BuildCalcs Screed Calculator
Estimate screed volume and the number of premixed bags required for a floor.
How the screed calculation works
Floor length × width × screed thickness gives the wet volume. The waste allowance is added and, if a bag yield is entered, the volume is divided by that yield and rounded up.
Screed thickness
Required thickness depends on the screed type, whether it is bonded, unbonded or floating, the substrate and the project specification.
Frequently asked questions
Why does bag yield matter?
Different premixed screeds produce different wet volumes per bag, so product-specific yield is needed for a reliable bag count.
Can I use this for an uneven floor?
For variable thicknesses, calculate using a realistic average depth or split the floor into areas with different depths.
Related floor-base calculator
If your screed sits above a new concrete slab or base, calculate that separately with the Concrete Calculator UK.
Why screed thickness matters
Screed volume is floor area multiplied by finished thickness, so even a small change in depth can have a large effect across a whole room. For example, 20 m² at 50 mm is 1.0 m³ before waste. If the existing base is uneven, a single nominal thickness may underestimate material; survey levels or divide the floor into zones where practical.
Use the premix bag yield
The bag count uses the wet litres produced by one bag. This figure varies between proprietary screeds and should come from the product data sheet. Do not substitute the dry bag volume or assume that two products of the same bag weight have the same yield.
Quantity is separate from screed specification
The calculator does not decide whether a screed should be bonded, unbonded, floating or suitable over underfloor heating, nor does it set minimum thicknesses. Those decisions depend on the system and substrate. Once the required build-up is known, enter the correct finished thickness and compare the calculated quantity with supplier guidance, allowing for practical mixing and placement losses.