Conversion chart
Inches down the side, eighths across the top, decimal feet in the middle. Printing this page gives you the chart on its own.
| Inch | — | 1/8 | 1/4 | 3/8 | 1/2 | 5/8 | 3/4 | 7/8 |
|---|---|---|---|---|---|---|---|---|
| 0" | 0.000 | 0.010 | 0.021 | 0.031 | 0.042 | 0.052 | 0.063 | 0.073 |
| 1" | 0.083 | 0.094 | 0.104 | 0.115 | 0.125 | 0.135 | 0.146 | 0.156 |
| 2" | 0.167 | 0.177 | 0.188 | 0.198 | 0.208 | 0.219 | 0.229 | 0.240 |
| 3" | 0.250 | 0.260 | 0.271 | 0.281 | 0.292 | 0.302 | 0.313 | 0.323 |
| 4" | 0.333 | 0.344 | 0.354 | 0.365 | 0.375 | 0.385 | 0.396 | 0.406 |
| 5" | 0.417 | 0.427 | 0.438 | 0.448 | 0.458 | 0.469 | 0.479 | 0.490 |
| 6" | 0.500 | 0.510 | 0.521 | 0.531 | 0.542 | 0.552 | 0.563 | 0.573 |
| 7" | 0.583 | 0.594 | 0.604 | 0.615 | 0.625 | 0.635 | 0.646 | 0.656 |
| 8" | 0.667 | 0.677 | 0.688 | 0.698 | 0.708 | 0.719 | 0.729 | 0.740 |
| 9" | 0.750 | 0.760 | 0.771 | 0.781 | 0.792 | 0.802 | 0.813 | 0.823 |
| 10" | 0.833 | 0.844 | 0.854 | 0.865 | 0.875 | 0.885 | 0.896 | 0.906 |
| 11" | 0.917 | 0.927 | 0.938 | 0.948 | 0.958 | 0.969 | 0.979 | 0.990 |
The whole trick is dividing by 12
A foot is twelve inches, so inches become decimal feet by dividing by twelve, and decimal feet become inches by multiplying by twelve. Everything else on this page is that one operation with the rounding done for you.
Where it gets people is that the two systems land on different numbers. 0.5 feet is six inches, not five. Half an inch is 0.042 feet. A tenth of a foot is 1.2 inches — just over 1 3/16 — so tenths and inches never line up on a tape, which is exactly why the mistake is so easy to make at speed.
A worked example
Say the plan calls for 3 feet 7 1/2 inches of cover. Seven and a half inches is 7.5 ÷ 12 = 0.625 feet. Add the three feet and you have 3.625 — which you'd write as 3.63 on a sheet kept in hundredths, or 3.6 if your crew works in tenths.
Going the other way, a cut sheet reading 2.45 is 2.45 × 12 = 29.4inches, which is 2 feet 5 3/8 inches on the tape — close enough to 5 7/16 that the rounding matters if you're setting a pipe invert.
How precise is precise enough
A hundredth of a foot is about an eighth of an inch. That's the working limit of a rod and a tape, and it's why most cut sheets stop at two decimals — a third decimal is a thousandth of a foot, roughly the thickness of a business card, and nobody grades to that.
- 0.1 ft — 1 3/16", the coarsest anyone writes
- 0.05 ft — 5/8", a common tolerance on subgrade
- 0.01 ft — 1/8", what a finish grade gets held to
Round at the end, not in the middle. Converting each piece to tenths and then adding them up will drift; add in inches, convert once, and the error stays where you put it.
Questions people ask
What is a tenth of a foot in inches?
One tenth of a foot is 1.2 inches, which is a shade over 1 3/16 on a tape. A hundredth of a foot is 0.12 inches, near enough to 1/8 that most grade work treats them as the same thing.
How do you convert inches to tenths of a foot?
Divide the inches by 12. Seven and a half inches divided by 12 is 0.625 feet, which reads as 0.6 in tenths or 0.63 in hundredths.
Why are cut sheets in decimal feet instead of inches?
Because the instruments are. A laser level, a rod, and a GPS rover all read in feet and hundredths, and adding decimals is faster and less error-prone than adding fractions all day. The tape is the only thing on site still in sixteenths.
Is a cut positive or negative?
It depends on the crew. Plenty of sheets write a cut as negative and a fill as positive, and plenty do the reverse. Confirm which convention your sheet uses before you set a stake off it.
This is one number on one line
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