Concrete Calculator
Your estimate
- Without waste
- 1.23 yd³33.3 ft³ · 0.94 m³
- Weight of concrete
- 4,877 lb2.44 tons · 2,212 kg
- Estimated cost
- $224
Works for slabs, walls, footings, round columns, sonotubes and steps — in feet and inches or in metres.
Updated 2026-10-20 · Based on: bag yields printed by concrete-mix makers (0.30–0.60 ft³ per bag) and a 1.54 dry-volume factor.
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How to use the Concrete Calculator
- Pick the shape: slab or wall, round column or hole, tube, or steps.
- Type the sizes. Each box has its own unit, so you can mix feet and inches (10 ft by 10 ft by 4 in).
- Keep the 10 % extra for waste unless your forms are perfectly level, then choose your bag size and the ready-mix price in your area.
- Read the cubic yards to order from a truck, or the number of bags to buy for a small pour.
Formula
Volume = length × width × thickness, with every size converted to feet first. Divide cubic feet by 27 to get cubic yards (1 yd³ = 27 ft³).
A round column is π × radius² × height; a tube subtracts the inner circle. Steps add one block per step: width × tread × (rise × step number), plus the top landing.
Bags: pre-mixed concrete weighs about 133 lb per cubic foot once mixed, so an 80 lb bag makes about 0.60 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³. In metric we use 2,130 kg per m³.
Worked example
With the values preset in the calculator above: (Slab / wall)
- Length
- 10 ft
- Width
- 10 ft
- Thickness / depth
- 4 in
- Quantity
- 1
- Extra for waste
- 10 %
- Bag size
- 80 lb bags
- Concrete needed (with waste)
- 1.36 yd³ (36.7 ft³)
- Without waste
- 1.23 yd³ (33.3 ft³)
- Bags
- 61
- Weight of concrete
- 4,877 lb (2.44 tons)
- Estimated cost
- $224
How much concrete do I need?
For a typical 10 × 10 ft patio poured 4 in thick you need 1.23 yd³ (33.3 ft³) of concrete. With 10 % extra that becomes 1.36 yd³, which is 61 bags of 80 lb mix.
For anything over about one cubic yard, ordering ready-mix is usually cheaper and much less work than mixing bags — most suppliers deliver in steps of ¼ or ½ yard, so round up.
How many bags of concrete do I need?
Divide the volume in cubic feet by the yield of the bag: 0.60 ft³ for 80 lb, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb bags. One cubic yard needs about 56 bags of 80 lb or 74 bags of 60 lb concrete.
Concrete thickness guide
- Walkways, patios and shed bases: 4 in (10 cm).
- Driveways for cars: 4–5 in; for trucks, RVs or heavy loads: 6 in (15 cm).
- Garage floors: 4–6 in with a thickened edge.
- Footings: at least as deep as your local frost line — check the building code.
Concrete for common slab sizes
| Length (ft) | Width (ft) | Thickness / depth (in) | Without waste | Bags (80 lb bags) |
|---|---|---|---|---|
| 10 | 10 | 4 | 1.23 yd³ | 56 |
| 12 | 12 | 4 | 1.78 yd³ | 80 |
| 10 | 20 | 4 | 2.47 yd³ | 111 |
| 20 | 20 | 4 | 4.94 yd³ | 222 |
| 24 | 24 | 4 | 7.11 yd³ | 320 |
| 20 | 20 | 6 | 7.41 yd³ | 333 |
| 30 | 30 | 4 | 11.11 yd³ | 499 |
Related guides
Frequently asked questions
How many yards of concrete for a 10×10 slab?
A 10 × 10 ft slab at 4 in thick is 1.23 yd³ (33.3 ft³). Order 1.36 yd³ to allow 10 % for waste and uneven ground.
How many 80 lb bags of concrete make a yard?
About 56 bags. One yard is 27 ft³ and each 80 lb bag yields roughly 0.60 ft³.
How much does a yard of concrete cost?
Ready-mix typically costs $150–$200 per cubic yard delivered in 2026, plus short-load fees for small orders. Enter your local price in the calculator to see the total.
Should I add extra concrete for waste?
Yes. Spills, form bulges and a subgrade that is a little low eat concrete quickly. 5–10 % is normal; use 15 % for holes dug in soft ground.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.