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Groundworks · slabs · bed · cost
How many slabs do I need for a patio?
Slabs, the mortar bed under them, jointing and a price at the bottom. Cuts are what kill a patio — 10% is sane, 15% if it is curved or laid diagonally.
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
Order list
The orange figure is the one you order to. Highlighted rows are the other numbers you order to. The small print under each row is the sum that produced it, so you can check it against your own take-off. Amber rows are warnings, not quantities.
How this one works
- Coverage per slab =
(slab length + joint) × (slab width + joint) - Slabs =
area ÷ coverage × (1 + cutting %) - Bed:
area × bed depth, then dry volume × 1.3 split by the mix ratio - Cost =
slabs × slab price + area × laying rate
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
| Rate | Default | Why |
|---|---|---|
| Bed mix — sand : cement | 5 :1 | Parts of sharp sand to one of cement. 5:1 is the normal bedding mix. |
| Sand density | 1.6 t/m³ | |
| Dry-to-wet factor | 1.3 | Dry sand and cement lose about a quarter of their volume once mixed. |
| Cement bag | 25 kg | |
| Jointing compound | 4.5 kg/m² | Brush-in compound at a 10 mm joint. Wider or deeper joints use more. |
| Jointing tub | 20 kg |
A worked example
With the defaults the page opens on:
- How long is the patio? 6 metres
- How wide is it? 4 metres
- Which slab size? 600 × 600
- How much spare for cuts? 10 per cent
- How deep is the mortar bed? 40 millimetres
- How wide are the joints? 10 millimetres
- What does one slab cost? £9.50
- What is the laying rate? 48 £/m²
Slabs
- Area: 24 m² — L × W
- Coverage per slab: 0.37 m² — slab + 10 mm joint
- Cutting waste: 10 %
- Slabs to order: 71 no. — area ÷ coverage, +10%
Bedding & jointing
- Bed volume: 0.96 m³ — 40 mm bed
- Sharp sand: 1.66 t — mix 5:1, dry volume × 1.3
- Cement: 14 bags — = 333 kg ÷ 25 kg
- Jointing compound: 108 kg — at 4.5 kg/m²
- Jointing tubs: 6 tubs — at 20 kg a tub
Budget
- Slabs: £675
- Laying: £1,152
- Materials + labour: £1,827 — excludes sub base, edgings and VAT
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How many slabs do I need for a patio?
Area ÷ the coverage of one slab including its joint, plus cutting waste. A 6 m × 4 m patio in 600 × 600 slabs with 10 mm joints and 10% for cuts needs 71 slabs.
How much cutting waste should I allow?
10% on a square patio, 15% on curves, diagonals or anything with a lot of edges. Cuts are what kill a patio budget.
How deep should the mortar bed be?
30-40 mm on a full bed is normal; 50 mm if the sub base is uneven. The calculator prices the bed at a 5:1 sharp sand to cement mix, which you can change.
What is not in the budget line?
The sub base, edgings and VAT. The budget covers slabs at your price each and laying at your rate per square metre; set the laying rate to 0 for materials only.
Calculators for the same job
Doing this for a whole job rather than one wall? SiteLedger keeps the valuations, supplier statements, invoices and variations against the project they belong to.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.