How many bags of concrete for a 10x10 slab?
A 10 x 10 ft slab poured 4 inches thick holds about 33.3 cubic feet (1.23 cubic yards) of concrete. Add 10% for waste and you are buying 62 bags of 80 lb mix, 82 bags of 60 lb, or 123 bags of 40 lb. Thicker slabs and the metric version are in the tables below.
Bags for a 10 x 10 ft slab
The table uses a 10% waste allowance and the yields printed on typical pre-mixed concrete bags: 0.60 cubic feet for 80 lb, 0.45 for 60 lb and 0.30 for 40 lb. Yields vary a little by brand, so check your bag and round up.
| Thickness | Volume (cu ft) | Volume (cu yd) | With waste (cu yd) | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|---|
| 4 in | 33.3 | 1.23 | 1.36 | 62 | 82 | 123 |
| 5 in | 41.7 | 1.54 | 1.7 | 77 | 102 | 153 |
| 6 in | 50 | 1.85 | 2.04 | 92 | 123 | 184 |
How the numbers are worked out
Volume is length x width x thickness, all in the same unit. Ten feet by ten feet by 4 inches is 10 x 10 x 0.333 = 33.3 cubic feet. Divide by 27 to get cubic yards, the unit ready-mix is sold in. Multiply by 1.10 for waste, then divide by the yield of one bag and round up to a whole bag.
The concrete calculator does the same arithmetic for any slab, footing, post hole or column, and lets you change the waste percentage and the bag size.
Bags or a ready-mix truck?
At 4 inches you are mixing 62 bags of 80 lb, which is about 4,960 lb of dry material. That is a long day for two people with a mixer, and the slab should be poured in one go so it cures as one piece. From roughly one cubic yard upwards, a ready-mix delivery is worth pricing: you pay for the volume plus a short-load fee on small orders, and the mix arrives at a consistent strength.
Metric: a 3 x 3 m slab
The closest metric slab is 3 x 3 m. Bags of 25 kg yield about 12 litres of concrete and 20 kg bags about 10 litres, again with 10% waste added.
| Thickness | Volume (m³) | With waste (m³) | 25 kg bags | 20 kg bags |
|---|---|---|---|---|
| 100 mm | 0.9 | 0.99 | 83 | 99 |
| 125 mm | 1.13 | 1.24 | 104 | 124 |
| 150 mm | 1.35 | 1.48 | 124 | 149 |
Before you pour
- The volume above is concrete only. A compacted sub-base (hardcore or crushed stone) and any reinforcement mesh are separate items.
- Measure the thickness in the middle of the formwork, not just at the edges; a slab that is 4 inches at the sides and 5 in the middle uses noticeably more mix.
- Have all the bags on site before you start mixing. A slab poured in two sessions has a cold joint and cracks along it.
Frequently asked questions
- How many 80 lb bags of concrete make a cubic yard?
- 45. A cubic yard is 27 cubic feet and an 80 lb bag yields about 0.60 cubic feet. For 60 lb bags it is 60, and for 40 lb bags 90.
- Is 4 inches thick enough for a 10x10 slab?
- For a patio, shed base or garden path, 4 inches (100 mm) on a compacted sub-base is the usual thickness. Anything that carries a vehicle or a building load is normally thicker and reinforced; follow your local code and, for structural work, an engineer's drawing.
- Should I really add 10% for waste?
- Yes for small jobs. Ground is never perfectly level, formwork bows outwards, and some mix stays in the mixer and on the tools. Running out with the pour half done is far more expensive than two spare bags.
- When is ready-mix cheaper than bags?
- 62 bags of 80 lb is roughly 2.5 US tons to carry and mix by hand. Around one cubic yard and above, ask a ready-mix supplier for a price including any short-load fee; the mix is also more consistent than hand-batched bags.
Last updated 2026-09-11. Figures are planning estimates from typical product data: check the yield, size or coverage printed on what you buy and round up.