How to calculate concrete
- Work out the volume. In cubic feet: length × width × thickness, all in feet.
- Add 5–10% extra. So you don't run out halfway through the pour.
- Divide by the bag yield. 80 lb = 0.60 cu ft, 60 lb = 0.45, 50 lb = 0.375, 40 lb = 0.30.
- Round up. You can't buy part of a bag: 58.3 bags means 59.
Worked example: a 3 × 30 ft walkway, 4 inches thick
- Volume: 3 × 30 × (4 ÷ 12) = 30.0 cu ft, plus 5% = 31.5 cu ft.
- 80 lb bags: 31.5 ÷ 0.60 = 52.5 → 53 bags, 4,240 lb to haul.
- 60 lb bags: 70 (4,200 lb). 40 lb bags: 105 (4,200 lb).
Square feet covered by one bag
| Bag | Yield | 2 in thick | 3 in | 4 in | 6 in |
|---|---|---|---|---|---|
| 80 lb | 0.6 cu ft | 3.6 | 2.4 | 1.8 | 1.2 |
| 60 lb | 0.45 cu ft | 2.7 | 1.8 | 1.4 | 0.9 |
| 50 lb | 0.375 cu ft | 2.3 | 1.5 | 1.1 | 0.8 |
| 40 lb | 0.3 cu ft | 1.8 | 1.2 | 0.9 | 0.6 |
Square feet per bag, no waste included.
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.
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 much does an 80 lb bag of concrete make?
About 0.60 cubic feet. That covers roughly 1.8 sq ft at 4 inches thick, or 2.4 sq ft at 3 inches.
Are 60 lb or 80 lb bags better?
The concrete is the same; 80 lb bags mean fewer bags to open and mix, 60 lb bags are easier to lift. Per cubic foot the price is usually close, so compare the cost column.
How many bags of concrete are in a yard?
45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb.
How much weight is that to carry?
A cubic yard in 80 lb bags is 3,600 lb of dry mix. The calculator shows the total for your job in the "To carry" column.
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.