How to calculate concrete
- Measure the form. Measure length and width in feet and thickness in inches. For round shapes measure the diameter.
- Convert inches to feet. Divide inches by 12. A 4 inch slab is 4 ÷ 12 = 0.333 ft thick.
- Multiply to get cubic feet. Length × width × thickness gives the volume in cubic feet. For a circle use π × radius² × thickness.
- Add waste. Multiply by 1.05 to 1.10 to add 5–10% for spillage and uneven ground.
- Convert to yards or bags. Divide cubic feet by 27 for cubic yards, or by the bag yield (0.60 cu ft for an 80 lb bag) for the number of bags, rounding up.
Worked example: a 12 × 14 ft patio, 4 inches thick
- Volume: 12 × 14 × (4 ÷ 12) = 56.0 cu ft.
- Plus 5% for waste: 58.8 cu ft, or 2.18 cubic yards (÷ 27).
- 80 lb bags at 0.60 cu ft each: 98.0, so 98 bags.
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]
- L-shaped slab
- (Overall length × overall width − notch length × notch width) × thickness × quantity
Cubic yards = cubic feet ÷ 27.
How many bags of concrete per cubic yard
| Bag size | Yield per bag | Bags per cu yd |
|---|---|---|
| 80 lb | 0.6 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 50 lb | 0.375 cu ft | 72 |
| 40 lb | 0.3 cu ft | 90 |
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
An 80 lb bag of premixed concrete yields about 0.60 cubic feet, so one cubic yard (27 cubic feet) takes 45 bags. With 60 lb bags (0.45 cu ft each) you need 60 bags, and with 40 lb bags (0.30 cu ft each) you need 90.
How much concrete do I need for a 10x10 slab?
A 10 ft × 10 ft slab 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. Adding 5% for waste brings it to about 1.30 cubic yards: 59 bags of 80 lb concrete, or 78 bags of 60 lb.
How much extra concrete should I order?
Plan on 5–10% extra. Uneven ground, forms that bow out and spillage all eat concrete, and running short in the middle of a pour is far worse than having a few bags left over.
Is it cheaper to use bags or order ready-mix?
Compare bag prices with a delivered ready-mix quote for your volume. Small-load charges vary by plant: they may be a flat fee, a charge for missing yards, or a minimum amount billed. Enter the supplier's unit price and the delivery charges quoted for this order; confirm the terms again when quantities change.
How much does a cubic yard of concrete weigh?
Normal-weight concrete weighs about 150 lb per cubic foot, so one cubic yard weighs roughly 4,050 lb, around two tons. Keep this in mind when hauling bags: 45 bags of 80 lb is 3,600 lb.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27, because a cubic yard is 3 ft × 3 ft × 3 ft. For example, 54 cubic feet is 2 cubic yards.
How we estimate prices
We will only show our own estimates once they are checked against real merchant prices, with the source and date.