Try one of these
Worked examples
| What | Sum | Answer |
|---|---|---|
| Add two lengths | 12' 4 3/8" + 7' 9 1/2" | 20' 1 7/8" |
| Take one off another | 10' 2" − 3' 7 3/4" | 6' 6 1/4" |
| Allow for the saw kerf | 8' − 1/8" | 7' 11 7/8" |
| Four of the same piece | 2' 7 1/2" × 4 | 10' 6" |
| Split a stair rise seven ways | 109 3/4" ÷ 7 | 15 11/16" (exactly 15 19/28") |
| How many 16" spaces in a wall | 20' ÷ 16" | 15 (studs at 16 inches on center) |
| Floor area of a room | 12' × 10' 6" | 126.00 sq ft |
| A board in three equal parts | 8' 5" ÷ 3 | 2' 9 11/16" (exactly 33 2/3") |
How to type a measurement
| Type | Means |
|---|---|
12' 4 3/8" | Feet, inches and a fraction |
12 ft 4-3/8 in | The same, with words and a dash |
4 3/8 or 4 3/8" | Inches and a fraction |
3/8 | A fraction of an inch |
7.5 ft or 7.5' | Decimal feet |
90.25 in | Decimal inches |
300 mm, 76.2 cm, 1.25 m | Metric, converted exactly (1 inch is 25.4 mm) |
4 | A plain number: inches when you add or subtract, a count when you multiply or divide |
Curly quotes and the prime marks a phone keyboard puts in work too, so you can paste a measurement from a text or a note.
How to add feet and inches by hand
Add the fractions first. Put them over the same bottom number: 3/8 + 1/2 is 3/8 + 4/8, which is 7/8. If the fractions come to more than an inch, carry the inch.
Then add the inches: 4 + 9 is 13. Twelve inches make a foot, so 13 inches is 1 foot and 1 inch. Carry the foot.
Last, add the feet with anything you carried: 12 + 7 + 1 is 20. So 12' 4 3/8" + 7' 9 1/2" is 20' 1 7/8".
To subtract, borrow the other way: a foot becomes 12 inches, and an inch becomes 16/16.
How the rounding works
The calculator keeps every value as an exact fraction and rounds once, when it shows the answer. That matters when you divide. 109 3/4" split into seven stair risers is exactly 15 19/28", which isn't on any tape. It shows as 15 11/16", the nearest sixteenth, and tells you it rounded.
A value exactly halfway rounds up, away from zero. A cut a sixteenth long can be trimmed. A cut a sixteenth short is scrap.
Pick 1/32 or 1/64 if you're doing finish work. The answer underneath doesn't change, only how finely it's shown.
On your phone, on the job site
This is the keypad from Handyman Calc, our free construction calculator. The app adds fifteen material calculators: concrete, drywall, decking, stairs, rafters, paint, tile and more. It also keeps a history, and it works with no signal.
▶ Try it in your browser ▶ Get it on Google Play
Charts worked out with the same math:
- how many bags of concrete a slab takes
- how many sheets of drywall a room takes
- a fraction to decimal chart for inches
- how much paint a room takes
- how much mulch a bed takes
- a roof pitch chart
- how many steps a rise takes
- the diagonal that squares a frame
- board feet for common lumber
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