Most concrete orders that come up short fail for one of eight reasons. The most common is mixing inches and feet in the same formula.
Eight mistakes and how to avoid them
- Mixing units. Convert thickness to feet before multiplying. Four inches is 0.333 ft.
- Forgetting the waste allowance. Add 5% to 10%.
- Trusting the nominal thickness. A 2 × 4 form is 3.5 inches tall. Measure the real depth in several places.
- Ignoring an uneven base. Low spots use extra concrete. Check the base with a string line.
- Leaving out thickened edges and footings. Add them as separate areas.
- Rounding down. Round bags up to whole bags and ready-mix up to the next quarter yard.
- Using the wrong bag yield. An 80 lb bag makes about 0.60 cubic feet. Specialty mixes differ, so check the bag.
- Treating an odd shape as a rectangle. Split L-shapes and curves into simple shapes and add them.
A quick check
For a 4 inch slab, divide the square footage by 81. If your calculated yards are far from that number, recheck your units.
What a small error costs
| Slab | Planned at 4 in | Actual at 4.5 in | Extra needed |
|---|---|---|---|
| 10 × 10 ft | 1.23 yd³ | 1.39 yd³ | 0.15 yd³ |
| 20 × 20 ft | 4.94 yd³ | 5.56 yd³ | 0.62 yd³ |
| 24 × 24 ft | 7.11 yd³ | 8.00 yd³ | 0.89 yd³ |
Half an inch of extra depth adds 12.5% to the order, which is why a 10% allowance is sometimes not enough on a rough base.
The concrete calculator keeps units straight for you: each field has its own unit, and the drawing shows what you entered.
Frequently asked questions
What is the most common concrete calculation mistake?
Mixing units. Entering thickness in inches in a formula that expects feet gives an answer twelve times too large, and the reverse gives one twelve times too small.
Why did my pour need more concrete than I calculated?
Usually because the base was lower or less even than planned, the forms moved, or no waste allowance was added.