How Much Concrete Do I Need?
“How much concrete do I need?” is the first question on every project, and the answer is always a volume — usually in cubic yards or in bags. This guide shows how to size the most common pours (slabs, footings, and posts), how to decide between bags and ready-mix, and how to avoid the two classic mistakes: ordering short and forgetting waste.
Start with volume
Whatever you are building, the amount of concrete depends on its volume. Find the volume in cubic feet by multiplying the dimensions in feet, then divide by 27 to get cubic yards. From there you can convert to bags if the job is small. The yardage calculator handles the conversion for any length, width, and depth.
Cubic yards = (Length ft × Width ft × Thickness ft) ÷ 27
How much concrete for a slab
Slabs are the most common pour. The volume is just area times thickness. Thickness matters as much as the footprint: going from 4 inches to 6 inches adds 50% more concrete. This table shows how much a slab needs at typical thicknesses:
| Slab size | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 8 × 8 ft | 0.79 yd³ | 0.99 yd³ | 1.19 yd³ |
| 10 × 10 ft | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ |
| 12 × 12 ft | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ |
| 20 × 20 ft | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
For a quick estimate on any size, use the slab calculator or the square footage calculator if you already know the area.
How much concrete for footings and posts
Footings and post holes are smaller but there are usually many of them, so the count matters. A footing is length × width × depth; multiply by the number of identical footings. A round post hole uses π × radius² × depth.
For example, a single fence-post hole 10 inches across and 2 feet deep holds about 1.1 cubic feet of concrete — roughly two 80 lb bags before you subtract the post. Across a 20-post fence that is more than 20 cubic feet, or about 0.8 cubic yards. The footing calculator totals every footing at once, and the Sonotube calculator covers round forms.
Bags or ready-mix?
Once you know the volume, the practical question is how to buy it. Bagged concrete is convenient for small jobs; ready-mix is delivered by the cubic yard and is better value once you need a lot. As a rule of thumb:
- Under about ½ cubic yard — use bags. That is roughly 23 × 80 lb bags or fewer.
- ½ to 1 cubic yard — bags are possible but heavy; compare prices.
- Over 1 cubic yard — order ready-mix. Mixing 45+ bags by hand is slow and inconsistent.
The bag calculator shows the count for 40, 60, and 80 lb bags so you can see when bagging stops making sense.
Worked example: a shed base
Suppose you are pouring a shed base that is 10 feet by 12 feet at 4 inches thick.
- Area: 10 × 12 = 120 square feet.
- Thickness in feet: 4 ÷ 12 = 0.333 ft.
- Volume: 120 × 0.333 = 40 cubic feet, or 40 ÷ 27 = 1.48 cubic yards.
- Add 10% waste: about 1.63 cubic yards.
At nearly 1.5 yards this is firmly ready-mix territory. If you bagged it anyway, you would need about 67 × 80 lb bags — a clear sign that delivery is the easier choice.
Next steps
Pick the calculator that matches your pour from the full list of concrete calculators, or read how to calculate concrete for the underlying method and how many bags of concrete are in a yard for the bag math.
Frequently asked questions
How much concrete do I need for a 10x10 slab?
A 10 × 10 ft slab at 4 inches thick needs about 1.23 cubic yards, or roughly 56 × 80 lb bags. At 6 inches it rises to about 1.85 cubic yards.
How much concrete do I need for a fence post?
A 10 inch wide, 2 foot deep hole holds about 1.1 cubic feet — around 2 × 80 lb bags per post before subtracting the post itself.
Is it better to order too much or too little concrete?
Slightly too much. A little leftover is cheap insurance; running short forces a second batch and a cold joint that weakens the finished pour.
How do I know whether to use bags or ready-mix?
Bags are practical up to about half a cubic yard. Beyond that, mixing dozens of bags by hand is slow and ready-mix delivery is usually cheaper and more uniform.