How to use the concrete calculator
- Pick a project type: slab/patio/driveway, footing, round column or post hole, or steps.
- Enter the dimensions. Lengths and widths default to feet; thickness and depth default to inches — change the unit next to any field if you measured differently.
- Press Add another item for each extra pour (a driveway plus a footing, or several post holes with different depths) and the totals add up automatically.
- Check the waste allowance (5–10% is typical) and pick a bag size if you're buying premix, or leave it as ready-mix.
- Read the total in cubic yards, cubic feet and cubic metres, plus bags needed and a rough number of ready-mix truck loads.
How much concrete do I need?
Cubic feet = length × width × thickness (all measurements converted to feet first), then cubic yards = cubic feet ÷ 27, because a cubic yard is 3 × 3 × 3 feet.
- A 10 ft × 10 ft slab, 4 inches thick: 100 sqft × (4 ÷ 12) ft = 33.3 cu ft, or 1.23 cu yd before waste.
- With a 10% waste allowance (the calculator's default): 36.7 cu ft, or 1.36 cu yd to order.
- A 12 ft × 20 ft patio, also 4 inches thick, with 10% waste: 3.26 cu yd.
Round up to the nearest quarter or half yard when you order ready-mix — suppliers don't deliver partial fractions of a yard, and most charge a short-load fee under a minimum (often 3–5 cu yd).
How many bags of concrete do I need?
Bag counts come from the yield figures printed on standard premix bag labels — the volume of mixed concrete one bag makes, not its dry weight. These are typical published yields for general-purpose and high-strength bagged concrete mix; always check the number printed on the bag you buy.
| Bag size | Typical yield (mixed) | Bags per cubic yard | Bags for the 10×10 ft, 4 in slab (10% waste) |
|---|---|---|---|
| 40 lb | ≈ 0.30 cu ft | 90 | 123 |
| 50 lb | ≈ 0.375 cu ft | 72 | 98 |
| 60 lb | ≈ 0.45 cu ft | 60 | 82 |
| 80 lb | ≈ 0.60 cu ft | 45 | 62 |
For anything over about 1–2 cu yd, ready-mix delivery is usually cheaper and far less labor than mixing dozens of bags — the calculator's ready-mix truck estimate helps you compare.
Footings, round columns, post holes and steps
Continuous footing
Footing volume = length × width × depth. A 40 ft footing, 16 inches wide and 12 inches deep needs about 58.7 cu ft (2.17 cu yd) with a 10% allowance. Footing width and depth are almost always in inches even when the run is measured in feet, so this calculator lets each dimension use its own unit.
Round columns and post holes
Volume per hole = π × (diameter ÷ 2)² × depth. Six post holes, 12 inches across and 24 inches deep, need about 10.37 cu ft with 10% waste — about 24 bags of 60 lb premix. Add a footing item instead if you're pouring a continuous strip rather than separate holes.
Steps
Solid concrete steps poured on grade are calculated as stacked treads, since the concrete under each step also sits under every step above it: cubic feet = width × run × rise × n(n + 1) ÷ 2, where n is the number of steps. Three steps with a 7-inch rise, 11-inch run and a 3 ft width need about 10.59 cu ft with 10% waste. This is the standard approximation most concrete-yardage calculators use for solid stairs; open-riser or precast stairs use less material.
Ready-mix or bagged concrete?
As a rough guide, bagged premix makes sense up to about 1 cu yd (roughly 45 bags of 60 lb mix); above that, ready-mix delivery is usually cheaper per yard and much less physical work. A typical mixer truck carries about 9–10 cu yd, so the calculator's truck estimate is rounded up to the nearest full load — most jobs order a single, slightly-over load rather than several exact ones.
Concrete calculator vs square footage calculator
This calculator gives you volume (cubic yards of concrete), which needs a thickness or depth. If you only need the flat area of a room, patio or lot in square feet — for flooring, paint or sod rather than concrete — use the square footage calculator instead.
Related home & garden tools
- Need a compacted base under the slab or footing? Use the gravel calculator.
- Topping the yard around a new patio or slab? See the mulch calculator and topsoil calculator.
- Measuring the room or lot first? The square footage calculator handles rectangles, circles, triangles and L-shaped rooms.
Frequently asked questions
How much concrete do I need?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. A 10 ft × 10 ft slab at 4 inches thick is 100 sqft × ⅓ ft = 33.3 cu ft, or 1.23 cu yd before any waste allowance. Add 5–10% waste and that rounds up to about 1.36 cu yd.
How many bags of concrete make a cubic yard?
It depends on the bag size, using each size's typical published yield: 90 bags at 40 lb, 72 at 50 lb, 60 at 60 lb, or 45 at 80 lb. Check the yield printed on your bag, since it varies a little by product.
How many 80 lb bags of concrete do I need for a 10×10 slab?
For a 10 ft × 10 ft slab 4 inches thick, with a 10% waste allowance, you need about 62 bags of 80 lb premix (or 82 bags of 60 lb, 123 bags of 40 lb). An 80 lb bag makes about 0.60 cu ft of mixed concrete; a 60 lb bag makes about 0.45 cu ft — figures from standard premix bag labels.
How much concrete do I need for fence post holes?
Volume = π × (diameter ÷ 2)² × depth, per hole. Six holes 12 inches across and 24 inches deep need about 9.42 cu ft (0.384 cu yd) before waste, or 24 bags of 60 lb premix with a 10% allowance. Concrete for posts is usually bagged rather than delivered, since the volume is small.
How do I calculate concrete for a footing?
Footing volume = length × width × depth, all converted to feet. A 40 ft footing, 16 inches wide and 12 inches deep, needs about 53.3 cu ft (2.17 cu yd) before waste, or 58.7 cu ft with a 10% allowance. Footing width and depth are usually measured in inches even when the length is in feet — this calculator lets you set each in its own unit.
How do I calculate concrete for steps?
Solid steps poured on grade are treated as stacked treads: cubic feet = width × run × rise × n(n+1)/2, where n is the number of steps (all measurements in feet). Three steps with a 7-inch rise, 11-inch run and 3 ft width need about 9.63 cu ft before waste. This approximation assumes solid stairs, not open risers or a separately framed stringer.
How much waste allowance should I add to a concrete order?
Most concrete calculators, including this one, default to 5–10%: a little extra for an uneven subgrade, spillage, and the difference between the "neat line" volume you calculated and what actually gets placed. Rougher or larger pours (footings dug by hand, irregular ground) usually need the higher end of that range.
How many yards of concrete are in a ready-mix truck?
A full mixer truck load is typically around 9–10 cubic yards, though it varies by supplier and local road limits. Most residential jobs are well under a full truck, and many ready-mix companies charge a "short load" fee below a minimum (often around 3–5 cu yd), so it can be worth combining smaller pours into one order.
Is this the same as the Quikrete calculator?
No — this is an independent calculator that uses the general yield figures printed on standard premix bag labels (Quikrete, Sakrete and similar products use very close to the same yields per pound). For an exact match to one product, use the yield printed on that specific bag, since it can vary slightly.
Does this calculator include gravel or a base layer?
No, it only covers the concrete itself. If your slab or footing needs a compacted gravel base first, use the gravel calculator to work out how much you need for the sub-base.