Enter the size of your slab, footing or post holes to see how much concrete you need, in cubic yards or cubic metres, and exactly how many bags to buy.
Last updated: October 24, 2026 · Bag yields checked against the manufacturer data sheet linked below.
Tip: use decimals for part-feet. 10 ft 6 in = 10.5 ft.
Estimates only. Bag yields are typical values; check the label on your bag.
How to use this concrete calculator
- Pick the job. Choose Slab or footing for patios, shed bases, paths and footings. Choose Post holes for fence posts, deck posts and mailbox posts.
- Pick your units. Use feet and inches, or metres and centimetres. Switching converts your numbers for you.
- Measure and enter. For a slab, enter length, width and thickness. For holes, enter the hole width, depth and how many holes you are digging.
- Set extra for waste. 10% is a sensible default. It covers spills, uneven ground and the concrete left in the mixer.
- Read the result. You get the total volume and the number of bags for each common bag size. Bag counts are always rounded up.
The formula: how much concrete for a slab
A slab is a box, so its volume is length × width × thickness. The only trick is keeping the units the same.
Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
Cubic yards = cubic feet ÷ 27
Worked example: a 10 × 10 ft patio, 4 inches thick
- 10 × 10 × 4 ÷ 12 = 33.3 cubic feet
- 33.3 ÷ 27 = 1.23 cubic yards
- Add 10% for waste: 1.23 × 1.10 = 1.36 cubic yards (36.7 cubic feet)
- In 80 lb bags (0.60 cu ft each): 36.7 ÷ 0.60 = 61.1, so buy 62 bags
Metric example: a 3 × 3 m slab, 10 cm thick
- 3 × 3 × 0.10 = 0.90 m³, plus 10% = 0.99 m³
- In 20 kg bags (about 0.0094 m³ each): 106 bags. In 25 kg bags (about 0.0117 m³ each): 85 bags.
How much concrete does one bag make?
Bagged concrete is sold by weight, but what matters is the volume it makes once mixed. These are the yields printed by a major US manufacturer for its standard concrete mix:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb (18.1 kg) | 0.30 cu ft (8.5 L) | 90 |
| 50 lb (22.6 kg) | 0.375 cu ft (10.6 L) | 72 |
| 60 lb (27.2 kg) | 0.45 cu ft (12.7 L) | 60 |
| 80 lb (36.2 kg) | 0.60 cu ft (17 L) | 45 |
Source: Quikrete Concrete Mix No. 1101 data sheet. Those figures work out to roughly 2,130 kg of dry mix per cubic metre of concrete, which is how we estimate metric bags: about 0.0094 m³ for a 20 kg bag (about 107 bags per m³) and about 0.0117 m³ for a 25 kg bag (about 86 bags per m³).
Yields vary between brands and products (fast-setting, high-strength and fibre mixes can differ). Check the yield printed on the bag you buy and adjust if it is different.
Concrete for post holes
A round hole is a cylinder: π × radius² × depth. If you set a post in the hole, the post takes up some of that space, so the calculator subtracts it.
Worked example: 8 fence posts
- Holes 10 inches wide and 30 inches deep: π × 5² × 30 = 2,356 cubic inches per hole
- 8 holes = 18,850 cubic inches = 10.9 cubic feet
- With 4×4 posts (3.5 × 3.5 in) in the holes, the concrete needed drops to 9.2 cubic feet
- Add 10% and divide by 0.60: 17 bags of 80 lb with posts in place (20 bags if you fill the holes solid)
A common rule of thumb is a hole about three times the width of the post, with roughly a third of the post’s length below ground. Frost depth and local rules can require deeper holes, so check before you dig.
Common slab sizes at a glance
4-inch slab, 10% extra included. Bag counts rounded up.
| Slab size | Cubic yards | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 4 × 4 ft | 0.22 | 14 | 10 |
| 8 × 8 ft | 0.87 | 53 | 40 |
| 10 × 10 ft | 1.36 | 82 | 62 |
| 10 × 12 ft | 1.63 | 98 | 74 |
| 12 × 12 ft | 1.96 | 118 | 88 |
| 16 × 16 ft | 3.48 | 209 | 157 |
| 20 × 20 ft | 5.43 | 326 | 245 |
Metric: 10 cm slab, 10% extra included.
| Slab size | Cubic metres | 20 kg bags | 25 kg bags |
|---|---|---|---|
| 1.2 × 1.2 m | 0.16 | 17 | 14 |
| 2.4 × 2.4 m | 0.63 | 68 | 55 |
| 3 × 3 m | 0.99 | 106 | 85 |
| 3 × 3.6 m | 1.19 | 127 | 102 |
| 3.6 × 3.6 m | 1.43 | 152 | 122 |
| 5 × 5 m | 2.75 | 293 | 236 |
| 6 × 6 m | 3.96 | 422 | 339 |
Tips before you buy
- Measure the real thickness. Ground is rarely flat. Check depth in several spots and use the average. An extra half inch across a large slab adds up quickly.
- Thickness depends on the job. 4 inches (10 cm) is a common thickness for patios, paths and shed bases. Areas that carry vehicles are usually thicker. Your local building code has the final say.
- Bags or ready-mix? Bags suit small jobs and post holes. For larger pours, compare the cost and effort of dozens of bags with a ready-mix delivery, which may have a minimum order.
- Do not forget the base. Most slabs sit on a compacted gravel base. That is a separate material to buy.
- Buy one or two extra bags. Running out halfway through a pour is worse than returning an unopened bag.
Frequently asked questions
How many 80 lb bags of concrete are in a yard?
About 45. One cubic yard is 27 cubic feet, and an 80 lb bag makes about 0.60 cubic feet (27 ÷ 0.60 = 45). For 60 lb bags it is 60, and for 40 lb bags it is 90.
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick with 10% extra, about 62 bags of 80 lb or 82 bags of 60 lb. At 6 inches thick, you would need about half as much again.
How many 20 kg bags of concrete make a cubic metre?
About 107, based on roughly 0.0094 m³ per 20 kg bag. For 25 kg bags it is about 86. Check the yield on your product’s bag, as it varies by brand.
How much extra concrete should I order?
Adding 5% to 10% is typical. Use the higher end for uneven ground, hand mixing or irregular shapes.
Is this calculator accurate enough to order from?
It gives a reliable estimate for planning and shopping, based on the dimensions you enter and published bag yields. Results are estimates only. For structural work, confirm thickness and reinforcement with your local building department or a professional. See our disclaimer.
Spotted something that does not look right? Let us know and we will check it.