Feet and Inches Calculator
8 ft 6 1/2 in plus 3 ft 9 3/4 in is 12 ft 4 1/4 in, 12.3542 decimal feet, 148.25 inches, and 3,765.55 mm.
Last updated . The operation starts on Add. Fractions start at 1/2 and 3/4. The inches box is whole inches, and the fraction is extra, down to 1/16.
A feet and inches calculator for a tape: add or subtract two readings, then read the result as feet and a reduced fraction, decimal feet, inches, and millimeters.
A running total of several lengths, or square feet turned into a length: linear feet calculator. Board feet from a nominal size: board foot calculator.
Add or subtract a tape
First measurement
Second measurement
The inches box is the whole inches. The fraction is added on top, down to 1/16. An inch entry of 14 is allowed and is carried into feet in the result. One inch is 25.4 mm. This is a tape reading, not board feet.
Results
How feet and inches are added
Each reading is stored in sixteenths: feet × 12 × 16, plus whole inches × 16, plus the fraction. Add or subtract those integers, then carry sixteenths into inches and inches into feet. The fraction in the result is reduced, so 4 sixteenths displays as 1/4 and 8 sixteenths displays as 1/2. An inches entry of 14, or 40, is allowed and is carried in the result.
Decimal feet = total inches ÷ 12, to four decimals. Millimeters = inches × 25.4, to two decimals. The default sum is 102.5 + 45.75 = 148.25 inches, which is 12 ft 4 1/4 in and 12.3542 ft. A result below zero is shown with a minus sign. It is a shortfall, not a cut length.
This is a tape reading. It is not board feet, and it is not a running total of more than two pieces.
Tape readings
| Readings | Feet and inches | Decimal feet | Inches | mm |
|---|---|---|---|---|
| 8 ft 6 1/2 in + 3 ft 9 3/4 in | 12 ft 4 1/4 in | 12.3542 | 148.25 | 3,765.55 |
| 10 ft 2 in − 3 ft 6 1/2 in | 6 ft 7 1/2 in | 6.6250 | 79.50 | 2,019.30 |
| 0 ft 40 in + 0 | 3 ft 4 in | 3.3333 | 40.00 | 1,016.00 |
| 2 ft − 3 ft | −1 ft 0 in | -1.0000 | -12.00 | -304.80 |
Worked example
Leave Add, 8 ft 6 1/2 in, and 3 ft 9 3/4 in. Feet and inches reads 12 ft 4 1/4 in. Decimal feet reads 12.3542 ft. Inches reads 148.25 in. Millimeters reads 3,765.55 mm.
Feet and inches questions
What is 8 ft 6 1/2 in plus 3 ft 9 3/4 in?
Leave the operation on Add. The first measurement stays 8 ft, 6 in, and 1/2. The second stays 3 ft, 9 in, and 3/4. Feet and inches reads 12 ft 4 1/4 in. Decimal feet reads 12.3542 ft. Inches reads 148.25 in. Millimeters reads 3,765.55 mm. The first reading is 102.5 inches and the second is 45.75 inches. 148.25 ÷ 12 = 12.354166…, which rounds to 12.3542. Millimeters are 148.25 × 25.4.
What is 10 ft 2 in minus 3 ft 6 1/2 in?
Set the operation to Subtract. First measurement: 10 ft, 2 in, fraction 0. Second measurement: 3 ft, 6 in, fraction 1/2. Feet and inches reads 6 ft 7 1/2 in. Decimal feet reads 6.6250 ft. Inches reads 79.50 in. Millimeters reads 2,019.30 mm. The difference is 79.5 inches, and 79.5 × 25.4 = 2,019.3.
How many feet is 40 inches?
Leave the operation on Add. Set the first measurement to 0 ft, 40 in, and fraction 0. Set the second measurement to 0 ft, 0 in, and fraction 0. Feet and inches reads 3 ft 4 in. Decimal feet reads 3.3333 ft. Inches reads 40.00 in. Millimeters reads 1,016.00 mm. Inches of 12 or more are carried into feet. 40 ÷ 12 = 3.3333… feet, and 40 × 25.4 = 1,016 mm.
What happens if the second measurement is longer?
Set the operation to Subtract. First measurement: 2 ft, 0 in, fraction 0. Second measurement: 3 ft, 0 in, fraction 0. Feet and inches reads −1 ft 0 in. Decimal feet reads -1.0000 ft. Inches reads -12.00 in. Millimeters reads -304.80 mm. A negative result is a shortfall, not a length you can cut.
Which fractions can I enter?
The fraction menu is in sixteenths: 0, 1/16, 1/8, 3/16, 1/4, 5/16, 3/8, 7/16, 1/2, 9/16, 5/8, 11/16, 3/4, 13/16, 7/8, and 15/16. The inches box is the whole inches. The fraction is extra. The result reduces the fraction, so 8 sixteenths displays as 1/2.
How do inches become decimal feet and millimeters?
Add the feet, the whole inches, and the fraction into a total in inches. Decimal feet = that total ÷ 12, shown to four decimals. Millimeters = inches × 25.4, shown to two decimals. One inch is 25.4 mm.
Is this board feet, or a list of linear feet?
No. This page adds or subtracts two tape readings. Board feet from a nominal size is the board foot calculator. A running total of several lengths, and square feet turned into linear feet, is the linear feet calculator.