How the volume and weight are worked out
The area is treated as a rectangle: length multiplied by width multiplied by depth, with the depth converted to the same unit as the length first. That gives the volume in cubic metres.
Weight is the volume multiplied by the density, which for screed is 2,000 kg per cubic metre. Loose material is less dense than the same material compacted, so this figure matches what a supplier loads rather than what it becomes in place.
The bag or bulk bag count is the weight divided by the pack size and rounded up, because you cannot buy part of a bag.
Worked example
A floor 4 metres by 3 metres at 50 mm is 0.60 cubic metres. At 2,000 kg per cubic metre that is 1.20 tonnes, which is 48 bags of 25 kg.
Change any input and all three outputs move together. The count always rounds up, so a job that needs 2.1 bulk bags is quoted as 3.
Bonded, unbonded and floating screeds
A bonded screed goes onto a prepared concrete base at 25 to 40 mm. An unbonded screed sits on a membrane at 50 mm or more, and a floating screed over insulation needs 65 to 75 mm to carry the load without cracking.
The thickness you enter should be the finished thickness. Screed does not compact like asphalt, so what you lay is close to what you get, but it does shrink slightly as it cures.
Bag counts above about 40 are worth pricing as a delivered bulk mix. On a large floor the bags alone take longer to handle than the screed takes to lay.
Limits of this calculator
It assumes a flat rectangular area at one depth. It does not account for a slope, a curved edge or a trench, all of which need the area measured separately.
It uses a typical loose density of 2,000 kg per cubic metre. Suppliers vary, and damp material weighs more, so treat the weight as an estimate and the volume as the figure to order on.
It does not include a price, delivery, a crane charge or waste from spillage. Add your own percentage if the site is awkward.
For a decorative surface layer rather than a base, use the UK gravel calculator.
Inputs for the screed calculator
Length
The long side of the area, in metres.
Width
The short side of the same area, in metres.
Depth
The layer thickness in millimetres, which is the finished depth, not the loose depth.
CALCULATOR QUESTIONS
UK screed calculator questions
How much does a cubic metre of screed weigh?
About 2,000 kg per cubic metre. The exact figure depends on moisture and grading, which is why suppliers quote a range.
Should I order extra?
Add 5 to 10 per cent for an irregular shape or an uneven base. For a simple rectangle at a constant depth, the calculated figure is enough.
Is the depth the loose or the finished depth?
Enter the finished depth. If the material compacts, the loose layer you lay will be thicker than the number you enter.
Does this work for a levelling a floor before the finish goes down?
It works for any flat area. For a levelling a floor before the finish goes down it gives the quantity; it does not specify the material grade, which your supplier should confirm.