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Use this free concrete calculator to work out exactly how much concrete you need for a slab, patio, driveway, footing, column or post hole. Enter your measurements in feet and inches or meters and centimeters and get the volume in cubic yards or cubic meters, plus the number of 40, 60 or 80 lb bags (or 20 and 25 kg bags) to buy.
How to Use the Concrete Calculator
- Choose the shape: a rectangular slab, footing or patio, or a round column, post hole or concrete tube.
- For a slab, enter the length, width and thickness. For a column, enter the diameter and the height or depth.
- Enter how many identical pours you need, for example 6 post holes or 2 footings.
- Set a waste allowance. Most contractors add 5–10% for spillage, uneven ground and form movement.
- Optionally enter the ready-mix price per cubic yard or cubic meter to estimate the cost.
Concrete Volume Formula
Concrete is ordered by volume. For a rectangular slab or footing the formula is:
Volume = Length × Width × Thickness
For a round column, pier or post hole, use the volume of a cylinder:
Volume = π × (Diameter ÷ 2)² × Height
Make sure every measurement is in the same unit before you multiply. In the US, convert inches to feet by dividing by 12, then divide cubic feet by 27 to get cubic yards. In metric, convert centimeters to meters by dividing by 100 and the result is already in cubic meters.
Worked Example: 10 × 10 ft Slab, 4 Inches Thick
- Thickness: 4 in ÷ 12 = 0.333 ft
- Volume: 10 × 10 × 0.333 = 33.3 cubic feet
- Cubic yards: 33.3 ÷ 27 = 1.23 cubic yards
- 80 lb bags: 33.3 ÷ 0.60 = 55.6, so you need 56 bags (62 bags with 10% extra)
Metric example: a 3 × 3 m slab that is 10 cm thick needs 3 × 3 × 0.10 = 0.9 m³ of concrete, or about 77 bags of 25 kg pre-mix.
How Much Does a Bag of Concrete Make?
Bag yields vary slightly by brand, but these figures are typical for standard pre-mixed concrete:
| Bag size | Approx. yield | Bags per cubic yard | Bags per m³ |
|---|---|---|---|
| 40 lb (18 kg) | 0.30 cu ft | 90 | ≈118 |
| 60 lb (27 kg) | 0.45 cu ft | 60 | ≈78 |
| 80 lb (36 kg) | 0.60 cu ft | 45 | ≈59 |
| 20 kg | ≈0.0094 m³ | ≈81 | ≈106 |
| 25 kg | ≈0.0117 m³ | ≈65 | ≈86 |
As a rule of thumb, bags are practical for jobs under about one cubic yard (0.75 m³). For anything larger, ready-mix delivered by truck is usually cheaper, faster and gives a more consistent result.
Recommended Slab Thickness
| Project | Typical thickness |
|---|---|
| Walkways, patios, shed bases | 4 in (10 cm) |
| Driveways for cars | 4–5 in (10–12.5 cm) |
| Driveways for heavy vehicles, RV pads | 5–6 in (12.5–15 cm) |
| Garage floors | 4–6 in (10–15 cm) |
These are common starting points only. Footing sizes, reinforcement and slab thickness are set by local building codes and soil conditions, so check with your building department or an engineer for structural work.
Tips for Ordering Concrete
- Measure the depth in several places. Low spots in the sub-base can add a surprising amount of volume.
- Order 5–10% more than the calculated volume. Running short mid-pour can leave a weak cold joint.
- Ready-mix suppliers often have a minimum order and charge a short-load fee for small deliveries.
- Have enough people and tools ready. Concrete starts to set within an hour or two, faster in hot weather.
- Preparing a gravel base first? Use our gravel calculator to size the sub-base.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab that is 4 inches thick needs about 1.23 cubic yards, which is 56 bags of 80 lb concrete or 75 bags of 60 lb concrete before adding extra for waste.
How many 80 lb bags are in a yard of concrete?
One cubic yard is 27 cubic feet. An 80 lb bag yields about 0.6 cubic feet, so you need about 45 bags of 80 lb concrete per cubic yard.
How do I calculate concrete for a round post hole?
Use the cylinder formula π × radius² × depth for each hole, multiply by the number of holes, and subtract the volume of the post if you want a more exact figure. Select the round column option in the calculator to do this automatically.
Should I add extra concrete for waste?
Yes. Add 5–10% to cover spillage, uneven ground, form bulging and concrete left in the mixer or truck. For irregular excavations, add closer to 10%.
Is it cheaper to use bags or ready-mix?
Bags are usually cheaper and easier for small jobs under about one cubic yard. For larger pours, ready-mix delivered by truck normally costs less per yard and saves a lot of mixing labor.
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