Screed Calculator

Start with your floor area: multiply the length by the width to get the square metres you need to cover. If your room isn't a simple rectangle, split it into rectangles, measure each one, and add the areas together. The same area drives a concrete calculator if the structural slab below the screed still needs pouring.

Next, multiply the area by the finished screed thickness (in metres, so a 70 mm pour is 0.07 m). That gives you the volume in cubic metres. Add your waste percentage on top to cover spillage, uneven substrate, and slight over-pours. Five percent is typical for a flat, bounded slab.

Converting volume to bags depends on the bag yield. A 25 kg bag of traditional sand-cement pre-mix yields around 0.012 m3 once mixed, so one bag covers about 1.2 m2 at 10 mm. A 40 kg bag scales to around 0.019 m3. A self-levelling 25 kg bag covers roughly 6 m2 at 3 mm. Mass uses screed density: 2000 kg per m3 for sand-cement, 2100 kg per m3 for anhydrite.

Use a tape measure and record the length and width in metres. Measure at several points along each wall because buildings are rarely square. Use the largest figure for each dimension so you don't come up short.

For L-shaped rooms or rooms with alcoves, break the floor into rectangles, measure each separately, and add the areas together. If you've got underfloor heating pipes already clipped to the insulation, measure the finished screed depth from the top of the pipe, not from the insulation.

Check the floor for high spots before you order. A screed evens out small dips, but a substrate that varies by 20 mm across the room means more material than the simple multiplication suggests. Walk the floor with a straight edge and note the low points.

Write the door-threshold heights on the same sketch, because the finished screed must sit flush with adjoining floors rather than ride proud of them. For underfloor heating, the minimum cover above the pipe varies from 30 to 45 millimetres depending on screed type; this figure drives the final thickness and therefore how much screed you actually need to order. A reliable screed calculator works best with all of these numbers, so jot the room plan, the cover depth and the perimeter expansion strip onto one piece of paper before you head to the merchant. In refurbishment work on timber joists, also check the structural load: a wet screed weighs roughly 40 to 50 kilograms per square metre once poured.

Under-ordering is the big one. A screed pour has to go down in a single session, so running short at 90% coverage leaves you with a cold joint that weakens the finished floor. A 5% to 10% waste margin is a cheap insurance policy.

Forgetting the underfloor heating minimum cover catches people out. DIN 18560 says you need 45 mm of sand-cement or 35 mm of anhydrite above the top of the pipe. On a 16 mm pipe that's a total pour depth of 61 mm or 51 mm respectively, not the nominal screed thickness you see in product brochures.

Pouring too thin causes cracking. A traditional screed needs at least 50 mm unbonded or 65 mm floating. An anhydrite liquid screed needs 30 to 35 mm minimum. Don't drop below those numbers to save a bag or two.

Finally, drying time is slower than people expect. A 70 mm traditional pour takes around 90 days under typical indoor conditions. Plan your tiling or timber install around that, not around the day the pour hardens. The same area then drives a tile calculator or a flooring calculator for the finished surface above the screed.