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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.
1
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.
2
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.
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
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? 9.5 pounds sterling
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.
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.
