QuantityBench Concrete Calculator
Enter the length, width, and thickness of your slab to estimate cubic yards for a ready-mix order, or how many bags of premix concrete you need — with a 10% waste allowance built in.
Estimated concrete needed
How this is calculated
This calculator converts a slab's length, width, and thickness into a volume, then converts that volume into either a cubic-yard order quantity or a count of premixed bags.
1. Volume: length × width × thickness (converted to feet) = cubic
feet. Divide by 27 for cubic yards.
2. Waste allowance: add 10% to cover spillage and an uneven sub-grade — the
same allowance used by this site's other material calculators.
3. Bags: divide the waste-adjusted cubic feet by a bag's yield — about 0.3 cu
ft for a 40 lb bag, 0.45 cu ft for a 60 lb bag, or 0.6 cu ft for an 80 lb bag — and round up,
since bags only come whole.
Assumptions
- Thickness is uniform across the whole slab — for a stepped or sloped pour, split it into sections and add the results.
- Bag yields are published figures for standard premixed concrete; specialty or fiber-reinforced mixes can differ slightly.
- The 10% waste allowance is a general guideline, not a guarantee — irregular forms may need more.
Pouring over a prepared gravel base? Use our Gravel Calculator to estimate the crushed-stone sub-base separately.
Worked example
A 10 ft × 10 ft slab at 4" thick:
| Area | 100 sq ft |
|---|---|
| Volume | 33 cu ft (1.235 cu yd) |
| Recommended order (+10% waste) | 1.358 cu yd (36.7 cu ft) |
| Bags needed | 123 × 40 lb, or 82 × 60 lb, or 62 × 80 lb |
In metric, the raw volume is about 0.94 m³ before the waste allowance.
Concrete calculator for a patio slab
A larger slab crosses the point where bagged concrete stops making sense. For a 12 ft × 12 ft patio at 6" thick:
| Area | 144 sq ft |
|---|---|
| Volume | 72 cu ft (2.67 cu yd) |
| Recommended order (+10% waste) | 2.93 cu yd |
| Bags needed (80 lb) | 132 bags |
At nearly 3 cubic yards and well over 100 bags, this is a textbook case for ordering ready-mix instead — most local suppliers will deliver a load this size for less than the cost and effort of mixing that many bags by hand.
Concrete calculator for post footings
Footings are long and narrow rather than a flat slab, but the same volume formula applies. A 20 ft run of 12" × 12" footing (entered as length 20 ft, width 1 ft, thickness 12"):
| Volume | 20 cu ft (0.74 cu yd) |
|---|---|
| Recommended order (+10% waste) | 0.81 cu yd |
| Bags needed (60 lb) | 49 bags |
For individual fence-post holes rather than a continuous footing, see the Fence Calculator, which uses a simpler 2-bags-per-post rule of thumb instead of a volume calculation.
Concrete vs. asphalt for a driveway
Concrete and asphalt suit different budgets, climates, and maintenance preferences — see Asphalt vs. Concrete Driveway: Cost & Tradeoffs for a full comparison, or use the Asphalt Calculator to estimate the alternative.
Frequently asked questions
How many bags of concrete do I need for a slab?
Multiply length × width × thickness (in feet) to get cubic feet, add the usual 10% waste allowance, then divide by a bag's yield. An 80 lb bag yields about 0.6 cu ft, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.3 cu ft. For a 10 ft × 10 ft slab at 4" thick, that's about 82 bags of 60 lb mix.
How many cubic yards of concrete do I need?
Cubic yards = (length × width × thickness in feet) ÷ 27. This calculator adds a 10% waste allowance on top before showing the "recommended order" figure, since ready-mix suppliers typically won't deliver a partial extra trip if you come up short.
Should I use bags or order ready-mix concrete?
For small pours (roughly under 1 cubic yard — a typical stepping-stone patio or a few fence-post footings) bagged premix is usually more practical. Beyond that, a ready-mix truck delivery is normally cheaper and far less labor than mixing dozens of bags by hand. This calculator flags that threshold once your project crosses it.
How thick should a concrete slab be?
This varies by project and isn't something this calculator determines for you — a shed pad or sidewalk is often 4", a driveway slab often 5–6", and footings or structural elements follow an engineer's or local code spec rather than a rule of thumb. Enter whatever thickness your plan specifies.
What does the 10% waste allowance cover?
Spillage, an uneven sub-grade, and minor over-excavation — the same standard trade allowance used by this site's other material calculators. It's shown as a separate line item, not folded silently into the base number.
Does this calculator account for rebar, mesh, or a gravel base?
No — it calculates concrete volume only. If you need to estimate the crushed-stone base layer that typically goes under a slab, see the Gravel Calculator linked below.