Paving slab calculator: slabs, MOT Type 1, sand and cement

How it works

Enter the patio area directly or enter its length and width. Select one-size 600 x 600 mm or 600 x 900 mm paving, or the sourced four-size sandstone project pack. Add the compacted sub-base depth, mortar-bed depth, a waste allowance, and the joint width. The result rounds whole slabs, project packs, bags and tubs up so the list contains full items to buy.

MOT Type 1 estimates use one named UK merchant’s stated approximate coverage and its 850 kg bulk-bag mass. Sharp-sand and cement estimates use the published bedding-mortar table for the selected 1 : 4 mix. Jointing compound quantities use the maker’s coverage table for the selected slab pattern and joint dimensions. Local products, moisture, ground conditions, cuts and installation methods change actual quantities, so confirm the delivered products and follow their instructions.

Formula

• Count individual slabs or project packs

  Order area = patio area × (1 + waste %) Slabs = ceil(order area ÷ ((slab length + joint) × (slab width + joint))) Project packs = ceil(order area ÷ pack coverage)

  Slab dimensions and pack coverage come from the selected product data.

• Estimate the bedding mortar

  Bedding volume = patio area × bed depth Cement bags = ceil(volume × 8 bags per m³) Sharp sand = volume × 1,000 kg per m³

  These rates use the cited 1 : 4 cement-to-sharp-sand bedding mix. The calculator rounds quantities up to whole packs.

• Estimate jointing compound and sub-base

  Joint tubs = ceil(order area ÷ maker coverage for your slab and joints) MOT bags = ceil(required volume ÷ merchant coverage at the chosen depth) Tonnes = whole MOT bags × 850 kg ÷ 1,000

  The MOT estimate uses the named merchant’s approximate coverage for its 850 kg bulk bag.

Worked example

1. 1

   Estimate the slabs

   ceil(20 m² ÷ (0.61 m × 0.61 m)) = 54 slabs

   The example uses 600 mm slabs, 10 mm joints and no added waste.

2. 2

   Estimate the sub-base

   ceil(20 m² ÷ 5 m² per bag) = 4 bulk bags

   At 850 kg each, the whole bags weigh about 3.4 tonnes in total.

3. 3

   Estimate the mortar bed

   20 m² × 0.04 m = 0.8 m³ 7 cement bags · 800 kg sharp sand

   This uses a 40 mm bed and the cited 1 : 4 bedding mix. The sharp sand fits in one approximate 850 kg bulk bag.

4. 4

   Estimate the jointing tubs

   Maker coverage for these slabs and joints → 2 × 15 kg tubs

   The selected FPM coverage table supplies the tub count, follow the maker’s joint-width and depth guidance.

5. 5

   Compare the four-size project pack

   2 project packs for the same 20 m² patio

   Each pack contains 48 slabs across four sizes. This option is counted by the pack’s stated area coverage.

Assumptions and limits

The single-size slab estimate adds the chosen joint width to both slab dimensions to estimate the repeating covered area. A real patio layout may require cuts, perimeter trimming or a different starting pattern, add a waste allowance that reflects your layout. The project pack uses its merchant-stated pack coverage and fixed composition. It is one sourced sandstone product example, not a universal mix pack.

The MOT Type 1 source lists approx. 5 m² at 100 mm and an 850 kg bulk bag. It separately states approx. 2 tonnes per m³, that density claim conflicts with the coverage and bag-weight figures, which imply about 1.7 tonnes per m³. To keep this result internally consistent, the calculator uses the stated coverage to count whole bags and derives tonnes from those whole bags x 850 kg. Treat both outputs as approximate product-specific estimates and confirm them with your supplier.

Bedding quantities assume the cited 1 : 4 cement-to-sharp-sand bedding mix and its approximate ingredient table. They do not include extra mortar for an uneven base, bedding variations, priming slurry, or a separate jointing mortar. The 15 kg jointing compound is one maker’s product example, check that the maker accepts your paving material and joint dimensions before use. Its published coverage is approximate.

Frequently asked questions