Concrete is ordered by volume — cubic yards from a ready-mix truck, or a count of 40, 60 or 80 lb bags from the store. This calculator turns the size of your slab, footing or post holes into both, with an allowance for spillage and an uneven base so you do not run short halfway through a pour.
How to use the concrete calculator
- Choose the pour type. “Slab” covers anything rectangular: patios, pads, footings, walls and steps (calculate each step as its own slab). “Round” covers post holes, sonotube columns and round pads.
- Enter the size. Length and width are usually measured in feet; thickness and hole sizes in inches. Each box has its own unit menu, so you can mix units freely.
- Set the quantity — for example 8 identical fence-post holes.
- Keep a waste allowance. 10% is the default. Lower it only if your forms are rigid and your base is flat and compacted.
The result shows the volume in cubic yards (switch to cubic feet or cubic meters with the menu) and how many bags you would need of each common size.
Concrete volume formulas
Round hole: V = π × (D ÷ 2)² × H
Cubic yards = cubic feet ÷ 27
Bags = ⌈ total cubic feet ÷ yield per bag ⌉
- V — volume; convert every measurement to feet first and the answer is in cubic feet
- L, W, T — length, width and thickness (4 in = 4 ÷ 12 = 0.333 ft)
- D, H — hole diameter and depth
- ⌈ ⌉ — round up to the next whole bag
Bag yields used
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
These are the yields printed on widely sold ready-mix concrete bags. Fast-setting, high-strength and fiber-reinforced mixes can yield a little less per bag — if your bag lists a different number, divide the cubic feet shown above by that yield instead.
Worked example: a 10 × 10 ft patio
A patio slab measures 10 ft × 10 ft and will be 4 inches thick.
- Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
- Volume: 10 × 10 × 0.333 = 33.33 cu ft.
- Add 10% waste: 33.33 × 1.10 = 36.67 cu ft.
- Cubic yards: 36.67 ÷ 27 = 1.36 cu yd.
- 80 lb bags: 36.67 ÷ 0.60 = 61.1 → 62 bags.
Sixty-two 80 lb bags weigh about 2.5 tons, so for a slab this size many people compare the price of a short-load ready-mix delivery before buying bags.
Concrete for common slab sizes
Cubic yards and 80 lb bags for square and rectangular slabs. “With 10%” includes the recommended waste allowance.
| Slab size | Thickness | Cubic feet | Cubic yards | 80 lb bags | 80 lb bags, with 10% |
|---|---|---|---|---|---|
| 4 × 4 ft | 4 in | 5.33 | 0.20 | 9 | 10 |
| 6 × 6 ft | 4 in | 12.00 | 0.44 | 20 | 22 |
| 8 × 8 ft | 4 in | 21.33 | 0.79 | 36 | 40 |
| 10 × 10 ft | 4 in | 33.33 | 1.23 | 56 | 62 |
| 10 × 10 ft | 6 in | 50.00 | 1.85 | 84 | 92 |
| 10 × 12 ft | 4 in | 40.00 | 1.48 | 67 | 74 |
| 12 × 12 ft | 4 in | 48.00 | 1.78 | 80 | 88 |
| 12 × 20 ft | 4 in | 80.00 | 2.96 | 134 | 147 |
| 20 × 20 ft | 4 in | 133.33 | 4.94 | 223 | 245 |
| 24 × 24 ft | 4 in | 192.00 | 7.11 | 320 | 352 |
Concrete for post holes
Volume of one hole 3 ft deep, and the bags it takes. This assumes the hole is filled completely; the post itself displaces a little concrete, which works in your favor.
| Hole diameter | Cubic feet (3 ft deep) | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 6 in | 0.59 | 1 | 2 |
| 8 in | 1.05 | 2 | 3 |
| 10 in | 1.64 | 3 | 4 |
| 12 in | 2.36 | 4 | 6 |
A common rule of thumb is a hole three times the width of the post — a 4 × 4 post (3.5 in actual) in a 10–12 in hole. Your local building code and frost depth decide how deep the hole must be.
Tips before you pour
- Check the thickness in several places. A base that sits one inch low across a whole 10 × 10 slab adds about 0.3 cubic yards (8.3 cu ft).
- Remember the gravel base. Slabs usually sit on 4 in of compacted gravel — use the Gravel Calculator for that layer.
- Order a little extra. A cold joint from running out mid-pour is weaker than the cost of a few spare bags.
- Bags are heavy. An 80 lb bag is a lot to lift repeatedly; 60 lb bags cost more per cubic foot but are easier to handle.
Measuring the area first? The Square Footage Calculator gives the footprint, and the Cubic Yard Calculator converts any volume to yards for mulch, soil or gravel.
Frequently asked questions
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab 4 inches thick holds 33.3 cubic feet (1.23 cubic yards). That is about 56 bags of 80 lb mix, or 62 bags with a 10% allowance for spillage and an uneven base.
How many cubic feet does an 80 lb bag of concrete make?
About 0.60 cubic feet. A 60 lb bag makes about 0.45 cu ft and a 40 lb bag about 0.30 cu ft. These are the yields printed on common ready-mix bags; check your brand’s label, since mixes vary slightly.
How do I convert cubic feet to cubic yards?
Divide by 27, because one cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. Ready-mix trucks sell concrete by the cubic yard.
Should I use bags or order a truck?
Bags make sense for small jobs — post holes, steps, a small pad. Above roughly 1 cubic yard (about 45 bags of 80 lb mix), many people find a ready-mix delivery easier, but local minimum orders and short-load fees vary, so ask your supplier.
How much extra concrete should I order?
Add 5–10% for spillage, form bulges and an uneven base. For a sub-base you leveled by hand, lean toward 10%. Running short mid-pour leaves a weak cold joint, so it is cheaper to have a little left over.