Hong Life CalculatorsUnderstanding the paving slab and block calculator — concept and principle
This calculator estimates how many paving blocks or slabs you need for a patio, path or driveway, together with the bedding sand and sub-base material underneath. Enter the paved area, the size of one paver, the sand bed thickness and the sub-base thickness. It gives you a complete order for the surface and the layers beneath it.
How it is calculated
- Paver face area (m²) = length (cm) × width (cm) ÷ 10,000.
- Pavers = ⌈area ÷ paver face area × 1.05⌉ (5% for cuts and breakage).
- Bedding sand (t) = area × sand bed (cm) ÷ 100 × 1.6.
- Sub-base (t) = area × sub-base (cm) ÷ 100 × 2.1.
The factors 1.6 and 2.1 are typical densities in t/m³ for bedding sand and compacted crushed-stone sub-base.
The principle
A paved surface is built in layers. The sub-base of compacted crushed stone spreads loads and drains water; the thin sand layer gives an even bed into which the blocks are vibrated; the pavers form the wearing surface. Each layer's volume is area × thickness, and multiplying by density converts it to the weight that suppliers sell. The paver count is the area divided by one block's face area, with an allowance because edge blocks are cut and some pieces are wasted.
Worked example
Default inputs: area 15 m², pavers 20 × 10 cm, sand bed 3 cm, sub-base 10 cm.
- Paver face area = 20 × 10 ÷ 10,000 = 0.02 m² (50 per m²).
- Pavers = 15 ÷ 0.02 × 1.05 = 787.5, rounded up to 788.
- Bedding sand = 15 × 0.03 × 1.6 = 0.72 t.
- Sub-base = 15 × 0.10 × 2.1 = 3.15 t.
| Paver length (cm) | Pavers (+5%) |
|---|---|
| 5 | 3,150 |
| 10 | 1,575 |
| 15 | 1,050 |
| 20 | 788 |
| 25 | 630 |
| 30 | 525 |
| 40 | 394 |
How to read the result
About 10 cm of compacted sub-base and 3–5 cm of sand suit a garden patio or path with foot traffic. Driveways and soft or clay soils need a thicker sub-base, often 15–20 cm or more. Joints between blocks are usually filled with fine jointing sand, which is a separate, small quantity. If the layout has many curves or a herringbone pattern with angled edges, increase waste to 7–10%. Order sub-base by weight, but remember it compacts, so the loose quantity delivered must be slightly higher than the finished layer.
Common mistakes
- Using nominal sizes that include joints, or forgetting joint width with large slabs.
- Skipping the sub-base or making it too thin for vehicles.
- Ordering exact quantities when later batches may differ in colour.
- Ignoring the fall (slope) needed to drain water away from buildings.
Frequently asked questions
How many 20 × 10 cm block pavers per square metre?
50, before adding waste.
How thick should the sand bed be?
Usually about 3–5 cm, screeded level before laying, never used to correct large dips.
Can I lay pavers directly on soil?
Not recommended; without a compacted sub-base the paving will sink and rut.
Sources: BS 7533 — Pavements constructed with clay, natural stone or concrete pavers (series)