The number, and why it feels wrong
A 25 kg bag of ready-mix concrete produces roughly 12.5 litres of wet concrete. One cubic metre is 1 000 litres. So you need about 80 bags — two tonnes — to fill it.
bags = 1 000 L ÷ yield per bag in litres
| Bag size | Approx. wet yield | Bags per m³ | Weight per m³ |
|---|---|---|---|
| 20 kg | 10 L | 100 | 2 000 kg |
| 25 kg | 12.5 L | 80 | 2 000 kg |
| 30 kg | 15 L | 67 | 2 000 kg |
| 40 kg | 20 L | 50 | 2 000 kg |
| 50 kg | 25 L | 40 | 2 000 kg |
The last column is the point. Whatever bag you choose, a cubic metre of concrete weighs about 2.4 tonnes when cured and takes roughly 2 tonnes of dry material to make. The bag size only changes how many times you lift it.
Why dry weight is not wet volume
The single most common mistake is assuming a 25 kg bag makes 25 litres. It does not, and the reason is worth understanding: dry aggregate is full of air. When you add water, the cement paste flows into the gaps between the stones, and the total volume ends up smaller than the loose dry material you started with. Concrete shrinks into itself.
A rough rule for any cement mix: wet volume is about 60-65% of loose dry volume. This is why bags list a yield in litres on the back — use that figure, never the weight, and never assume.
Yields vary between manufacturers by 10% or more. The figure printed on the bag you are actually buying beats every table, including this one.
Mixing from cement, sand and gravel
If you are buying the ingredients separately, the classic general-purpose mix is 1 : 2 : 3 — one part cement, two parts sand, three parts gravel, by volume. For one cubic metre of finished concrete:
| Mix | Cement | Sand | Gravel | Typical use |
|---|---|---|---|---|
| 1 : 2 : 3 | ~350 kg (14 × 25 kg) | ~700 kg | ~1 050 kg | General purpose, slabs, footings |
| 1 : 2 : 4 | ~300 kg (12 × 25 kg) | ~600 kg | ~1 200 kg | Mass fill, non-structural |
| 1 : 1.5 : 3 | ~400 kg (16 × 25 kg) | ~600 kg | ~1 200 kg | Higher strength, thin slabs, posts |
| 1 : 3 : 0 (mortar) | ~450 kg | ~1 350 kg | — | Bedding, rendering, blockwork |
Water is roughly half the cement weight — about 175 litres for a 350 kg cement mix. Add it last and add it slowly. Excess water is the fastest way to weaken concrete: it raises workability and destroys strength at the same time, and the damage is invisible until the slab cracks.
A worked example
A garden shed base, 3.0 × 2.5 m, 100 mm thick.
volume = 3.0 × 2.5 × 0.10 = 0.75 m³
with 10% for over-dig and spillage = 0.83 m³
25 kg bags = 830 L ÷ 12.5 = 67 bags
Sixty-seven bags is 1.7 tonnes. That is a pallet and a half, a full day of mixing for two people, and a delivery your driveway may not thank you for. At this volume a ready-mix truck is usually cheaper per cubic metre and always faster — most suppliers have a minimum of 1 m³ or charge a part-load fee, which 0.83 m³ sits awkwardly against.
Note the 10% allowance. Sub-grade is never flat: the ground eats concrete in the low spots and you will not notice until the mix runs out three-quarters of the way across.
Bags or truck?
- Under 0.2 m³
- Bags, easily. A few fence posts or a small pad. Mixing by hand in a barrow is realistic up to about 0.1 m³ per person per hour.
- 0.2 to 0.5 m³
- Bags with a hired mixer. Around 16 to 40 bags. Still a manageable half day.
- 0.5 to 1 m³
- The awkward zone. 40-80 bags is genuinely hard work, but most ready-mix suppliers charge a part-load fee below 1 m³. Compare the total, not the unit price.
- Over 1 m³
- Ready-mix truck, almost always. Cheaper per cubic metre, consistent strength, and one pour instead of thirty batches. Continuous pours also avoid cold joints, which matter structurally on a slab.
Concrete for anything structural — foundations, retaining walls, load-bearing slabs — should be specified by an engineer, not by a table. The quantities here are for planning and budgeting a domestic job.
Practical notes
- Measure the depth in several places before ordering. A slab that averages 120 mm instead of 100 mm needs 20% more concrete, and averages are what you get on real ground.
- Order the truck slightly under and finish with bags, rather than over-ordering. Surplus wet concrete is your problem to dispose of and it sets on schedule regardless of your plans.
- Concrete cannot be paused. Once mixing starts you are committed for the whole pour, so have everything — formwork, reinforcement, tools, help — in place first.
- Cold and hot weather both change the timing significantly. Below 5 °C the cure stalls; above 30 °C you may have twenty minutes of working time instead of an hour.
- Keep the finished slab damp for several days. Concrete does not dry, it cures, and letting it dry out early costs real strength.