How The Calculation Works
This calculator uses a standard 1 : 2 : 3 concrete mix ratio — 1 part cement, 2 parts sharp sand, 3 parts gravel (by volume). This general-purpose mix is suitable for shed bases, paths, footings and most domestic groundwork.
Mix Ratios at a Glance
| Material |
Ratio |
Per m³ of concrete |
| Portland Cement |
1 part |
~320 kg (13 × 25kg bags) |
| Sharp Sand |
2 parts |
~640 kg (0.64 tonnes) |
| Gravel / Aggregate |
3 parts |
~960 kg (0.96 tonnes) |
Wet concrete is about 2,400 kg per cubic metre. The calculator adds a 10% waste buffer automatically — you'll always have slightly more material than the mathematical minimum, which saves a trip to the builders' merchant.
Recommended Slab Depths
| Application |
Depth |
| Garden path / stepping stones | 75 mm (3″) |
| Standard shed or summerhouse | 100 mm (4″) |
| Heavy shed / workshop with machinery | 125–150 mm (5–6″) |
| Garage or vehicle-bearing slab | 150–200 mm (6–8″) |
Frequently Asked Questions
Can I use ready-mix concrete instead?
Yes. For bases larger than about 2m³ it's usually cheaper and faster to order ready-mix delivered by truck. Below that volume, bags of cement mixed on-site are more practical. This calculator gives you the bag-by-bag breakdown for the DIY approach.
Do I need a sub-base under the concrete?
For most shed bases, yes. Lay 75–100mm of compacted MOT Type 1 or hardcore beneath the slab. This improves drainage and stops the slab settling unevenly. The calculator covers only the concrete layer itself — the sub-base is additional material.
How long does the concrete take to set?
Concrete reaches initial set in 24–48 hours and is safe to walk on. Full structural strength (around 95%) takes about 28 days. Avoid placing heavy loads or building on the slab for at least 3–7 days, and keep it damp in hot weather to prevent cracking.
What's the difference between cement and concrete?
Cement is the powder (Portland cement) that acts as the binder. Concrete is the finished product — cement mixed with sand, gravel and water. When people say "bags of cement for a shed base," they usually mean bags of cement to make concrete.
Is a 1:2:3 mix strong enough for a shed?
Yes, it produces roughly a C20 grade concrete which is more than sufficient for domestic shed bases, paths and patios. For structural footings or load-bearing walls, a stronger mix or structural engineer's advice may be needed.
What's the difference between 20kg and 25kg bags?
The cement inside is identical — it's just the bag size that differs. Some retailers (Wickes, B&Q) sell 25kg bags, while others stock 20kg. The 25kg bags are slightly better value per kilo but heavier to carry. Use the bag size toggle above to match what your local store sells.
How accurate is the cost estimate?
The cost estimate uses average UK prices: roughly £6 per 25kg bag of cement (or £5 per 20kg bag), £50–60 per tonne of sharp sand, and £45–55 per tonne of gravel. Actual prices vary by region and supplier. It's meant as a rough budget guide — get a quote from your local builders' merchant for exact pricing.