The formula
Concrete is sold by volume. In the US that means cubic yards for ready-mix and cubic feet per bag for premixed bags; elsewhere it's cubic meters. Every estimate comes down to three steps:
- Convert every measurement to feet (or meters). Thickness is the one people get wrong: 4 inches is 4 ÷ 12 = 0.333 ft.
- Multiply to get cubic feet. For a slab that's length × width × thickness. For a round post hole it's π × radius² × depth.
- Divide by 27 to get cubic yards, since a cubic yard is 3 × 3 × 3 = 27 cubic feet. Then add a waste allowance.
For bags, divide the cubic feet by the yield printed on the bag and round up. The Concrete Calculator does all of this for slabs, footings, round and square holes, walls and steps, in imperial or metric, and lets you add several pours together.
Worked example: a 10 × 10 ft patio slab
A patio 10 ft by 10 ft and 4 inches thick:
- 10 × 10 × 0.333 = 33.3 ft³
- 33.3 ÷ 27 = 1.23 yd³
- Plus 10% waste = 1.36 yd³ (36.7 ft³)
If you buy bags, that's:
| Bag size | Yield per bag | Bags to buy | Total weight |
|---|---|---|---|
| 80 lb | 0.60 ft³ per bag | 62 | 4,960 lb |
| 60 lb | 0.45 ft³ per bag | 82 | 4,920 lb |
| 40 lb | 0.30 ft³ per bag | 123 | 4,920 lb |
That's over two tons of bags. If you order ready-mix instead, suppliers usually sell in quarter-yard steps, so you'd order 1.5 yd³. At this size, ready-mix or a mixer rental is worth pricing.
More examples
Fence posts
Twelve holes, each 10 inches across and 36 inches deep, hold 19.6 ft³ of concrete (0.73 yd³) before you subtract the posts. With 10% extra that's 36 × 80 lb, 48 × 60 lb, 72 × 40 lb bags, or about 3 bags of 80 lb mix per hole. Hand-dug holes are rarely neat cylinders, so be generous with the allowance.
A strip footing
A footing 40 ft long, 16 inches wide and 12 inches deep is 40 × 1.333 × 1 = 53.3 ft³, or 1.98 yd³. With 10% extra it's 2.17 yd³, so you'd order 2.25 yd³ of ready-mix.
Steps
Solid steps are stacked blocks, so the bottom step is the full height of the stair. Three steps 4 ft wide with a 7-inch rise and 11-inch run come to 12.8 ft³. With 10% extra that's 24 bags of 80 lb mix or 32 of 60 lb.
Metric slab
A 3 × 3 m slab 10 cm thick is 3 × 3 × 0.10 = 0.90 m³. With 10% extra that's 71 × 30 kg, 85 × 25 kg, 107 × 20 kg bags.
How thickness changes the total
Thickness drives volume as much as area does. Going from 4 to 6 inches adds 50% more concrete. For a 20 × 20 ft slab, before waste:
| Thickness | Cubic yards | 80 lb bags (no waste) |
|---|---|---|
| 4 in | 4.94 | 223 |
| 5 in | 6.17 | 278 |
| 6 in | 7.41 | 334 |
Measure thickness in several places once the base is prepared. A sub-base that dips by half an inch across a slab adds about 12% to a 4-inch pour.
How much extra to order
Running short mid-pour leaves a cold joint, a visible and weaker seam where fresh concrete meets concrete that has started to set. That's why everyone orders extra:
- 5% for formed slabs on a flat, compacted base.
- 10% for most DIY jobs, and when you're unsure. It's the default in our calculator.
- 15% or more for hand-dug holes, uneven ground or footings dug straight into soil.
Leftover bags keep if stored dry; leftover ready-mix doesn't, so have a small form ready for a pad or a couple of stepping stones.
Bags or ready-mix?
Bag yields vary by product. Our figures come from the QUIKRETE Concrete Mix data sheet: 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb. An 80 lb bag weighs 36 kg. Always check the label on what you buy, since high-strength and fast-setting mixes can yield differently.
As a rule of thumb, bags make sense up to about one cubic yard (45 bags of 80 lb). Beyond that, mixing by hand gets slow, and the first batches can start to set before you finish. Ready-mix arrives in one truck at a consistent strength; ask about minimum orders, short-load fees and whether the truck can reach your pour.
Before you pour
- Measure the area. For irregular shapes, split them into rectangles and circles; the Square Footage Calculator adds them up.
- Prepare and compact the base. Most slabs sit on a few inches of compacted gravel; the Gravel Calculator works out tons for that layer.
- Measure the actual depth of your forms in several places and use the average.
- Run the numbers, add your allowance, and round up.
For structural work such as foundations and footings, follow your plans and local building code; this guide covers quantities, not design. Planning more outdoor work? Our home improvement calculators cover mulch, paint, decking and roofs too.