Patio calculator: slabs, sand and cement
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| What are you laying? | One slab size |
| Patio area | rectangle 4 m × 3 m |
| Slab length | 600 mm |
| Slab width | 600 mm |
| Joint width | 10 mm |
| Waste allowance | 5% |
| Mortar bed depth | 40 mm |
| Bed mix: parts sharp sand to 1 part cement | 5 parts |
| Jointing | Brush-in compound |
| Fall: 1 in … | 60 (1 in n) |
| Distance the water runs across the patio | 3 m |
| Waste allowance | 10% |
| Dry-volume factor | 1.3 × |
| Sharp sand density | 1.6 t/m³ |
| Cement density | 1,440 kg/m³ |
| Cement bag size | 25 kg |
| Bulk bag size | 0.85 t |
Order: 34 slabs, 7 × cement bags and 0.92 t of sharp sand
| Result | Value |
|---|---|
| Slabs | 34 |
| Cement bags | 7 |
| Sharp sand | 915.2 kg |
| Sharp sand bulk bags | 2 |
| Drop across the patio for the fall | 50 mm |
| Fall as a percentage | 1.67% |
| Mortar bed (wet volume, with waste) | 0.528 m³ |
| Cement | 164.7 kg |
| Slabs per m² (no waste) | 2.69 |
| Patio area | 12 m² |
Show the working
- Area: 12 m²
- Slabs per m²: 1 ÷ ((0.6 + 0.01) × (0.6 + 0.01)) = 2.687
- Slabs: 12 × 2.687 × 1.05 (5% waste) = 33.86, rounded up = 34
- Mortar bed: 12 m² × 40 mm × 1.1 (10% waste) = 0.528 m³
- Dry volume: 0.528 × 1.3 = 0.686 m³
- Cement: 0.686 ÷ (1 + 5) × 1,440 kg/m³ = 164.7 kg
- Sharp sand: 0.686 × 5 ÷ (1 + 5) × 1.6 t/m³ = 0.92 t
- Cement bags: 164.7 kg ÷ 25 kg, rounded up = 7
- Sharp sand bulk bags: 0.92 t ÷ 0.85 t, rounded up = 2
- Fall: 3 m ÷ 60 = 50 mm drop (1.67%)
How it works
- Slabs per m² = 1 ÷ ((slab length + joint) × (slab width + joint)), all in metres
- Slabs = area × slabs per m² × (1 + waste), rounded up; project packs = area × (1 + waste) ÷ m² per pack, rounded up
- Mortar bed (wet) = area × bed depth × (1 + mortar waste); dry volume = wet × dry-volume factor
- Cement (kg) = dry ÷ (1 + r) × cement density; sharp sand (t) = dry × r ÷ (1 + r) × sand density, for r parts sand to 1 cement
- Mortar joints (wet) = area × joint share × slab thickness × (1 + mortar waste); joint share = 1 − L·W ÷ ((L + j)(W + j))
- Jointing compound (kg) = area × kg per m² from the tub; tubs rounded up
- Drop for the fall = distance the water runs ÷ n, for a fall of 1 in n
A full mortar bed and the fall away from the house are laying details: follow the slab maker's laying guide and BS 7533.
Project packs mix several slab sizes; the pack says how many m² it covers.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Slab length | 600 mm | Common UK size Not yet checked against the source | From your product brief v2 L1 (PA1: 600 × 600 slab). Slabs vary; check the product. |
| Slab width | 600 mm | Common UK size Not yet checked against the source | From your product brief v2 L1 (PA1: 600 × 600 slab). Slabs vary; check the product. |
| Joint width | 10 mm | Common UK size Not yet checked against the source | From your product Standard UK mortar joint. |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L1 PA1 and L2 BP1 (5%). Researcher to check against a laying guide. |
| Mortar bed depth | 40 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L1 PA2 (40 mm bed). Researcher to confirm against a laying guide (e.g. Marshalls) and BS 7533. |
| Bed mix: parts sharp sand to 1 part cement | 5 parts sand to 1 cement | Our assumption Not yet checked against the source | Our choice: change it to suit your job Check your maker's or builder's spec; the right ratio depends on the job. |
| Jointing compound coverage | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 L1: brush-in jointing compound coverage comes from the maker's tub (null). |
| Fall: 1 in … | 60 1 in n | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L1 PA3 (fall 1:60). Brief says the default comes from a sourced laying guide (e.g. Marshalls): Researcher to source. |
| Waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Droppings and mixing loss. |
| Dry-volume factor | 1.3 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job Dry materials shrink when mixed with water. Researcher to check against cement makers' guidance. |
| Sharp sand density | 1.6 t/m3 Sources range 1.6–1.7 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) Damp sand bulks; order by weight. |
| Cement density | 1440 kg/m3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Typical planning figure with no source yet. Researcher to source it and reclassify as typical. |
| Cement bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common UK cement bag. |
| Bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
Regulations and standards
- BS 7533 (pavements of flags and pavers): see the explainer for laying guidance
Explained: BS 7533 (paving).
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.