Concrete Calculator
Slabs, columns, footings, walls, stairs, post holes, curb and gutter, ramps. Enter the dimensions and get the volume to order plus the number of bags to buy. Openings, hollow cores and the post in its hole are all subtracted — most calculators charge you for them. The result updates as you type.
Bags or ready-mix — price it yourself
No prices are filled in for you. Concrete is priced locally and changes constantly, so any figure printed here would be wrong for most people reading it. Enter what your supplier charges and the comparison becomes yours rather than ours.
Often the figure that decides it.
Below it you pay for the minimum anyway.
Assumptions used, and where they come from
One 80 lb bag yields 0.6 ft³ of mixed concrete. Yields vary slightly between manufacturers and mix types. Check the bag you buy.
Read at the sourceBag yields from Sakrete, read .
Weight assumes normal-weight concrete at 2400 kg/m³ (150 lb/ft³). Normal-weight concrete. Lightweight and heavyweight mixes differ significantly.
Secondary sourceDensity from ACI 318 / ASTM C138, read .
- Bag counts are rounded up: a partial bag still has to be bought.
- No price is assumed anywhere. Any cost shown is the one you entered, applied to the volume above.
The formula
Every measurement is converted to the same unit first. That is where most hand calculations go wrong: a slab is measured in feet but poured to a thickness in inches, and a pier is measured in inches but stands feet tall.
footing = run × width × depth × runs
column = π ÷ 4 × (diameter² − core²) × height × columns
stairs = rise × run × n(n+1) ÷ 2 × width
curb = (curb height × curb depth + gutter width × flag thickness) × run
wall = (length × height − openings) × thickness × walls
post hole = (π ÷ 4 × hole² − post area) × depth × holes
ramp = length × width × (thick end + thin end) ÷ 2
order = volume × (1 + waste ÷ 100)
bags = ⌈ order ÷ bag yield ⌉
Three of these subtract what is not concrete, which is where most estimates go wrong: a hollow core out of a column, doors and windows out of a wall, and the post out of its hole. The stair formula is a triangular number because the top step is one rise deep, the next two, and so on down to the ground — that is why a fourth step costs two thirds more concrete than the third did.
Bags are rounded up because a partial bag still has to be bought — and a pour that runs short sets before you can finish it.
Worked example — a slab
A 20 ft × 12 ft patio slab, 4 in thick, with a 10% waste allowance:
- 4 in ÷ 12 = 0.333 ft
- 20 × 12 × 0.333 = 80 ft³
- 80 × 1.10 = 88 ft³ to order
- 88 ÷ 27 = 3.26 yd³
- 88 ÷ 0.6 = 146.7 → 147 bags of 80 lb
At 147 bags, ready-mix delivery is the sensible choice. The bag count is there to tell you when to stop considering bags at all.
Worked example — four deck piers
Four 12 in tube forms, each 8 ft tall, with a 10% waste allowance. A pier is a cylinder, so the diameter is squared — doubling it quadruples the concrete.
- 12 in ÷ 12 = 1 ft diameter
- π ÷ 4 × 1² × 8 = 6.28 ft³ per pier
- 6.28 × 4 = 25.13 ft³
- 25.13 × 1.10 = 27.65 ft³ to order
- 27.65 ÷ 0.6 = 46.1 → 47 bags of 80 lb
The per-pier figure is the one you work to on site: it is what a single batch has to produce, and it is what tells you whether your mixer can do one pier at a time.
Worked example — three porch steps
Three steps, 7 in rise and 11 in run, 4 ft wide, with 10% waste. The cross-section is a triangular number: 1 + 2 + 3 = 6 stacked rises.
- 3 × 4 ÷ 2 = 6 stacked rises
- (7 ÷ 12) × (11 ÷ 12) × 6 = 3.21 ft² of section
- 3.21 × 4 = 12.83 ft³
- 12.83 × 1.10 = 14.12 ft³ to order
- 14.12 ÷ 0.6 = 23.5 → 24 bags of 80 lb
Add a fourth step and the section jumps to 10 stacked rises, not 8 — two thirds more concrete for one more step. This is the calculation people get wrong by eye.
Worked example — six fence posts
Six 10 in holes, 30 in deep, each taking a 4×4 post. A nominal 4×4 actually measures 3½ in, and it occupies part of the hole you would otherwise fill.
- π ÷ 4 × (10 ÷ 12)² = 0.545 ft² of hole
- (3.5 ÷ 12)² = 0.085 ft² taken by the post
- 0.545 − 0.085 = 0.460 ft² of concrete
- 0.460 × 2.5 × 6 = 6.91 ft³
- 6.91 × 1.10 = 7.60 ft³ → 13 bags of 80 lb
The post takes up 16% of the hole. Price the hole as if it were empty and you buy concrete for timber you already own — which is what every other calculator does.
Bag yields
How much mixed concrete each bag size produces, and how many fill one cubic yard.
| Bag size | Yield | Bags per yd³ |
|---|---|---|
| 40 lb | 0.3 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.6 ft³ | 45 |
Source: Sakrete — High-Strength Concrete Mix — product specifications, checked 2026-08-10.
The 50 lb bag is not listed: the yield usually quoted for it could not be confirmed at any manufacturer, so it is left out rather than guessed.
Questions
- How many 80 lb bags of concrete make a cubic yard?
- A cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.6 cubic feet, so you need 45 bags per cubic yard. At that point ready-mix delivery is usually cheaper and far less work.
- How thick should a concrete slab be?
- 4 inches is standard for patios, walkways and shed bases. Driveways carrying cars are usually 5 to 6 inches. Anything carrying heavy vehicles or structural load should be designed by an engineer.
- How much waste should I allow?
- 10% is the usual allowance. It covers spillage, an uneven subgrade and forms that flex slightly under the pour. On a rough or hand-dug base, 15% is safer — running out mid-pour leaves a cold joint.
- When should I order ready-mix instead of bags?
- Around one cubic yard, roughly 45 bags of 80 lb. Beyond that, mixing by hand becomes slower than the concrete sets, and you risk an inconsistent mix.