How this feet and inches calculator works
Every tape calculation is the same three moves: convert to inches, do the arithmetic, convert back to the nearest sixteenth:
result = A (+ − × ÷) B → round to 1/16″
At the defaults — 7′ 3 3/8″ plus 2′ 11 13/16″ — that is 87.375 + 35.8125 = 123.1875 in, which the tape reads as 10′-3 3/16″ (3,129 mm). The exact decimal always shows beneath the rounded answer, so nothing is lost when results chain into the next calculation. It parses whatever you type — 7' 3 3/8, 87 3/8, or 7ft 3.375in all land on the same number.
Sixteenths, and how the marks work
The fraction system on a tape is binary — each mark family halves the last, and the line length tells you which family you’re in:
- Inch lines are longest, then 1/2, 1/4, 1/8 and 1/16 in descending size.
- Read past the inch: count small marks after the last inch line — three sixteenths past 3″ is 3 3/16″.
- Fractions reduce: 8/16 is 1/2, 12/16 is 3/4 — the calculator always shows the reduced form.
- The black diamonds every 19.2″ lay out five joists per 8 ft sheet — engineered-joist spacing.
- The hook floats by its own thickness on purpose, so pushed and hooked measurements both read true.
When a hooked measurement isn’t possible — inside a window opening, say — carpenters “burn an inch”: hold the tape at the 1″ line instead of the hook, then subtract that inch from the reading. It trades the sloppy hook for a crisp line, and the subtraction is exactly the kind of tape math this calculator runs without a stray mental slip.
When to work in fractions, decimals, or metric
- Fractions are for the tape and the saw — every answer here rounds to the 1/16 you can actually mark.
- Decimal inches are for chaining math — the exact figure carries no rounding error between steps.
- Decimal feet show up on engineer’s tapes and site plans — 0.25 ft is 3″, 0.1 ft is 1 3/16″.
- Millimetres (× 25.4) are for plans drawn metric — 10′-3 3/16″ is 3,129 mm.
- Division is layout: ÷ by a count is even spacing — the same math the baluster calculator automates with the gap rule.
Sixteenths to decimals, and decimal feet to inches
Table 1 is the full sixteenth family; Table 2 translates the decimal feet an engineer’s tape shows back to carpenter marks.
| Fraction | Decimal | Fraction | Decimal |
|---|---|---|---|
| 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 |
| Decimal ft | Tape reads |
|---|---|
| 0.1 ft | 1 3/16″ |
| 0.2 ft | 2 3/8″ |
| 0.25 ft | 3″ |
| 0.5 ft | 6″ |
| 0.75 ft | 9″ |
| 0.9 ft | 10 13/16″ |
How to run tape math without mistakes
- Type lengths the way the tape reads them — feet, inches and the fraction, in any of the accepted forms.
- Pick the operation — for × and ÷ the second entry becomes a plain number, not a length.
- Put the larger length first when subtracting — a cut can’t be negative.
- Chain with the exact decimal, not the rounded fraction, when a result feeds the next step.
- Mark the rounded sixteenth — it’s the finest line the tape and pencil can hold anyway.
Worked example: one board, four operations
The defaults, worked line by line — A = 7′ 3 3/8″ (87.375 in), B = 2′ 11 13/16″ (35.8125 in):
subtract = 87.375 − 35.8125 = 51.5625 → 4′-3 9/16″
multiply: 35.8125 × 4 = 143.25 → 11′-11 1/4″
divide: 92 5/8 ÷ 3 = 30.875 → a mark every 2′-6 7/8″
The division line is the sleeper: splitting a 92 5/8″ precut stud into three equal pieces puts the marks at 2′-6 7/8″ — no guessing, no walking a compass down the board. Any time a layout says “evenly spaced,” it is this calculation wearing work clothes.
What this tape calculator doesn’t do
- Areas and volumes. Square footage and cubic yards have their own calculators.
- Angles and diagonals. Rafter and stringer trig live on the rafter and stringer pages.
- Finer than 1/16. Cabinetmakers working in 32nds should carry the exact decimal instead.
- Negative results. A subtraction that goes below zero flags the order rather than signing the answer.
- Unit surveying. Chains, rods and links are out of scope — decimal feet covers the engineer’s tape.
Feet and inches FAQ
How do you add feet and inches?
Convert everything to inches, add, then convert back: 7′ 3 3/8″ is 87.375 in and 2′ 11 13/16″ is 35.8125 in, so together they are 123.1875 in — 10′-3 3/16″. The calculator does the conversion both ways and rounds the answer to the nearest 1/16, the finest mark on most tapes.
How do you subtract tape measurements?
Same trick in reverse: 7′ 3 3/8″ minus 2′ 11 13/16″ is 87.375 − 35.8125 = 51.5625 in, which reads as 4′-3 9/16″ on the tape. Enter the larger length as A — a cut length can’t be negative, so the calculator flags the order instead of returning a signed number.
How do I divide a measurement into equal parts?
Divide the length by the number of spaces: a 92 5/8″ run split into 3 equal bays is 92.625 ÷ 3 = 30.875 in — a mark every 2′-6 7/8″. This is the layout math behind even shelf spacing, balusters and pickets; switch the operation to ÷ and B becomes the count.
What is 5/8 as a decimal?
5/8 is 0.625. The whole sixteenth family converts cleanly: 1/16 = 0.0625, 1/8 = 0.125, 3/16 = 0.1875, 1/4 = 0.25, and so on up to 15/16 = 0.9375 — each sixteenth adds 0.0625. The reference table on this page lists all fifteen so you can jump between a tape and a decimal drawing.
How do I read the fraction marks on a tape measure?
Most tapes mark sixteenths: the longest lines are inches, then descending lengths for 1/2, 1/4, 1/8 and 1/16. Count the small marks past the last inch line — three short marks past 3″ on the 1/16 scale is 3 3/16″. Many tapes print a diamond every 19.2″ (five per 8 ft) for engineered-joist layout.
How do I convert decimal feet to inches?
Multiply by 12: 0.1 ft is 1.2 in, which rounds to 1 3/16″ on a tape; 0.25 ft is exactly 3″; 0.5 ft is 6″. Decimal feet show up on engineer’s tapes, site plans and laser levels, and the conversion table on this page translates the common values back to carpenter fractions.
Why are precut studs 92 5/8 inches?
Because 92 5/8″ plus a bottom plate and a doubled top plate (three 1 1/2″ plates = 4 1/2″) makes a 97 1/8″ rough wall — which lands finished ceilings at the standard 8 ft once flooring and drywall take their share. It is tape math baked into the lumberyard: the calculator and the lumber size chart both carry it.
Sources and standards
- Exact conversions: 12 inches per foot; 25.4 mm per inch (the international inch, exact by definition).
- Sixteenth rounding: results round to the nearest 1/16 and reduce — the tape’s own convention.
- 19.2″ diamonds: the five-per-8-ft layout marks printed on most US tapes.
- Precut studs: 92 5/8″ + three 1 1/2″ plates = the standard 8 ft wall — see the lumber size chart.
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