Concrete Calculator: Bags & m3 For Slabs, Footings & Posts
Concrete is cheap until you get the quantity wrong. Order too little and you end up with a cold joint where the second pour meets the first. Order too much and you are paying to get rid of it. This calculator works out the volume you need for a slab, a strip footing or a set of fence posts, then turns that into 20kg bags or a ready mix order.
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Pick your shape, measure in metres, and use the matching formula. Always add a waste allowance, usually 5% to 10%.
| Shape | Formula for volume in m³ |
|---|---|
| Slab or base | length × width × depth |
| Strip footing | length × width × depth |
| Round post hole | 3.14 × radius × radius × depth, per hole |
| Square post hole | width × width × depth, per hole |
Then multiply by (1 + waste). For example, with 10% waste, multiply by 1.1.
Turning volume into bags
A 20kg bag of ready mixed dry concrete makes roughly 0.01m³ once mixed, so there are about 100 bags in a cubic metre. Bag yields vary a little by brand, so check the yield printed on the bag you buy and adjust.
Bags = volume in m³ ÷ 0.01
Worked example: shed base
A shed base 3m × 2.4m at 100mm (0.1m) deep:
- Volume: 3 × 2.4 × 0.1 = 0.72m³
- With 10% waste: 0.79m³
- Bags: 0.792 ÷ 0.01 = 79.2, so buy 80 bags of 20kg
That is a lot of mixing by hand. At this size, it is worth pricing ready mix or a mixer hire against the bags. See our guide to building a shed for base options.
Worked example: fence posts
Ten fence posts in round holes 300mm across (radius 0.15m) and 600mm deep:
- Per hole: 3.14 × 0.15 × 0.15 × 0.6 = 0.042m³
- Allow for the post itself taking up space, then add 10% waste, and you are close to 0.04m³ per hole
- Total: 10 × 0.04 = 0.4m³, or about 40 bags
Many fencers use a rapid setting post mix instead. Our comparison of postcrete vs concrete explains when each one makes sense.
Ready reckoner for slabs
| Slab size | 100mm deep | 150mm deep |
|---|---|---|
| 2m × 2m | 0.44m³ (44 bags) | 0.66m³ (66 bags) |
| 3m × 3m | 0.99m³ (99 bags) | 1.49m³ (149 bags) |
| 4m × 3m | 1.32m³ (132 bags) | 1.98m³ (198 bags) |
| 5m × 4m | 2.2m³ (220 bags) | 3.3m³ (330 bags) |
Figures include 10% waste.
Mix ratios
If you are mixing from scratch rather than using bagged ready mix, these are common rule of thumb ratios by volume (cement : sharp sand : aggregate):
| Use | Typical mix |
|---|---|
| Fence posts, general bases | 1 : 2 : 4 |
| Shed and garage bases, paths | 1 : 2 : 3 |
| Foundations | specified by the engineer or building control |
For anything structural, such as an extension footing, the mix and depth are set by the engineer's design and checked by building control, so do not rely on a rule of thumb. Our guide to laying foundations covers the process.
Add water gradually. Too much water makes concrete easy to place but weaker once it cures.
Ready mix vs bags
Bagged concrete is easy for small jobs: a few posts, a step, a small pad. Above roughly half a cubic metre, the mixing becomes the hard part. Ready mix arrives by lorry, mixed to a consistent strength, and is poured in one go.
| Method | Best for | Watch out for |
|---|---|---|
| 20kg bags | Under about 0.5m³, posts, repairs | Labour of mixing, consistency between batches |
| Mixer hire and loose materials | 0.5m³ to 1.5m³ | Measuring by volume accurately |
| Ready mix lorry | Over about 1m³, slabs, driveways | Access for the lorry, minimum orders, part load charges |
| Mix on site (volumetric) | Uncertain quantities | Usually a minimum charge |
Ready mix suppliers usually charge per cubic metre plus delivery, and many add a part load charge for small orders, so call two or three local suppliers with your volume and ask for a delivered price. Prices vary by region and change often.
Cost
To compare the two routes, price your job both ways:
- Bags: number of bags × price per bag at your local merchant.
- Ready mix: volume × price per m³, plus delivery and any part load charge, plus pump hire if the lorry cannot reach.
On larger jobs, labour often decides it. A concrete driveway or a big base is usually a job for a groundworker. Our concrete driveway guide gives typical installed prices.
FAQs
How many 20kg bags of concrete are in a cubic metre?
About 100, based on a yield of roughly 0.01m³ per bag. Check the bag for the exact yield.
How deep should a concrete shed base be?
Around 75mm to 100mm is common for a garden shed on firm ground, often over a compacted hardcore layer. Larger outbuildings and garages need more.
How long before I can walk on new concrete?
Usually after a day or two for light foot traffic. Concrete keeps gaining strength for weeks, so wait longer before putting heavy loads on it.
Do I need reinforcement mesh?
Mesh helps control cracking in larger slabs and driveways. For a small garden pad it is often skipped, but anything that will carry a vehicle should be designed properly.
Can I pour concrete in cold weather?
Avoid it when frost is forecast. Freezing water in fresh concrete weakens it, so cover the pour or wait for milder weather.