How do you add up measurements in feet and inches?

Put everything in one unit before you add. Convert feet to inches, turn fractions into decimals, add the column, then convert the total back. Mixed units are where most measuring math goes wrong.

Step by step

1. Feet to inches: 6 ft 4 in = 6 × 12 + 4 = 76 in.
2. Fractions to decimals: 3/8 = 0.375.
3. Add all the inches.
4. Back to feet: divide by 12. 245.5 ÷ 12 = 20.46, so 20 ft and 0.46 × 12 = 5.5 in.

Example: trim for a room

Wall	Measured	In inches
North	12 ft 3 1/2 in	147.5
East	8 ft 9 in	105
South	12 ft 3 1/2 in	147.5
West, minus door	5 ft 6 3/8 in	66.375
Total		466.375 in = 38 ft 10 3/8 in

If you add 10% for cuts and waste: 466.375 × 1.1 = 513 in, about 42 ft 9 in.

Inch fractions as decimals

1/8	1/4	3/8	1/2	5/8	3/4	7/8
.125	.25	.375	.5	.625	.75	.875

On the job

A construction calculator with feet-inch keys handles fractions directly, and for a big job it’s the right tool. I build Voice Calculator, which does plain numbers: you say “147.5 plus 105 plus 147.5 plus 66.375 equals” with a tape measure in hand and see each value on the tape before the total. It won’t convert units for you.

Related: scaling a recipe by voice, why the tape matters and the Voice Calculator guides.