
| Unit | Inches per unit (k) |
|---|---|
| Foot | 12 |
| Yard | 36 |
| Millimeter | 0.03937 |
| Centimeter | 0.3937 |
| Meter | 39.3701 |
Example Problems
Example 1: Adding two lengths. Add 5 ft 8 1/2 in and 2 ft 9 3/4 in. Convert each to inches: the first is (5 * 12) + 8 + 0.5 = 68.5 inches, and the second is (2 * 12) + 9 + 0.75 = 33.75 inches. The total is 68.5 + 33.75 = 102.25 inches. Convert back: floor(102.25 / 12) = 8 feet, and 102.25 - 96 = 6.25 inches. The answer is 8 ft 6 1/4 in.
Example 2: Converting a metric length. Convert 1.75 meters to feet and inches. Total inches = 1.75 * 39.3701 = 68.90 inches. Then floor(68.90 / 12) = 5 feet, and 68.90 - 60 = 8.90 inches. Rounded to the nearest 1/16 inch this is 8 7/8 in, so 1.75 m is about 5 ft 8 7/8 in.
Frequently Asked Questions
How many inches are in a foot? There are exactly 12 inches in 1 foot. That is why converting to total inches uses a factor of 12, and why the answer wraps back into feet whenever the inch total reaches 12.
How do you add feet and inches together? Convert each measurement to total inches, add the inch totals, then convert the sum back to feet and inches. Working in a single unit avoids mistakes when the inches add up to more than 12.
How do you turn a decimal foot into inches? Multiply the decimal part of the foot value by 12. For example, 0.75 feet equals 0.75 * 12 = 9 inches, so 3.75 feet is 3 ft 9 in.
