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Screed Depth Volume Calculator

Work out how much floor screed to order for your area, depth and wastage allowance.

How it is calculated

The tool takes floor area A in m², screed depth d in millimetres, bag size b in kg and a wastage allowance w in percent. Volume is V = A × (d ÷ 1000) × (1 + w ÷ 100) in m³, so the depth is converted to metres and the allowance added on top. Weight is V × 2100 kg, assuming a sand-cement screed density of 2100 kg/m³. Bags are ⌈weight ÷ b⌉, always rounded up to whole bags. Area, depth and bag size must be above zero, and the wastage field must be filled in: 0 is allowed, a negative value or a blank field shows no result. A depth above 500 mm is refused as a mistyped unit. A flowing screed or a different mix has another density, so weight and bag count would change.

Worked example

A 20 m² floor, screed 65 mm deep, 25 kg bags, 10 % wastage. Depth in metres: 65 ÷ 1000 = 0.065. Volume: 20 × 0.065 × 1.10 = 1.43 m³. Weight: 1.43 × 2100 = 3,003 kg. Bags: 3,003 ÷ 25 = 120.1, rounded up to 121. Result: 1.430 m³ of screed, 3,003 kg, 121 bags. Without the allowance it would be 1.300 m³ and 110 bags, so the 10 % adds 11 bags.

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Common mistakes

  • Entering the depth in centimetres or metres. The field is millimetres, so 6.5 for a 65 mm screed gives a layer a tenth as thick and far too little material; a value such as 650 (mm mistyped from cm) is above 500 and shows no result.
  • Leaving the wastage field empty. The tool needs a number there, and 0 means no allowance; it does not fall back to a default.
  • Using the bag size of a different product. The bag count depends directly on the kilograms per bag, so a 40 kg bag gives far fewer bags than a 25 kg one.
  • Measuring a sloping or uneven floor at its thinnest point. Use the average depth, or the volume will be short in the low spots.
  • Treating the bag count as exact. The weight uses a fixed 2100 kg/m³ for the mix, so check the manufacturer's stated yield per bag before ordering.

About this calculator

Screed volume follows the same simple prism formula as concrete — floor area × screed depth — but the calculation typically adds a wastage allowance, commonly 5–10%, to cover spillage, uneven subfloor levels and trimming, since running short mid-pour on a screed job is far more disruptive than on most other building materials. Multiplying the resulting volume by the screed mix's density (commonly 2,000–2,100 kg/m³ for traditional sand-cement screed, though flowing/liquid screeds differ; this calculator uses 2,100 kg/m³) gives the total weight, which is then divided by the weight of one bag to get the bag count.

Builders and flooring contractors run this calculation before ordering because screed is typically laid in a single continuous session for a given area — mixing a new batch to finish an area that ran short can create a visible joint or weak point — and because bagged screed materials are a meaningful cost that's worth getting right rather than over-ordering by a wide margin.

Setting the wastage allowance to 0 removes it from the bag count altogether.

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