How much concrete do I need? Volumes, bags and mix ratios explained
Work out concrete for a slab, path, shed base or post holes in cubic metres, then convert to bags of pre-mix or cement, sand and gravel to mix yourself.
By Revivista Editorial TeamUpdated 3 min read
Safety note: parts of this topic involve electrical, structural or gas work. Rules differ by country and some work must be carried out or signed off by a qualified professional.
To work out how much concrete you need, multiply length × width × depth in metres. That gives the volume in cubic metres (m³). Add a margin for waste and uneven ground, then decide whether to buy bags of pre-mix, mix your own from cement, sand and gravel, or order ready-mixed concrete.
Our concrete calculator does every step below for slabs, paths and round post holes.
Step 1: Calculate the volume
Slab, path, shed base or strip:
Volume (m³) = length (m) × width (m) × thickness (m)
Convert thickness from centimetres to metres by dividing by 100: 10 cm = 0.10 m.
Example: a 3 × 3 m shed base, 10 cm thick = 3 × 3 × 0.10 = 0.9 m³.
Round post hole or column:
Volume (m³) = π × (diameter ÷ 2)² × depth
Example: a 30 cm hole, 60 cm deep = 3.1416 × 0.15² × 0.6 ≈ 0.042 m³ — so 10 such holes need about 0.42 m³.
Step 2: Add a margin
Ground is rarely perfectly level, formwork bows slightly, and some concrete is always lost in the mixer and barrow. A margin of around 5–10% is common for small jobs. On uneven sub-bases, measure the depth in several places and use the average.
Step 3: Choose how to buy it
Bags of pre-mix (dry ready-mixed concrete)
Best for small jobs: post holes, small bases, repairs. Each bag says how much volume it makes once mixed — use that figure. As a rough guide, dry pre-mix fills around 2,150 kg per cubic metre of finished concrete, so:
Bags = volume (m³) × 2,150 ÷ bag weight (kg)
Example: 0.945 m³ (0.9 m³ + 5%) ÷ 20 kg bags ≈ 102 bags — which is about 2 tonnes to carry. At that size, mixing your own or ordering ready-mix is usually more practical.
Mixing your own from cement, sand and gravel
Mixes are described as a ratio by volume: cement : sand : gravel (aggregate).
| Mix | Typical use |
|---|---|
| 1 : 1.5 : 3 | Stronger mixes |
| 1 : 2 : 4 | General use: paths, bases, post holes |
| 1 : 3 : 6 | Mass fill, blinding layers |
Dry materials shrink in volume when water is added. A common rule of thumb is to multiply the wet volume by 1.54 to get the dry volume, then split it by the ratio. For a 1 : 2 : 4 mix, that makes 7 parts:
- Cement = dry volume × 1/7, converted to weight at about 1,440 kg per m³
- Sand = dry volume × 2/7
- Gravel = dry volume × 4/7
For 0.945 m³ of 1 : 2 : 4 concrete, that is roughly 299 kg of cement, 0.42 m³ of sand and 0.83 m³ of gravel. Builders’ merchants often sell sand and gravel in bulk bags, or as ready-combined “ballast”.
These ratios and factors are rules of thumb for nominal mixes — not a structural mix design.
Ready-mixed concrete from a truck
For larger slabs, ready-mix is delivered to order. Suppliers usually sell by the cubic metre and may have a minimum order or charge extra for small loads, so get quotes. Check access for the truck, and how far the concrete has to be moved on site.
When you need a professional
Garden paths, small shed bases and fence posts are typical DIY jobs. Foundations for extensions, suspended slabs, retaining walls and anything that carries a structural load must be designed by a structural engineer and approved under the Building Regulations. If you are unsure whether your project needs approval, ask your local authority’s building control team before you start.
Quick checklist
- Measured length, width and depth in several places
- Volume calculated in m³ with a 5–10% margin
- Sub-base compacted and formwork set
- Method chosen: bags, own mix or ready-mix
- Gloves and eye protection — wet cement can burn skin
