What to measure
How to use the concrete calculator
- Choose the shape. Slabs, footings and walls are rectangles; columns and post holes are round.
- Enter the measurements in whatever units you have: inches, feet, yards, centimeters or meters. Each field converts on its own, so 10 ft × 10 ft × 4 in is fine.
- Set the quantity if you’re pouring several identical pieces, like eight fence-post holes.
- Keep 10% waste unless you have a reason to change it.
The result shows cubic yards (how ready-mix is sold) and bag counts rounded up to whole bags.
The formula
- Rectangle: length × width × thickness
- Round: π × (diameter ÷ 2)² × depth
Multiply by the quantity, add the waste percentage, then divide by 27 for cubic yards or by the bag yield (0.60 cu ft for an 80 lb bag) for bags.
Worked example: four post holes
Four holes, each 12 in across and 4 ft deep:
- One hole: π × (0.5 ft)² × 4 ft = 3.14 cu ft
- Four holes: 12.57 cu ft
- Plus 10%: 13.82 cu ft = 0.51 cubic yards
- 13.82 ÷ 0.60 = 23.04 → 24 bags of 80 lb (or 31 bags of 60 lb)
Common thicknesses
| Project | Typical thickness |
|---|---|
| Walkway, patio | 4 in |
| Driveway (cars) | 4–5 in |
| Driveway (trucks, RVs) | 6 in |
| Shed or garage floor | 4–6 in |
Footings and anything structural follow local code, so check with your building department.
Bags per post hole
One hole, 10% extra, rounded up to whole bags. The post itself takes up some of the space, so these counts leave a small margin.
| Hole (width × depth) | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|
| 6 in × 2 ft | 1 | 1 | 2 |
| 8 in × 2 ft | 2 | 2 | 3 |
| 8 in × 3 ft | 2 | 3 | 4 |
| 10 in × 3 ft | 3 | 4 | 6 |
| 12 in × 3 ft | 5 | 6 | 9 |
| 12 in × 4 ft | 6 | 8 | 12 |
Common mistakes
- Mixing up inches and feet. A 4 ft deep hole entered as 4 in comes out 12 times too small. Check the unit next to every field.
- Dividing by 9 instead of 27. A square yard is 9 square feet, but a cubic yard is 27 cubic feet.
- Measuring the plan, not the hole. Dug holes and trenches usually end up wider than planned. Measure what you actually dug before you buy.
- Ordering the exact volume. Forms bow and ground is uneven. Keep the waste allowance so you don’t run short mid-pour.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
45. One 80 lb bag of premixed concrete yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet (27 ÷ 0.6 = 45). For 60 lb bags it's 60, and for 40 lb bags 90.
Should I buy bags or order ready-mix?
Bags are practical up to roughly one cubic yard (about 45 × 80 lb bags). Above that, a ready-mix truck is usually cheaper and far less work, but many suppliers charge a short-load fee for small orders, so ask for their minimum.
How much extra concrete should I order?
5–10% is typical. Uneven ground, soft spots and forms that bow add up quickly, and running short mid-pour is worse than a few leftover bags. The calculator adds 10% by default; change it in the waste field.
How do I measure a post hole?
Measure the hole's width (diameter) and depth. Choose "Round column or post hole", enter both, and set how many holes you have. If the post sits in the hole, the concrete needed is slightly less, so the result leaves a small margin.
How many bags of concrete do I need per fence post?
An 8 in hole 2 ft deep takes 2 bags of 80 lb, and a 10 in hole 3 ft deep takes 3, both with 10% extra. For a whole fence, enter the hole size once and set the quantity, which rounds the bags for the job instead of per hole (ten 10 in × 3 ft holes take 30 bags).
How much concrete do I need for a footing?
Use the slab, footing or wall shape. A footing 20 ft long, 2 ft wide and 1 ft deep needs 1.63 cubic yards with 10% extra, or 74 bags of 80 lb, which is a ready-mix job.
How much concrete fills a 12 inch round column form?
A 12 in column 8 ft tall takes 0.26 cubic yards with 10% extra, or 12 bags of 80 lb. Choose "Round column or post hole" and enter the form's inside diameter.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.