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. For what actually drives the price of either one, see What Affects Material Cost.
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.