It's the most-asked question on any concrete job. The answer comes from just two numbers: the volume you're filling and what one bag actually makes.
What one bag makes
| Bag | Yield | Bags per m³ |
|---|---|---|
| 25 kg pre-mixed concrete | ~0.0115 m³ | ~87 |
| 20 kg pre-mixed / post-mix | ~0.0095 m³ | ~105 |
Yields vary a little by brand, so check the bag. These are the figures our calculators use by default.
Quick slab table
Bags of 25 kg mix per m² of slab: about 8.7 at 100 mm thick, 10.9 at 125 mm and 13 at 150 mm, before waste.
Post holes
Post holes use less than you'd expect, because the post takes up part of the hole. A 75 mm post in a 250 mm × 600 mm hole needs roughly 0.025 m³, about 2–3 bags of 20 kg post-mix.
Add waste
Add 10% for spills, uneven ground and the half-bag that gets wet. On anything over a cubic metre, seriously consider ready-mix: see bags vs ready-mix.
Water
A 25 kg bag takes about 2.5–3 litres of water. Too much water is the most common way to weaken bagged concrete. Add it gradually until the mix is workable, not soupy.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the Building Regulations and Building Control for your project.