How to calculate concrete
- Work out the volume. Length × width × thickness in feet, plus 5–10% extra, then ÷ 27 for cubic yards.
- Count the cement. 5 to 6 bags of 94 lb portland cement per cubic yard: 1.30 cubic yards need 7 to 8 bags.
- Measure sand and gravel. For each bag of cement (1 cu ft), 2 cu ft of sand and 3 cu ft of gravel: the same bucket works for all three.
- Add water slowly. Just enough for a workable mix: too much water makes weak concrete.
Worked example: a 10 × 10 ft slab, 4 inches thick
- Volume: 10 × 10 × (4 ÷ 12) = 33.3 cu ft, plus 5% = 35.0 cu ft (1.30 cubic yards).
- Cement: 1.30 × 5 to 6 bags = 7–8 bags of 94 lb.
- Sand: 14–16 cu ft. Gravel: 21–24 cu ft.
Cement, sand and gravel by volume of concrete
| Cubic yards | 94 lb cement bags | Sand (cu ft) | Gravel (cu ft) | Or 80 lb premix bags |
|---|---|---|---|---|
| 0.25 | 2 | 4 | 6 | 12 |
| 0.50 | 3 | 6 | 9 | 23 |
| 1.00 | 5–6 | 10–12 | 15–18 | 45 |
| 1.50 | 8–9 | 16–18 | 24–27 | 68 |
| 2.00 | 10–12 | 20–24 | 30–36 | 90 |
| 3.00 | 15–18 | 30–36 | 45–54 | 135 |
1 : 2 : 3 mix, 5 to 6 bags of cement per cubic yard (QUIKRETE). No waste included: add 5–10%.
Concrete formulas by shape
All measurements in feet; divide inches by 12 first. Results are in cubic feet.
- Slab
- length × width × thickness
- Round slab
- π × (diameter ÷ 2)² × thickness
- Continuous footing
- total length × width × depth
- Pad footing
- length × width × depth × number of footings
- Monolithic slab
- length × width × thickness + perimeter band × (edge depth − thickness), where perimeter band = length × width − (length − 2 × edge width) × (width − 2 × edge width)
- Bell footing
- π × r² × pier height + π × bell height ÷ 3 × (R² + R × r + r²), with r = pier radius and R = bell radius
- Wall
- length × height × thickness
- Column / pier
- π × (diameter ÷ 2)² × height
- Post hole
- (hole area − post area) × depth; round hole π × (diameter ÷ 2)², square hole width²
- Tube / ring
- π × [(outer ÷ 2)² − (inner ÷ 2)²] × height
- Curb & gutter
- [curb depth × (curb height + flag thickness) + gutter width × flag thickness] × length
- Steps (solid, n steps)
- width × rise × run × (1 + 2 + … + n). With a landing, the landing is the top step: width × rise × [run × (1 + … + (n − 1)) + landing depth × n]
Cubic yards = cubic feet ÷ 27.
Frequently asked questions
What is the ratio for mixing concrete?
1 part portland cement, 2 parts sand and 3 parts gravel, by volume. QUIKRETE gives this mix for general concrete work, with 5 to 6 bags of 94 lb cement per cubic yard.
How many bags of cement for a yard of concrete?
5 to 6 bags of 94 lb portland cement, plus about 10 to 12 cu ft of sand and 15 to 18 cu ft of gravel.
Is mixing my own concrete cheaper than premix bags?
Usually, for bigger jobs: one cubic yard takes 45 bags of 80 lb premix, against 5 to 6 bags of cement plus sand and gravel. For a few post holes premix is simpler. Enter your prices in the calculator to compare.
How much water do I add?
Add it a little at a time until the mix holds its shape, like thick oatmeal. Extra water makes the concrete weaker and more likely to crack.
About the prices
Costs are estimated ranges with source and date, never a quote. Enter your local prices under "Use your own prices", or read how we estimate prices.