How to use it
- Pick the material: bagged screed, site-mixed sand and cement, flowing anhydrite or self-leveling compound.
- Enter the floor area and thickness; for a leveler, the thinnest and thickest point.
- Tick radiant floor heating if there are pipes underneath.
- Check the consumption and waste from the bag, then read the result and export it.
What this screed calculator covers
A screed calculator has to handle more than one product. This one covers four:
- bagged sand and cement screed or topping mix: add water and go,
- site-mixed screed from portland cement and sand (the UK-style traditional screed),
- flowing anhydrite (calcium sulfate) screed, ordered by volume,
- self-leveling concrete or underlayment, for thin layers of varying depth. Use it as a floor leveler calculator.
You get pounds, bags or cubic yards, plus the perimeter isolation strip along the walls, the polyethylene sheet underneath and a rule-of-thumb drying time. With radiant heating ticked, the tool warns you if the screed is thinner than 45 mm (about 1-3/4 in) over the pipes.
US units are the default; one click switches to metric. Consumption values, allowances and unit prices are editable, the result comes with a drawing, and you can export it to PDF or Excel for free. No sign-up; everything runs in your browser.
Formula and worked example: 200 sq ft at 2 in
Formula:
- Volume = area × thickness
- Material (lb) = area × thickness (in) × consumption (lb per sq ft per inch) × (1 + waste %)
- Bags = material ÷ bag weight, rounded up
Example: a 200 sq ft floor, 2 in of sand-cement screed, 10.4 lb per sq ft per inch from the bag (2.0 kg/m² per mm), 80 lb bags and 5% waste.
- Volume: 200 × 2 ÷ 12 = 33.3 cu ft = 1.23 cu yd (0.94 m³)
- Material: 200 × 2 × 10.4 × 1.05 ≈ 4,370 lb (1,982 kg)
- Bags: 4,370 ÷ 80 = 54.6 → 55 bags
- Drying: 2 in is about 5 cm → about 6 weeks
For flowing anhydrite, the same floor is an order of 1.23 cu yd, or about 1.3 cu yd with the allowance. For a site mix, the tool turns the volume into cement, sand and water. At 350 kg of cement per m³ with four times its weight in sand, one cubic yard takes about 590 lb of cement (6.3 bags of 94 lb), 2,360 lb of sand and 35 gallons of water.
Self-leveling concrete: average of min and max depth
An old floor is rarely flat, so a self-leveling concrete calculator needs the thinnest and thickest point. The tool uses their average.
Example: 120 sq ft, depth from 1/8 in to 1/2 in, so 5/16 in on average. The bag says 8.32 lb per sq ft per inch (1.6 kg/m² per mm), 50 lb bags, 5% waste:
- Material: 120 × 0.3125 × 8.32 × 1.05 ≈ 328 lb
- Bags: 328 ÷ 50 = 6.6 → 7 bags
If the floor has one deep low spot rather than an even slope, fill it first with patching compound or calculate it separately, otherwise the average underestimates. Prime the substrate as the manufacturer specifies, and stay within the minimum and maximum depth printed on the bag.
Drying time and radiant heating
The calculator uses a rule of thumb at about 68 °F (20 °C) and 65% relative humidity:
| Material | Up to 4 cm (about 1-1/2 in) | Beyond 4 cm | Example |
|---|---|---|---|
| Cement screed | 1 week per cm | 2 weeks per cm | 2 in → about 6 weeks |
| Anhydrite screed | 1 week per cm | 1.5 weeks per cm | 50 mm → 5.5 weeks |
Thick screeds dry disproportionately slower, and cold, damp or unventilated rooms slow everything down. Test the moisture content before laying wood, vinyl or tile; the flooring manufacturer sets the limit.
Over radiant floor heating, cement screed needs at least 45 mm of cover above the pipes, and the system should be brought up to temperature gradually following the installer's start-up procedure.
Common mistakes
- No edge strip. The screed must be separated from walls and columns or it can crack. The tool sizes the strip from the room perimeter.
- No plastic sheet over insulation. A floating screed needs a polyethylene separation layer, lapped at the joints.
- Using the planned thickness on an uneven slab. Take levels at several points and use the real average.
- Too thin over heating pipes. Check the cover, not just the total thickness.
- Laying flooring too early. Moisture trapped under flooring causes cupping and adhesive failure.
For thicker slabs use the concrete calculator; for the finish floor, the tile calculator.
Frequently asked questions
How much self-leveling concrete do I need?
Area × average depth × the bag's consumption. For 120 sq ft at an average 5/16 in, at 8.32 lb per sq ft per inch plus 5%, that's about 328 lb, or 7 bags of 50 lb.
How many bags of screed per square foot?
At 10.4 lb per sq ft per inch, a 2 in screed needs about 21 lb per sq ft, so one 80 lb bag covers roughly 3.8 sq ft before waste.
How long does screed take to dry?
As a rule of thumb, cement screed takes about one week per cm up to 4 cm and two weeks per cm beyond that, so 2 in (5 cm) takes about 6 weeks.
How thick should screed be over underfloor heating?
Cement screed needs at least 45 mm (about 1-3/4 in) above the top of the pipes. The calculator flags thinner layers.
Can I use metric units?
Yes. Switch to metric with one click and the calculator works in m², mm, kg and m³.
Last updated: November 13, 2026