Concrete slab calculator
Calculate concrete volume, bag counts, ready-mix orders, sub-base and mesh for slabs, bases and pads in cubic metres or cubic yards.
Your project
Quick start
Slab dimensions
100 mm suits patios and shed bases; 125–150 mm for driveways.
Use this for repeated pads such as fence-post bases or pier footings.
How the concrete is supplied
Practical up to roughly 1 m³ (1.3 yd³) before mixing becomes the job.
5–10% covers spillage, an uneven excavation base and over-dig. Running out mid-pour causes a cold joint.
Build-up beneath the slab
Estimate updated. 68 25 kg bags of Ready-to-use concrete — 25 kg bags required.
Concrete slab calculator
United Kingdom · Metric units · 30 July 2026
Shopping list
2 lines- 6825 kg bagsReady-to-use concrete — 25 kg bags0.72 m³ (0.94 yd³) of placed concrete
- 3bulk bagsMOT Type 1 sub-base100 mm compacted beneath the slab
Estimate results
What you measured
- Slab area
- 7.20 m²
- 77.50 ft²
- Thickness
- 100 mm
- Concrete volume
- 0.72 m³ (0.94 yd³)
- Concrete weight
- 1.73 tonnes (3,810 lb)
What to buy
10% waste included| Material | Buy | Weight |
|---|---|---|
Ready-to-use concrete — 25 kg bags 0.72 m³ (0.94 yd³) of placed concrete Rounding: The calculation requires 61.02 25 kg bags. Adding 10% waste gives 67.12. Rounded up to 68 25 kg bags. Main material | 6825 kg bags | 1,926 kg |
MOT Type 1 sub-base 100 mm compacted beneath the slab Rounding: The calculation requires 2.2 bulk bags. Adding 10% waste gives 2.42. Rounded up to 3 bulk bags. Base and membrane | 3bulk bags | 2,550 kg |
Assumptions behind each line
Ready-to-use concrete — 25 kg bags
- Bag yield derived from the published rate of 0.00047 m³ per kg of dry mix — a 25 kg bag yields about 12 L.
- Yields vary between manufacturers. Check the bag before ordering a large quantity.
MOT Type 1 sub-base
- Includes a 1.3× compaction allowance.
- Density taken as 2,000 kg/m³.
Slab cross-section
Plan size: 3.00 m × 2.40 m
- Formwork should be set to the finished slab level and braced before the pour.
How this was calculated
Slab area
3.00 m × 2.40 m
= 7.20 m² (77.50 ft²)
Concrete volume
7.20 m² (77.50 ft²) × 100 mm thickness
= 0.72 m³ (0.94 yd³)
Bags required
The calculation requires 61.02 25 kg bags. Adding 10% waste gives 67.12. Rounded up to 68 25 kg bags.
= 68 25 kg bags
Concrete weight
0.72 m³ (0.94 yd³) × 2,400 kg/m³
= 1.73 tonnes (3,810 lb)
- Wet, normal-weight concrete. Useful for checking access and barrow runs.
Sub-base volume
7.20 m² (77.50 ft²) × 100 mm × 1.3 compaction
= 0.94 m³ (1.22 yd³)
Weight and delivery
Approximate weight
4.48 tonnes
Likely delivery
tipper or grab lorry
- 4.48 tonnes needs a tipper or grab lorry. Confirm access width, overhead clearance and where the driver can tip.
- Check that the delivery route can take the weight — block paving, drainage covers and soft verges are easily damaged.
Safety and compliance
Material planning only
This estimate is for material planning only. Structural dimensions, foundations and compliance requirements should be checked by a qualified professional.
This is not a structural design
Slab thickness, reinforcement, joint layout and ground-bearing capacity are design decisions. A slab supporting a building, a retaining structure or a vehicle load must be designed by a qualified engineer.
Check for buried services before digging
Before digging, check for buried services. Free utility plans are available from the relevant network operators, and a cable avoidance tool (CAT) scan is strongly recommended.
Check whether approval is required
Slabs forming part of a building, garage base or extension are usually notifiable. Confirm the requirements with your local authority Building Control team before you start.
Data behind this estimate
| Assumption | Value | Source |
|---|---|---|
Wet concrete Normal-weight concrete. Lightweight mixes are considerably lighter. | 2,400 kg/m³range 2,300–2,500 | British Standards InstitutionReviewed 2026-07-01 |
Bagged ready-to-use concrete yield Derived from the published 80 lb (36.3 kg) ≈ 0.60 ft³ (0.0170 m³) yield. | 0 m³ per kg of dry mixrange 0–0.001 | Bagged concrete manufacturers (Quikrete, Sakrete)Reviewed 2026-07-01 |
MOT Type 1 sub-base Graded crushed rock to the Series 800 specification. | 2,000 kg/m³range 1,900–2,100 | National Highways (UK)Reviewed 2026-07-01 |
Compaction allowance for granular sub-base Order about 30% more loose material than the finished compacted depth. | 1.3 × loose volumerange 1.2–1.4 | National Highways (UK)Reviewed 2026-07-01 |
Default waste allowance Covers cutting, breakage, setting-out error and the odd unusable unit. | 10 %range 5–15 | Project EstimateReviewed 2026-07-01 |
Cost
Enter the prices your supplier quoted. This site does not hold or estimate supplier prices, so totals appear only once you fill these in.
UK standard-rate VAT is 20%. Builders' merchant prices are often quoted excluding VAT.
No prices entered yet. Everything else in this estimate — quantities, weight, delivery and the shopping list — works without them.
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How this calculator works
Every figure below is derived rather than looked up, so you can check it against your own numbers. Nothing is hidden behind a black box.
Volume
Volume is length × width × thickness, multiplied by the number of identical slabs. Thickness is entered in millimetres or inches because that is how it is specified, and converted internally to metres.
volume = length × width × thickness × number of slabs
Bag counts
Bagged concrete is sold by dry weight but used by volume. The engine uses a yield of 0.00047 m³ per kilogram of dry mix, derived from the published figure of 0.60 ft³ from an 80 lb bag.
That gives 0.0170 m³ for an 80 lb bag, 0.0127 m³ for a 60 lb bag, 0.0118 m³ for a 25 kg bag and 0.0094 m³ for a 20 kg bag. Manufacturers vary slightly, so check the bag before placing a large order.
bags = volume with waste ÷ yield per bag, rounded up
Ready-mix
Ready-mix is rounded to the increment the plant sells in — 0.1 m³ in the UK and 0.25 yd³ in the US — with a supplier minimum applied. Short loads normally attract a surcharge.
Sub-base
Sub-base volume is the slab area multiplied by the sub-base depth and by a 1.3 compaction factor, because granular material loses volume as it is whacked down.
sub-base = area × depth × 1.3
Reinforcement
Mesh is estimated by effective coverage after lapping, not by sheet size. A UK A142 sheet is 11.52 m² gross but only about 10.13 m² once a 300 mm lap is allowed on two edges.
This is a quantity estimate only. Mesh grade, position and cover are design decisions.
Common questions
- How many bags of concrete do I need for a cubic metre?
- About 85 bags of 25 kg ready-to-use concrete, or 108 bags of 20 kg. In US terms a cubic yard needs roughly 45 bags of 80 lb. This is why anything over about a cubic metre is usually ordered as ready-mix.
- How thick should a concrete slab be?
- 75 mm (3 in) for footpaths and light shed bases, 100 mm (4 in) for patios and general slabs, and 125–150 mm (5–6 in) for driveways and garages, usually with reinforcement. Anything carrying a building load needs a designed foundation.
- Is ready-mix or bagged concrete cheaper?
- Bagged concrete is cheaper for very small pours because there is no delivery minimum. Ready-mix becomes cheaper per cubic metre well before 1 m³, and it saves hours of mixing. The crossover point depends on your local delivery charges.