Details
Type feet and inches like 12'6" or just 12.5. Everything is saved on this device, so your list is still here in the shop.
About the concrete calculator
Whether you are pouring a shed base, a patio, a path or setting fence posts, this calculator tells you how much concrete you need — in cubic yards (or m³) — and how many bags of ready-to-mix concrete that is, in the size you buy.
Add each part of the job as a slab (length × width × thickness) or a post hole (diameter × depth × how many). The calculator adds them, adds a little extra for spills and uneven ground, and rounds the bags up.
How to use it
- Choose Feet & inches or Metres & cm
- For a slab, patio or path: enter the length, width and thickness
- For posts: choose Post hole or column, enter the hole diameter, depth and how many
- Pick your bag size and the extra to add
- Read the cubic yards and the bags — and press Copy the list
How concrete is worked out
Volume = length × width × thickness, with all three in the same unit.
For a 10 × 10 ft slab, 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.3 cubic feet. A cubic yard is 27 cubic feet, so that is 1.23 cubic yards.
For a round post hole: 3.1416 × (diameter ÷ 2)² × depth. A 10 in hole, 36 in deep, holds 1.6 cubic feet — before the post goes in.
How much one bag makes
| Bag | Makes | Bags for 1 cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
| 25 kg | about 12 litres | about 83 per m³ |
| 20 kg | about 10 litres | about 105 per m³ |
The exact yield is printed on the bag — it changes a little between brands.
Concrete for common slabs
4 inches thick, without extra:
| Slab | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 5.3 | 0.20 | 9 | 12 |
| 10 × 10 ft | 33.3 | 1.23 | 56 | 75 |
| 10 × 12 ft | 40.0 | 1.48 | 67 | 89 |
| 12 × 12 ft | 48.0 | 1.78 | 80 | 107 |
| 10 × 20 ft | 66.7 | 2.47 | 112 | 149 |
| 20 × 20 ft | 133.3 | 4.94 | 223 | 297 |
How thick should the slab be?
| Job | Usual thickness |
|---|---|
| Path, shed base, small patio | 4 in (10 cm) |
| Patio with a hot tub or heavy grill | 5–6 in (12–15 cm) |
| Driveway for cars | 5–6 in (12–15 cm) |
| Fence post holes | about 3 × the post width across, ⅓ of the post length deep |
Local building rules can ask for more — check them for driveways and anything structural.
Bags or ready-mix?
Bags are best for small jobs: post holes, steps, a small pad. Above about 1 cubic yard (45 bags of 80 lb) mixing by hand is hard work, and a ready-mix truck is usually cheaper and gives a stronger, more even slab. Many suppliers have a minimum order, so ask for the "short load" price.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
At 4 inches thick: about 56 bags of 80 lb, or 75 bags of 60 lb. With 10% extra, plan on 62 bags of 80 lb. That is 1.23 cubic yards — a size where ready-mix is worth a phone call.
How many 80 lb bags make a cubic yard?
45 bags. One 80 lb bag makes about 0.6 cubic feet, and a cubic yard is 27 cubic feet.
How much concrete do I need for a fence post?
For a 4 × 4 post in a 10 inch hole, 3 ft deep: the hole holds 1.6 cubic feet, the post takes about 0.3, so about 1.3 cubic feet — just over two 80 lb bags. The calculator ignores the post, which gives you a small safety margin.
How much extra concrete should I order?
Add 5–10% for spills, uneven ground and the forms bowing out. Running short in the middle of a pour is far worse than a few bags left over.
How do I convert cubic feet to cubic yards?
Divide by 27. 54 cubic feet is 2 cubic yards.
Is concrete measured in cubic metres?
Yes, outside the US. 1 cubic yard is 0.765 m³, and 1 m³ is 1.31 cubic yards. Switch to Metres & cm to get the result in m³ and litres.
Good to know
- Wear gloves and eye protection — wet concrete can burn the skin
- Your slabs and holes are saved in this browser for next time
- Nothing is sent anywhere — it works in your browser
Related tools
- Square Footage Calculator — the area of a patio or yard
- Mulch, Soil & Gravel Calculator — gravel for the base
- Tile & Flooring Calculator — tiles for the finished floor
- Paint Calculator — paint for walls
- Wallpaper Calculator — rolls for a room
- Unit Converter — feet, metres, pounds and kilograms
