How to Calculate Concrete for a Slab
The math behind "how much concrete do I need" — volume, waste, bags vs. ready-mix — with worked examples.
The formula
A slab is a box: length × width × depth. Keep the units straight — measure the footprint in feet and the depth in inches, then divide by 12.
Volume (ft³) = Length (ft) × Width (ft) × Depth (in) ÷ 12
Example: a 10′ × 12′ slab at 4″ thick = 10 × 12 × (4 ÷ 12) = 40 ft³.
Add waste — yes, really
Ground is never perfectly level, forms flex, and mix sticks to the wheelbarrow. Add 10% on top of the theoretical volume. For the example above: 40 × 1.10 = 44 ft³. This is the number you buy.
Bags or ready-mix?
Bagged concrete yields are standardized:
- 80-lb bag ≈ 0.60 ft³ (0.022 yd³)
- 60-lb bag ≈ 0.45 ft³ (0.017 yd³)
- 40-lb bag ≈ 0.30 ft³ (0.011 yd³)
Divide your waste-adjusted volume by the bag yield and round up. For 44 ft³ with 80-lb bags: 44 ÷ 0.60 = 74 bags. That's 74 × 80 = 5,920 lb of concrete — you will feel every bag.
Ready-mix is ordered by cubic yard (1 yd³ = 27 ft³). 44 ft³ = 1.63 yd³, so you'd order 1.75 or 2 yards depending on the plant's minimum and your confidence in the forms. Most plants have a delivery minimum around 1–3 yd³ — below that, bags are cheaper.
Skip the arithmetic
Use the concrete calculator — enter your dimensions, depth, and waste, and it returns cubic feet, yards, bag counts, and a cost estimate.
Common mistakes
- Forgetting waste — a 10% allowance is the difference between one more trip and a cold joint.
- Ordering ready-mix by feet — plants speak in yards. 27 ft³ = 1 yd³.
- Mixing depth units — 4″ is 0.333 ft, not 4 ft.
- Not accounting for grade — a sloped site needs more concrete at the low end; measure the average depth.
Concrete slab FAQ
How do I calculate how much concrete I need for a slab?
Measure the footprint in feet and the depth in inches, then compute Length × Width × Depth ÷ 12 for cubic feet. Example: a 10′ × 12′ slab at 4″ thick = 10 × 12 × (4 ÷ 12) = 40 ft³. Add 10% waste, so you'd order 44 ft³.
How many bags of concrete do I need?
Divide your waste-adjusted volume by the bag yield and round up: 80-lb bag ≈ 0.60 ft³, 60-lb bag ≈ 0.45 ft³, 40-lb bag ≈ 0.30 ft³. For 44 ft³ with 80-lb bags: 44 ÷ 0.60 = 74 bags.
Bags or ready-mix — which is cheaper?
Under about 1 cubic yard, bagged mix usually wins on cost and control. Over about 2 cubic yards, ready-mix saves your back and your schedule. Most plants have a 1–3 yd³ delivery minimum, so below that bags are cheaper.
How many cubic feet are in a cubic yard of concrete?
27 cubic feet. Ready-mix is ordered by cubic yard, so 44 ft³ = 1.63 yd³, and you'd order 1.75 or 2 yards depending on the plant's minimum.
Before you pour
- Check your local frost depth — footings go below it.
- Verify minimum slab thickness for your use (4″ is common for patios and shed floors; driveways often need 4–6″ with rebar or wire mesh).
- Call before you dig — utility locates are free.