Concrete calculator
Slab, shed pad, patio or footing — enter dimensions and thickness to get cubic yards for ready-mix or exact bag counts for mixing it yourself.
Result
Concrete needed
How much concrete do I need?
Formula
ft³ = length × width × thickness (in) ÷ 12 · yd³ = ft³ ÷ 27
bags = ft³ ÷ yield (80 lb → 0.60 · 60 lb → 0.45 · 40 lb → 0.30)
Example — 10 × 10 ft shed pad at 4 inches: 100 ft² × 4/12 = 33.3 ft³ = 1.23 yd³ = fifty-six 80 lb bags mixed by hand (a full weekend and a mixer) — or one short-load ready-mix delivery.
Bag counts per cubic yard (standard yields)
| Bag size | Yield | Bags per yd³ |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
How thick should concrete be?
- 4 inches: patios, walkways, shed bases on graded gravel.
- 5–6 inches: residential driveways, hot tub pads (check spa weight), heavy shed footprints.
- Footings: depth set by local frost line and code — check with your building department.
Rule of the thumb none of us should skip: always place concrete on 2–4 in of compacted gravel, use wire mesh or rebar for slabs over 8 ft wide, and order 10% extra — a short pour on the last wheelbarrow is a heartbreak (the calculator's buffer is on by default).
Setting fence posts instead of a slab? Use the post hole concrete calculator.
Frequently asked questions
How many 80 lb bags of concrete for a 10×10 slab?
At 4 inches: 33.3 ft³ ÷ 0.6 = 56 bags. At 6 inches it's 84 bags — that's ready-mix territory.
How much concrete for a 12×12 patio at 4 inches?
48 ft³ = 1.78 yd³ = 80 of the 80 lb bags, or a 2-yard ready-mix load (order 2 yd³ — waste factor included).
How many bags of quikrete per fence post?
A 4×4 post in a 10-inch hole 30 in deep takes ~1.6 ft³ = three 80 lb bags or four 50 lb bags — the post hole calculator does post counts too.
Is it cheaper to mix your own concrete?
Under about 1 yd³ (45×80 lb), bagged + mixer rental is competitive. Above that, delivered ready-mix is cheaper per yard and infinitely less work — short-load fees included in this calculator's default price.
How long does bagged concrete last?
Kept dry, ~12 months. A lumpy hard bag has absorbed moisture — don't trust it for structural work.