Calculating Quantities

If you need help deciding how much of your landscaping material to use, we will happily estimate quantities for you. All we need to know are the measurements and maybe a rough sketch and we can work out the number of slabs or walling blocks required plus the volume of aggregates needed to lay them.

Some useful guides on helping you calculate your own quantities are outlined below.

How Many Paving Slabs Do I Need?

Decide on what size slabs and what laying pattern you are going to use.

For popular slab sizes, the list below gives the number of slabs needed per m² (approx.)

  • 1200 x 600mm = 1.4 slabs
  • 1200 x 300mm = 2.8 slabs
  • 900 x 600mm = 1.8 slabs
  • 600 x 600mm = 2.7 slabs
  • 600 x 450mm = 3.6 slabs
  • 600 x 400mm = 4.1 slabs
  • 600 x 300mm = 5.4 slabs
  • 450 x 450mm = 4.7 slabs
  • 450 x 300mm = 7.0 slabs
  • 300 x 300mm = 10.4 slabs

Note that some slabs that have a wider/narrower joint width then less/more slabs maybe required.

If using more than one size you can calculate the number of each size slab as follows:

  • 3 Contempo sizes; 900 x 600, 600 x 600, 600 x 400mm = Divide the m²by 1.14
  • 3 sizes; 600 x 600, 600 x 300, 300 x 300mm = Divide the m² by 0.65
  • 3 sizes; 600 x 450, 450 x 450, 450 x 300mm = Divide the m² by 0.63
  • 4 sizes; 900 x 600, 600 x 600, 600 x 300, 300 x 300mm = Divide the m² by 1.1
  • 6 sizes; 600 x 600, 600 x 450, 600 x 300, 450 x 450, 450 x 300, 300 x 300mm = Divide the m² by 1.23

If you are not using these common size combinations, you can calculate how many of each size slab you need as per the example below:
20m² patio using just 600 x 600 and 600 x 300mm sizes
600 x 600 (2.7 slabs per m²), so 1 slab will cover 1 ÷ 2.7 = 0.37m²
600 x 300 (5.4 slabs per m²), so 1 slab will cover 1 ÷ 5.4 = 0.185m²
1 of each size slab will cover 0.37m² + 0.185m² = 0.555m²
20m² patio divided by 0.555m² = 36 of each of the two sizes.

If you need to work to a plan where you need a different number of each size slab, your best bet is to sketch the design out on graph paper.

Experienced landscapers should be able to work out a random pattern using a roughly equal quantity of each size.

How Much Aggregate Do I Need?

To calculate the volume of aggregates, multiply width x length x depth of the area you are covering


Area is 5m x 4m = 20m²
To cover a depth of 100mm = 20m² x 0.1m
Total Required = 2 cubic metres.

A 50mm covering is generally recommended for shingle/gravel covering or a mortar bed.
A 100mm to 150mm is generally recommended for a sub-base.

A bulk bag (approx 900kg or 0.56 cubic metres) tends to cover between 10-14 square metres at a depth of 50mm
A poly bag (approx 20-25kg or 0.015 cubic metres) tends to cover between 0.4-0.5 square metres at a depth of 50mm

How Much Cement Do I Need

The amount of cement you need of course requires on the strength mix.

The ratio required, for example 3:1 would mean 3 parts aggregate to 1 part cement. So to make a tonne of mix at a 3:1 ratio, you would require 750kg of aggregate and 250kg of cement (10 bags). To make a tonne of mix using a 6:1 ratio, you would require 850kg of aggregate and 150kg of cement (6 bags).

Bricklaying: A ratio of 3:1 using building (soft) sand is normally used for underground brickwork, or 5:1 for brickwork above ground.

Pointing: A ratio of 3:1 is normally used with building (soft) sand.

Concrete: A ratio of 5:1 is normally used for domestic concreting, using ballast.

Laying Slabs: A ratio of 3:1 for heavy vehicular use, 6:1 for medium/domestic use or 10:1 for domestic light duty, using sharp sand.