Offcut WorksConcrete base calculator
Enter the length, width and thickness of the slab. You get the volume, a ready-mix order, a range of bags of dry mix, and the cement, sand and aggregate to mix it on site.
How the numbers are worked out
Volume = length × width × thickness. A 3 m × 2 m slab at 100 mm is 3 × 2 × 0.1 = 0.6 m³. Adding 10% gives 0.66 m³.
Bags = volume ÷ the volume one bag makes. Bags of 20 kg make about 9 to 13 litres, depending on the product, so the answer is shown as a range until you enter your own yield.
Site-mixed: dry materials take up about 54% more space than the wet concrete they make, so the dry volume is 1.54 × the wet volume. That is split by the mix ratio. Cement is taken as 1,440 kg per cubic metre.
Volume per square metre
| Thickness | Volume per m² |
|---|---|
| 75 mm | 0.075 m³ |
| 100 mm | 0.100 m³ |
| 150 mm | 0.150 m³ |
Bags, site mixing or ready-mix?
Bags are easy to carry and mix in small amounts but cost more per cubic metre, and each bag takes effort. Mixing cement, sand and aggregate yourself is cheaper but needs a mixer and storage. Ready-mixed concrete suits larger jobs, because you pour it all in one go, but you need access and people ready to place it. Bag counts get very large for a big slab, so the ready-mix figure above is worth a call to a supplier.
What goes under the concrete
A concrete base normally sits on compacted hardcore or sub-base, not directly on soil. Use the gravel and aggregate calculator for the sub-base. The amount and thickness depend on the ground and the load, so check the guide for the job, such as the shed maker's base instructions.
Thickness
The default of 100 mm is a common figure for light garden bases and paths, and many DIY guides quote it. Heavy loads, vehicles and weak ground need advice. The calculator does not decide thickness for you.
Ordering tips
- Measure the actual excavation, not the finished slab, if the formwork leaves a gap.
- Order a little over. Running short mid-pour leaves a cold joint.
- Ask the supplier which mix suits the job, and whether they deliver small loads.
Questions
How much concrete do I need for a shed base?
Multiply the length, width and thickness in metres. A 3 m × 2 m base at 100 mm needs 0.6 m³, or about 0.66 m³ with 10% extra.
How many bags of concrete make a cubic metre?
It depends on the product. A 20 kg bag of mix makes about 9 to 13 litres, so roughly 77 to 111 bags make one cubic metre. Check the yield on the bag you buy.
What mix should I use for a concrete base?
Common site mixes are 1 part cement to 2 of sharp sand and 4 of coarse aggregate, or 1 part cement to 5 of all-in ballast. Ask your supplier which suits your job.
Should I use ready-mix or bags?
Bags suit small amounts. For larger slabs, ready-mix is usually quicker and more consistent, but you need access and help to place it.
Where the defaults come from
Show the 2 sources
- Tarmac Postcrete data sheet (PDF). A UK manufacturer's table of 20 kg bags by post and hole size. It is where the 13 litre upper figure comes from.
- Tarmac Postmix concrete. One 20 kg bag covers about 0.09 m² at 100 mm deep, which is about 9 litres. That is the lower figure.
- The 9 and 13 litre yields come from Tarmac post-setting products. Other bagged mixes, such as general-purpose or ballast mixes, may yield differently, so enter the yield from your bag.
- The 1.54 dry-to-wet factor is a common rule of thumb, not a figure from a standard.
- The mix ratios are common DIY mixes, not a recommendation for your job. We did not find an official source.
- The 100 mm thickness is a typical figure from DIY guides. It is not a standard.
- The 10% waste and 1,440 kg per cubic metre for cement are our own working values.