Screed volume:
how much mix a floor of a given depth takes

A screed layer is a thin box

Screed is the smooth, level layer of sand and cement, or a liquid alternative, that goes over a structural slab or insulation before the final floor finish. However complicated the building, the quantity is simply a flat box: the area of the floor multiplied by the depth of the layer.

That makes the sum quick, but quick sums hide unit slips. Area is normally in square metres and thickness in millimetres. If you multiply 70 by 50 you get 3,500, which is neither cubic metres nor anything useful. Convert 50 mm to 0.05 m first, then multiply.

Plan the order in cubic metres because that is how ready-mixed and pumped screed is sold. Bagged screed mixes are sold by weight, so you will convert the volume to tonnes using the mix density that the manufacturer states.

The formula and a worked figure

Thinking in litres can be useful for small areas. A single square metre at 50 mm takes 50 litres, so every square metre of floor at that depth swallows exactly 50 L of mix.

Dealing with several rooms and different depths

A whole-house screed rarely has one thickness. A kitchen may have underfloor heating and need 65 mm, while a hallway gets by with 50 mm and a thin threshold area is ramped between them. The simplest route is to list each zone separately, calculate its own volume, and then add the totals.

The weighted average thickness is 3.84 ÷ 70 = 0.0549 m, so the total is almost the same as the single 50 mm figure, but it is distributed very differently. Ordering by zone also helps the crew plan the sequence of pours and the position of day joints.

Why the thickness assumption matters most

Doubling the depth doubles the volume, and thickness is the number most likely to be guessed. A bonded screed on a prepared slab can be thin, while an unbonded or floating screed over insulation or underfloor heating pipes needs to be thicker to stay strong. The mix designer or manufacturer sets the minimum, so use their figure instead of a rule of thumb.

The table shows how fast the quantity grows. Going from 50 mm to 75 mm adds 1.75 m³, which is half as much again, and that jump is a significant extra load on both the structure and the budget.

Allowances for waste and uneven slabs

The base calculation assumes a perfectly flat slab and perfect filling. Real structural floors are not flat, and a screed is laid to a level datum, so low spots take more material. Some mix is also left in the mixer, the pump line and the barrow, and some is lost at the edges.

• A slab that is out of level by 10 mm over a room adds 10 mm of average depth to the part that is low.
• Door thresholds and level changes between rooms change the depth locally.
• Underfloor heating pipes are covered by a minimum depth above the pipe, which adds to the base thickness.
• Waste is higher on a small job done in several mixes than on one continuous pour.

From cubic metres to bags and tonnes

A supplier delivering ready-mixed screed will quote in cubic metres, but a DIY job using bagged material is priced by weight. The link between the two is the density of the mix, which depends on the sand, the cement ratio and the water content. Use the number printed on the product data sheet, since two mixes can differ by several hundred kilograms per cubic metre.

Three hundred bags is a heavy carry and a long mixing day, which is why larger floors are normally pumped. The same number also tells you whether a floor structure can take the temporary load of stacked bags before they are mixed.

• Check the stated yield per bag, which already accounts for the water added.
• Round up to whole bags, then add a few extra for a mixing mistake.
• Stack bags near a wall or beam, not in the middle of a suspended floor span.
• Store them dry; damp bags cure in the bag and are useless.

Checks before you order

Because the calculation is so short, almost every error comes from an input. Measure the area to the actual screeded surface, deducting service ducts and large openings. Measure the slab levels with a level, not by eye, so the thickness reflects the finished datum rather than the plan depth.

A concrete volume calculator uses exactly the same length × width × depth principle, so it is a convenient cross-check when you want to test different thicknesses quickly. Remember that curing time and drying time depend on the depth and mix, and these are set by the screed product, not by the quantity.