What a concrete estimate is actually measuring
Concrete is ordered by volume, not by weight and not by area. However the pour is shaped — a patio slab, a strip footing, a round post hole — the only question that matters is how many cubic feet of space you are about to fill. Everything else on this page is that one number converted into the units a supplier or a hardware store will actually understand.
That is why a slab and a footing use identical arithmetic despite looking nothing alike. A slab is a wide, shallow box; a footing is a narrow, deep one. Both are plan area multiplied by thickness. Only a round column breaks the pattern, and only because its area comes from a diameter rather than a length and a width.
Using this concrete calculator
Enter the footprint and the thickness, and the volume appears immediately in cubic yards (or cubic metres, if you switch units) along with the bag count, the raw cubic feet, and the approximate weight. The 4-inch default thickness is the standard for patios and walkways.
Pour shape and unit system both live in the customize panel — the sliders icon at the top right of the calculator — because they are preferences that should persist rather than fields you retype. Alongside the rectangle you will find a rectangle border, circle, circle border, triangle and trapezoid for awkward footprints, plus Round column / post hole, which swaps thickness for a diameter and a depth.
Setting six identical deck posts? Enter the dimensions of one and put 6 in “How many”. And note that the waste allowance is already applied to every figure shown — the default 10% is in the bag count, not something to add afterwards.
The formula, worked through
Plan area in square feet, times thickness converted from inches to feet, divided by 27 to reach cubic yards:
A 10 ft × 10 ft patio at 4 inches is 100 sq ft × (4 ÷ 12) = 33.3 cubic feet. Divided by 27, that is 1.23 cubic yards. Add the default 10% waste and it becomes 1.36 cubic yards, or about 82 bags of 60 lb mix.
The single most common error in that chain is leaving thickness in inches — a 4-inch slab is one third of a foot, not four, and treating it as four overstates the order twelvefold. The second is forgetting that bag counts come from yield, the volume of wet concrete a bag of dry mix produces, not from the bag's weight. A 60 lb bag yields 0.45 cubic feet, so a cubic yard takes 60 of them; an 80 lb bag yields 0.60 cubic feet and needs only 45.
Bags, ready-mix, and knowing which you need
The bag count is the number that usually decides the job. Below roughly half a cubic yard, bagged mix is straightforward. Past about a cubic yard — some 45 eighty-pound bags — hand mixing stops being sensible: that is over 3,600 lb to lift, open and combine before the first of it starts setting, and ready-mix delivery is generally cheaper as well as easier.
Weight is worth a second look before you plan a trip. Cured concrete runs about 150 lb per cubic foot, so a cubic yard is roughly 4,050 lb, and even a modest 10×10 slab is around 5,000 lb of material — well beyond what most pickups are rated to carry in one load.
Two things this calculator deliberately does not decide for you: reinforcement, and the base underneath. Most slabs sit on 4 inches of compacted gravel, which you can size from the same footprint with the gravel calculator. Mesh, rebar and control joints depend on span, load and local code, and no estimator can substitute for that.
FAQ
How many bags of concrete are in a cubic yard?
It depends only on the bag's yield — the volume of wet concrete one bag of dry mix produces. A cubic yard is 27 cubic feet, so:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Every major brand of dry mix publishes the same yields. Once you're past about a cubic yard — roughly 45 eighty-pound bags — mixing by hand stops making sense and ready-mix delivery is usually cheaper as well as easier.
How do you calculate concrete for a slab?
Multiply the plan area by the thickness, converting thickness from inches to feet, then divide by 27 to get cubic yards:
The most common mistake is leaving thickness in inches. A 4-inch slab is one third of a foot, not four.
How much extra concrete should I order for waste?
Between 5% and 10% for most jobs. The concrete itself doesn't shrink — the allowance covers spillage, an over-excavated subgrade, and forms that aren't perfectly true. Running short mid-pour is far more expensive than a little leftover, because a cold joint forms where fresh concrete meets partly set concrete and the slab is permanently weaker there.
Go toward 10% on ground that's uneven or freshly dug, and toward 5% on a compacted base inside solid forms. This calculator defaults to 10%; the result already includes it.
How much does a cubic yard of concrete weigh?
About 4,050 lb, or just over 2 US tons. Normal-weight concrete runs 140–150 lb per cubic foot, and there are 27 cubic feet in a yard.
That number matters more than people expect. A modest 10×10 ft, 4-inch slab is roughly 5,000 lb of material, which is well past what most pickup trucks are rated to carry — worth checking before you plan to haul ninety bags home in one trip.
Can I use this for footings, post holes, or round columns?
Yes. Open the customize panel (the sliders icon at the top right of the calculator) and change the pour shape. Footings and walls are just long, narrow rectangles. Post holes and tube forms are round columns — pick that shape and enter the diameter and depth, and the diagram will redraw.
Setting several identical posts? Enter the count in “How many” rather than running the calculator once per hole.
What else do I need to estimate for the job?
Most slabs sit on a compacted base of gravel or crushed stone, typically 4 inches deep. Our Gravel Calculator will size that base from the same footprint and shape you entered above.
If you're building up a garden bed or path alongside the pour, the Mulch Calculator uses the same area-and-depth math.