Concrete Calculator
Cubic yards, bags, and which of the two is cheaper for your pour.
Estimates only. Textured walls, high ceilings, sprayers and major colour changes can all increase the amount you need. Always buy slightly more than the calculation suggests.
Concrete for common pours
| Pour | Volume | Cubic yards | Bags | Cheaper |
|---|---|---|---|---|
| Post hole, 8" x 24", one post | 0.6 cu ft | 0 cu yd | 1 × 80 lb | Bags |
| Fence, 20 posts at 8" x 24" | 11.6 cu ft | 0.4 cu yd | 20 × 80 lb | Bags |
| Patio, 12 x 14 at 4" | 61.6 cu ft | 2.3 cu yd | 103 × 80 lb | Ready-mix |
| Shed base, 10 x 12 at 4" | 44 cu ft | 1.6 cu yd | 74 × 80 lb | Ready-mix |
| Driveway, 20 x 24 at 5" | 220 cu ft | 8.1 cu yd | 367 × 80 lb | Ready-mix |
| Sonotube, 12" x 48", one tube | 3.5 cu ft | 0.1 cu yd | 6 × 80 lb | Bags |
10 % waste, 80 lb bags at $6 and ready-mix at $165 per cubic yard. The bag or ready-mix figure shows which comes out cheaper once the supplier’s one-yard minimum is applied.
Two numbers, because you buy concrete two ways
Concrete is the only material on this site with two completely different purchase routes, and the calculator gives you both.
- Ready-mix arrives in a truck, is priced by the cubic yard and is measured by volume. It is fast, consistent and needs somewhere for the truck to stand.
- Bagged mix is priced per bag, and each bag yields a fixed volume — an 80 lb bag makes about 0.6 cubic feet. It is slower and heavier but needs no delivery.
Both figures come from the same volume calculation. Only the last step differs.
The one-yard minimum changes the answer
This is the part most calculators get wrong. Ready-mix looks cheaper on paper for almost any job, because $165 a yard beats $6 a bag on pure arithmetic.
But almost every supplier bills for a minimum of one cubic yard, or adds a short-load fee that comes to the same thing. Nobody sends a truck with 0.4 yards and charges you for 0.4 yards.
So for 20 fence posts — 0.43 cubic yards — ready-mix is billed as a full yard at $165, while 20 bags cost $120. The bags win. On a 20x24 driveway at 8.15 yards, ready-mix at $1,345 beats $2,202 in bags by a mile.
The crossover sits somewhere around one cubic yard, which is roughly what a 10x10 slab at 4 inches needs.
Why the waste allowance is not optional
Concrete is unforgiving. You cannot stop halfway, go and buy more, and carry on — the first pour will have started to set.
- Subgrade is never perfectly level. A slab that is half an inch deeper than planned over 200 square feet uses an extra 8 cubic feet.
- Forms leak and move. Ground conditions and the weight of the concrete both work against you.
- Spillage and washout cost real volume, especially when you are mixing bag after bag.
- Running short is catastrophic. A cold joint in the middle of a slab is a permanent weak point, and it looks wrong forever.
Five per cent is enough for a carefully formed slab on a flat, well-compacted base. Ten is the normal figure. Use fifteen on uneven ground or with timber forms that may bow.
Frequently asked questions
How much concrete do I need for a 10x10 slab?
A 10x10 slab at 4 inches needs about 36.7 cubic feet with waste, which is 1.36 cubic yards, or 62 bags of 80 lb mix.
How many bags of concrete are in a cubic yard?
An 80 lb bag yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet, so it takes 45 bags. With 60 lb bags it takes 60, and with 40 lb bags it takes 90.
Is it cheaper to mix concrete in bags or order ready-mix?
Bags are cheaper for anything under about one cubic yard, which is the minimum most ready-mix suppliers will bill. Above that, ready-mix wins clearly — often by hundreds of dollars.
How much waste should I allow for concrete?
Ten per cent is typical. Five is enough for a carefully formed slab on a flat, compacted base, and fifteen is sensible on uneven ground or with timber forms.