Concrete Slab Calculator
Find how much concrete a slab, base or pad needs, in cubic metres or 20 kg bags.
Measure your project
Your answer
That is about 14 wheelbarrow loads. We added 10% extra.
If you buy bags
Blue Circle says each 20 kg bag makes about 0.01 m³ of concrete.
If you order a lorry (ready-mix)
Ask for 1.00 m³. Rock Solid Concrete takes home orders from 1 m³, so we price at least that.
This is a guide. For a garage, drive or floor, check your local building rules or ask a builder or engineer.
Which costs less?
You also skip mixing 40 bags by hand. Have the edges ready before it comes.
UK middle prices from October 2026, with 20% VAT. Your supplier's price may differ.
Quick answers
- A 2.4 × 1.8 m shed base
- Make the base 2.5 × 1.9 m and 75 mm thick. That is 0.39 m³ with 10% extra, or 40 bags of 20 kg.
- Shed base thickness
- At least 75 mm of concrete on 75–100 mm of packed-down hardcore, says Heidelberg Materials.
- Garage floor
- At least 100 mm of concrete on 100 mm of hardcore, says NHBC.
- Joints in a plain slab
- Usually no more than 4.5 m apart, says the Concrete Society.
- Walk on it
- After 24–48 hours, says B&Q. Take the timber edges off after about 5 days, say Heidelberg and Cemex.
How to work it out
- Measure the length and width in metres. Multiply them for the area.
- For a shed, make the base 100 mm wider and longer than the shed.
- Pick the thickness in mm. Divide it by 1,000 for metres.
- Area × thickness is the volume in m³. Add 10% extra.
- Divide by 0.01 for 20 kg bags, and round up.
Shed 2.4 × 1.8 m → base 2.5 × 1.9 m
2.5 × 1.9 × 0.075 = 0.356 m³
0.356 + 10% = 0.392 m³
0.392 ÷ 0.01 = 39.2 → 40 bags
Concrete shed base: how thick and how much
Three UK sources agree on about 75 mm of concrete for a garden shed base.
- Heidelberg Materials: at least 75 mm of concrete on 75–100 mm of packed-down hardcore. Make the base 100 mm wider and longer than the shed.
- Cemex: about 3 in of hardcore, then about 3 in of concrete.
- B&Q: dig 150 mm for most sheds. That is 75 mm of MOT Type 1 and 75 mm of concrete.
| Shed | Base | m³ (+10%) | 20 kg bags |
|---|---|---|---|
| 6 × 4 ft | 1.93 × 1.32 m | 0.21 | 21 |
| 8 × 6 ft | 2.54 × 1.93 m | 0.40 | 41 |
| 10 × 8 ft | 3.15 × 2.54 m | 0.66 | 66 |
| 12 × 8 ft | 3.76 × 2.54 m | 0.79 | 79 |
Each base is 100 mm wider and longer than the shed. Shed sizes are in feet; we work in metres. 8 × 6 ft is 2.44 × 1.83 m, a little bigger than the 2.4 × 1.8 m example above.
So the 2.5 × 1.9 m base weighs about 0.9 tonnes, at Cemex's 2.4 tonnes per m³.
Heidelberg's shed base guide mixes 1 part cement to 5 parts ballast, a 1 : 5 ratio. To mix your own, our ballast and cement calculator counts the bags. For the hardcore, use our Type 1 calculator.
Concrete slab thickness and sub-base by job
Pick your job, then type that thickness in the calculator. The sub-base is the layer of hardcore or Type 1 under the slab.
| Job | Concrete | Under it |
|---|---|---|
| Shed base | 75 mm or more | 75–100 mm hardcore |
| Patio slabs on a concrete bed | 50–80 mm bed | 50–80 mm hardcore |
| Garage floor | 100 mm or more | 100 mm hardcore |
| House floor | 100 mm or more | up to 600 mm hardcore |
| Car drive | 100 mm (150–200 mm for heavier use) | about 150 mm Type 1 (see note) |
Sources: Heidelberg (shed), Cemex (patio), NHBC (garage) and Approved Document C (house). Drive concrete: Pavingexpert, a trade guide. The 150 mm is from a UK government guide on paving front gardens, written for drives that let water through. We found no UK figure for under a concrete drive.
For a garage, drive or house floor, check your local building rules or ask a builder or engineer.
Mesh, rebar and joints in a concrete slab
Steel mesh (reinforcement)
Pavingexpert, a trade guide, says A142 mesh is often used in 100 mm slabs for paths, drives and garage bases. It also says mesh is not essential in a plain slab, though it suggests it.
Hy-Ten sells A142 in sheets of 4.8 × 2.4 m, about 25.57 kg each. Pavingexpert overlaps sheets by at least 350 mm. It keeps at least 50 mm of concrete all round the steel.
Joints
The Concrete Society says joints ease the stress as concrete warms, cools, wets and dries. In a slab with no steel, they are usually no more than 4.5 m apart. Pavingexpert spaces them at about 30 times the thickness. It cuts these crack-control joints 25–33% of the way into the slab.
| Slab | 30 × thickness | Joints every (4.5 m at most) |
|---|---|---|
| 75 mm | 2.25 m | 2.25 m |
| 100 mm | 3 m | 3 m |
| 150 mm | 4.5 m | 4.5 m |
| 200 mm | 6 m | 4.5 m |
NHBC says a garage floor over 16 m² needs steel, or joints that split it into bays under 16 m².
Before you pour, and after
- Lay the hardcore and pack it down.
- Heidelberg adds a layer of building sand, then a damp-proof sheet (DPM). Overlap and tape its joints.
- Fix timber edges (formwork) round the slab, and have helpers ready.
- Blue Circle says to keep its bagged concrete covered for at least 7 days.
- Take the timber edges off after about 5 days (Heidelberg and Cemex).
- Cemex says concrete takes 28 days to reach full strength.
Questions people ask
How much concrete do I need for a shed base?
For a 2.4 × 1.8 m shed, make the base 2.5 × 1.9 m. At 75 mm thick, that is 0.39 m³ with 10% extra. That is 40 bags of 20 kg.
Does a concrete slab need a sub-base?
The UK guides we read all lay hardcore under it. Heidelberg uses 75–100 mm for a shed base. NHBC uses 100 mm under a garage floor.
Does a concrete slab, patio or driveway need rebar?
Pavingexpert, a trade guide, says steel mesh is not essential in a plain slab, though it suggests it. It says A142 mesh is often used in 100 mm drives, paths and garage bases. For a drive, check your local building rules or ask a builder or engineer.
Does a concrete slab need expansion joints?
The Concrete Society says a slab with no steel needs joints, usually no more than 4.5 m apart. They are needed early, usually within 24 hours.
How much does it cost to lay a concrete base?
MyBuilder, the only cost guide we found, says £40–£80 per m² for a shed base, VAT not stated. Our concrete cost calculator lists more laid prices.
How much does it cost to concrete a driveway?
Only 3 cost guides give a price: Checkatrade about £95 per m², MyBuilder £80–£100 and MyJobQuote £85–£135. None says if VAT is in. Our concrete cost calculator lists more laid prices.
How long before I can walk or drive on new concrete?
Walk on it after 24–48 hours, says B&Q; Cemex says 3 days. Tarmac says a car can park on a drive after 5–7 days.
Where our numbers come from
- Shed base: Heidelberg Materials guide (75 mm concrete minimum)
- Shed base: Cemex guide (about 3 in concrete, 3 in hardcore)
- Shed base: B&Q guide (75 mm concrete on 75 mm Type 1)
- Bag size: Blue Circle Multi-Purpose Concrete data sheet (20 kg makes about 0.01 m³)
- Bag price: the middle of 4 prices from 3 UK stores for 20 kg concrete (Oct 2026)
- Ready-mix price: the middle of 5 prices from 3 UK suppliers (Oct 2026, with VAT)
- Rock Solid Concrete price guide (Midlands ready-mix)
- Full loads, barrows and mixes: Heidelberg Materials UK merchant guide
- Patio build-up: Cemex guide to garden walls and paving
- Garage floors: NHBC Standards 2026, chapter 10.1 (at least 100 mm)
- House floors: Approved Document C (at least 100 mm)
- Concrete drives: Pavingexpert (a trade guide; 100 mm for cars, 150–200 mm for heavier use)
- UK government guide to paving front gardens
- Mesh laps: Pavingexpert trade guide (lap at least 350 mm)
- Mesh sheets: Hy-Ten A142 (4.8 × 2.4 m, 25.57 kg)
- Joint spacing: Concrete Society (4.5 m max, unreinforced slabs)
- Paving joints: Concrete Society (cut within 24 hours)
- Slab joints: Pavingexpert trade guide (about 30× slab thickness)
- Driving on new concrete: Tarmac help page (5–7 days)
- Laid concrete prices: MyBuilder guide
- Concrete drive cost: Checkatrade guide (about £95 per m²)
- Concrete drive cost: MyBuilder guide (£80–100 per m²)
- Tarmac per tonne, resurfacing and drive surfaces: MyJobQuote (VAT not stated)
- Concrete weight: Cemex concrete FAQs (about 2.4 tonnes per m³)
- Extra for spills: Sakrete slab calculator (10%)
- Unit sizes: NIST Handbook 44, Appendix C (1 ft = 0.3048 m; 1 US gallon = 231 cubic inches)
Made by BuildersCount. Last checked 4 Oct 2026.
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