How to calculate concrete for a slab
Concrete is ordered by the cubic yard (27 cubic feet). Whether you are pouring a patio, sidewalk, or footing, the math is the same:
- Measure length and width in feet.
- Convert thickness from inches to feet: divide inches by 12.
- Multiply: length × width × thickness (ft) = cubic feet.
- Divide cubic feet by 27 to get cubic yards.
This calculator adds your waste allowance on top — useful for uneven excavations, spill, and subgrade that is not perfectly level.
Worked example: 12×10 ft patio at 4 inches
- Volume: 12 × 10 × (4 ÷ 12) = 40 cu ft
- Cubic yards: 40 ÷ 27 = 1.48 cu yd (before waste)
- With 10% waste: 1.63 cu yd → order a short-load truck or round up bags accordingly
Use the calculator above with your exact dimensions — do not guess from square footage alone, because thickness drives volume.
When to order a truck vs buy bags
| Volume | Typical approach |
|---|---|
| Under 1 cu yd | Bagged mix from a home center |
| 1–10 cu yd | Short-load ready-mix delivery |
| 10+ cu yd | Full ready-mix truck |
Ready-mix usually requires a minimum order (often ~1 cu yd). Call local suppliers for short-load fees — they vary by market and can add $50–$150 per partial load.
Bagged mix rule of thumb: one 80 lb bag yields about 0.6 cu ft. A full cubic yard needs roughly 45 bags — practical only for small pads, post holes, or repairs.
Standard thickness guide
| Project | Typical thickness |
|---|---|
| Patio / sidewalk | 4 in |
| Driveway (cars) | 4–6 in |
| Driveway (trucks) | 6+ in |
| Footings | Per local code |
Thicker pours cost more in material and labor. Do not default to 6 inches for a light-use patio unless your climate, soil, or code requires it.
Prep, base, and reinforcement (often forgotten)
Volume math is only half the job. Most failed slabs trace back to base and drainage, not short pours:
- Subgrade: compact soil or gravel base before the pour; soft spots cause cracking later.
- Gravel base: many patios use 4–6 in of compacted gravel under the slab — that is separate from the concrete volume this calculator measures.
- Control joints: plan cuts every 8–12 ft on large slabs to control random cracking.
- Reinforcement: wire mesh or #3/#4 rebar on driveways and load-bearing pads — does not change yardage but affects quotes.
If you are hiring out, contractors price demo, forms, gravel, finish, and seal separately from raw concrete yards.
Common ordering mistakes
- Measuring thickness wrong — siding or turf often hides true depth; verify with a probe or test hole.
- Ignoring waste — running short mid-pour is expensive; 10% on flat work is standard.
- Mixing bag count with truck math — suppliers think in yards; home centers sell bags.
- No call-ahead — ready-mix plants need lead time; summer Saturdays book fast.
Planning a patio budget?
After you know cubic yards, see our concrete patio cost guide for 2026 installed price ranges per square foot — material, labor, finish, and add-ons.
Pouring footings for a deck? Pair with deck building cost for the full outdoor project picture.