Floor leveling screed volume:
formula and worked example

What a levelling screed volume has to account for

A levelling screed is a layer of cement-sand or similar mix laid over a rough or uneven base to create a flat surface for flooring. It is ordered by volume, since a mixer or a delivery truck deals in cubic metres, and the quantity decides how many materials and labour days are needed.

The area is straightforward to measure. The depth is not, because the whole reason for a levelling screed is that the base is not level. The depth input must therefore represent the average thickness actually laid.

The exercise also tells you how much weight is going onto the slab. A screed adds load per square metre equal to its thickness times its density, which for a thick levelling layer can be material for an older or lightly designed floor. If the structure is in question, ask a structural engineer before ordering several cubic metres of mix.

The formula and its inputs

• Area: the floor area to be screeded in square metres, net of columns, built-in fittings and anywhere the screed stops.
• Depth: the average finished thickness in metres, so 50 mm is entered as 0.05.

If the thickness varies in clear zones, work each zone separately and add the volumes rather than averaging by eye.

Worked example for 75 m² at 50 mm

A room set measuring 75 m² needs a screed averaging 50 mm. Illustrative values.

1. 1Convert depth to metres: 50 mm = 0.05 m.
2. 2Multiply by the area: 75 × 0.05 = 3.75 m³.
3. 3If you add a 5% allowance for waste and thickness variation, 3.75 × 1.05 ≈ 3.94 m³.

From finished volume to materials

The result is a wet, placed volume. Dry ingredients occupy more space than the finished mix, because the water and the mixing fill voids between sand grains, so material quantities are calculated using the mix's yield factor or the supplier's coverage tables. For a cement-sand mix, that means calculating cement bags and sand tonnage from the volume and the mix ratio.

Also consider the screed's own limits. Bonded, unbonded and floating screeds each have a minimum practical thickness, and these depend on the mix and the specification. If a floor survey shows depths below that minimum in places, the answer is to raise the base or change the system, not to use thinner screed than is permitted.

Falls change the maths. A wet-room floor needs a gentle slope to the drain, so the screed is thinner at the high point and thicker at the low point. The average thickness is then roughly the mean of the two extremes. Calculate the volume with that mean over the sloped area, and handle the flat areas separately.

How area growth scales the order

Volume is linear in area when depth is held constant.

A 10% larger floor increases the order from 3.75 to 4.125 m³, which is 0.375 m³ more. Depth works the same way, but a change of 5 mm on a 50 mm screed is also 10%, and that is easily within the variation that a rough slab produces, so depth needs more attention than area.

Where screed estimates go wrong

• Taking the design depth as the average when the slab actually dips or falls, so the real average is higher.
• Entering depth in millimetres, which produces a volume a thousand times too big.
• Not allowing for the extra thickness at falls to drains or in wet rooms.
• Treating the finished volume as the dry material quantity.

Before the screed arrives

1. 1Set the finished floor level with datum marks around the room.
2. 2Survey the base and calculate average depth by zone.
3. 3Compute volume, add a chosen allowance and convert to material quantities.
4. 4Plan the pour in bays so the screed can be compacted and finished before it stiffens.

Curing needs planning too, since a screed that dries too fast can crack and curl.

Mixing capacity limits what can be laid in one go. A screed has a limited working life after mixing, so a large floor is divided into bays that the team can place, compact and finish before the mix stiffens. Share the 3.75 m³ across bays, and check the mixer can supply each bay at the rate the finishers work.