How to Calculate Concrete: Formulas for Every Shape
To calculate concrete, multiply length × width × thickness in feet to get cubic feet, divide by 27 for cubic yards, and add 10% for waste.
Advanced options: waste, prices
NRMCA advises ordering 4 to 10 percent over the estimate.
Concrete needed (with 10% extra)
1.96 yd³
1.50 m³ · 48 ft³ before extra
Slab: 12 ft × 12 ft × 4 in. Over about 1 yd³: order ready-mix.
Premix bags
- 80 lb88
- 60 lb118
- 50 lb141
- 40 lb176
- Ready-mix cost
- $313
- Weight
- 7,200 lb
Full breakdown
| Volume before extra | 1.78 yd³ · 1.36 m³ |
|---|---|
| With 10% extra | 1.96 yd³ |
| Weight (150 lb/ft³) | 7,200 lb · 3.60 tons |
| Pallets of 40 lb (80 bags each) | 3 |
| Pallets of 60 lb (56 bags each) | 3 |
| Pallets of 80 lb (42 bags each) | 3 |
| Truckloads at 12 yd³ | 1 |
| Truckloads at 8 yd³ | 1 |
Bag counts round up, as Quikrete's own calculator does. Cost uses your prices; delivery and short-load fees are extra.
Key takeaways
- Volume = L × W × T (ft) ÷ 27 = yd³.
- Convert inches to feet first (÷ 12).
- Add 4–10% waste (NRMCA).
- Bags = ft³ × 1.1 ÷ yield, rounded up.
How to Calculate How Much Concrete You Need
Calculate how much concrete you need in five steps.
- Measure length and width in feet.
- Convert thickness from inches to feet (divide by 12).
- Multiply length × width × thickness for cubic feet.
- Divide by 27 for cubic yards.
- Add 10% for waste (this calculator's own rule, inside NRMCA's 4–10 percent).
A 12×12 ft slab at 4 in: 12 × 12 × 0.333 = 48 ft³; 48 ÷ 27 = 1.78 yd³; with 10%, 1.96 yd³.
Concrete Formulas by Shape
Each shape has its own volume formula; the examples come from the calculator's engine.
| Shape | Formula (ft³) | Example | Result |
|---|---|---|---|
| Slab | L × W × T | 12×12 ft, 4 in | 48 ft³ |
| Footing | L × W × D | 20 ft × 12 in × 6 in | 10 ft³ |
| Sonotube | 3.1416 × r² × H | 12 in × 4 ft | 3.14 ft³ |
| Steps | W × tread × riser × n(n+1)/2 | 3 steps, 4 ft wide | 12.83 ft³ |
| Wall | (L × H − openings) × T | 20×4 ft, 8 in | 53.33 ft³ |
| Triangle | ½ × base × height × T | 10×12 ft, 4 in | 20 ft³ |
| Curb and gutter | L × (curb area + gutter area) | 100 ft standard | use the calculator |
How to Calculate Concrete in Cubic Meters
Multiply length × width × thickness in meters to get cubic meters; one cubic yard equals 0.7646 m³ (NIST). A 4×4 m slab 10 cm thick holds 1.6 m³.
How to Calculate Bags of Concrete Needed
Divide the cubic feet (with 10% extra) by the bag yield and round up: 0.6 ft³ per 80 lb bag, 0.45 per 60 lb. See the bagged concrete calculator.
How Much Waste to Add When Calculating Concrete
Add 4 to 10 percent for waste, as NRMCA advises; this calculator uses 10%. A slab poured 1/8 in thicker than a 4 in design uses 3 percent more concrete, and hardened concrete is about 2 percent smaller than fresh (NRMCA CIP 8).
Common Concrete Calculation Mistakes
Four mistakes cause most short orders.
- Inches not converted: 4 in is 0.333 ft, not 4.
- No waste allowance: uneven subgrade uses extra concrete.
- Uneven forms: a thicker pour than planned adds volume fast.
- Wrong bag size: 60 lb and 80 lb bags yield different volumes.
Why Divide by 27?
You divide by 27 because one cubic yard is 3 × 3 × 3 = 27 cubic feet.
How Much Will 1 Yard of Concrete Pour?
One cubic yard pours 81 sq ft at 4 in thick or 54 sq ft at 6 in; more on concrete yard calculator.
Worked Examples: Four Shapes
Slab 24×24 ft at 4 in. 24 × 24 × 0.333 = 192 ft³ ÷ 27 = 7.11 yd³; with extra, 7.82 yd³.
Footing 40 ft × 16 × 8 in. 40 × 1.333 × 0.667 = 35.6 ft³ ÷ 27 = 1.32 yd³; with extra, 1.45 yd³.
Cylinder 10 in × 4 ft. 3.1416 × 0.417² × 4 = 2.18 ft³; with extra, 4 bags of 80 lb.
Triangle 10 ft base × 12 ft height at 4 in. ½ × 10 × 12 × 0.333 = 20 ft³ ÷ 27 = 0.74 yd³.
How to Calculate Concrete in a Circle
Use 3.1416 × radius² × thickness: a 10 ft circle 4 in thick holds 26.18 ft³, or 0.97 yd³ before extra. See the circular slab calculator.
Is 1 Yard the Same as 1 Cubic Yard?
Yes, in concrete a yard always means a cubic yard of 27 ft³.
How to Calculate the Waste Factor
Multiply your volume by 1 + the waste percentage: 5% waste is × 1.05, 10% is × 1.10. NRMCA advises 4 to 10 percent; this calculator uses 10%. The separate 1.54 factor converts wet volume to dry materials when you mix your own (see concrete mix ratio calculator).
How to Calculate Concrete FAQ
How many yards of concrete do I need for a 24×24×4 slab?
A 24×24 ft slab at 4 in needs 7.11 cubic yards, or 7.82 yd³ with 10% extra.
How many 60 lb bags of concrete equal 1 cubic yard?
60 bags of 60 lb concrete make one cubic yard.
How much is a 1 yard truck of concrete?
One yard of ready-mix costs $160–195 (Concrete Network), but small loads often carry short-load fees; ask the supplier.
What is the acceptable wastage of concrete?
NRMCA advises ordering 4 to 10 percent over the estimate.
How do you calculate 5% wastage?
Multiply the volume by 1.05; for example, 10 yd³ × 1.05 = 10.5 yd³.
What is the 1.54 factor in concrete?
It converts wet concrete volume to the dry volume of cement, sand and gravel needed (a common rule of thumb).
How many cubic feet will a 50 lb bag cover?
A 50 lb bag makes 0.375 ft³ of concrete.
Is it cheaper to buy bags of concrete or mix your own?
For small jobs premix bags are simplest; mixing your own from cement, sand and gravel needs about 7 bags of 94 lb cement plus aggregate per yard, and above about 1 yd³ ready-mix is usually cheapest. See bags vs truck comparison.
Does a 4-inch concrete slab need rebar?
Not by code: the 2021 IRC treats slab-on-ground reinforcement as optional (R506.2.4); see the rebar calculator for slabs.
Next step
Use the calculator above for your shape, then read how to order ready-mix concrete or count bags on the concrete bag calculator.
Official sources
Last reviewed 2026-11-05. Numbers come from our tested engine; see how we calculate and our sources.