The trick is the 12: carry it, borrow it
Feet and inches add like time on a clock, not like plain numbers. Inches roll over at 12, not 10. Add the inches; whenever they hit 12, take away 12 and add 1 to the feet. Subtracting works the same in reverse: if you run short on inches, borrow a foot and count it as 12 inches.
The reliable method: turn each measurement into total inches, add or subtract, then split back — feet is the total divided by 12, inches is the remainder.
Worked both ways
- Adding. 3 ft 8 in + 2 ft 7 in → 44 in + 31 in = 75 in → 75 ÷ 12 = 6 ft, remainder 3 in → 6 ft 3 in.
- Subtracting with a borrow. 10 ft 4 in − 6 ft 9 in → 124 in − 81 in = 43 in → 3 ft 7 in.
- Going negative. Subtracting a larger length just flips the sign: 4 ft − 5 ft 6 in = −1 ft 6 in.
Feet-and-inches to metric
| Feet & inches | Centimeters | Meters |
|---|---|---|
| 5'0" | 152.4 | 1.52 |
| 5'6" | 167.6 | 1.68 |
| 5'9" | 175.3 | 1.75 |
| 5'10" | 177.8 | 1.78 |
| 6'0" | 182.9 | 1.83 |
| 6'2" | 188.0 | 1.88 |
The exact factors: 1 inch = 2.54 cm and 1 foot = 30.48 cm, both fixed by international agreement in 1959. So 5'10" = 5×30.48 + 10×2.54 = 177.8 cm.
The mistakes that ruin a cut list
- Decimal feet are not inches. 5.6 ft is 5 ft 7.2 in, not 5 ft 6 in. Multiply the decimal part by 12 to get real inches.
- Inches over 12 need normalizing. "5 ft 14 in" is really 6 ft 2 in — carry the extra foot before you use the number.
- The notation. An apostrophe means feet, a double quote means inches: 5'10" is five foot ten.
Common questions
How do I add feet and inches together?
Add the inches first. If they reach 12 or more, subtract 12 and carry 1 to the feet. Then add the feet. So 3 ft 8 in plus 2 ft 7 in: 8 + 7 = 15 in, which is 1 ft 3 in, giving 6 ft 3 in.
How do I subtract feet and inches?
If the top inches are smaller than the bottom inches, borrow 1 foot as 12 inches and add it before subtracting. For 10 ft 4 in minus 6 ft 9 in: borrow to make it 9 ft 16 in, then 16 - 9 = 7 in and 9 - 6 = 3 ft, giving 3 ft 7 in.
Is 5.6 feet the same as 5 feet 6 inches?
No. Decimal feet are tenths, not inches. 5.6 ft is 5 ft 7.2 in, because 0.6 x 12 = 7.2. To write 5 ft 6 in as decimal feet you would use 5.5.
