How this self-levelling calculator works
Floor levelling compounds are sold by weight, but the amount you use depends on area × depth. Almost every UK brand (Mapei Ultraplan, Bostik Screedmaster, Dunlop, Setcrete, Instarmac) uses between 1.5 and 1.7kg of powder per m² per mm of thickness. The calculator uses this formula:
For example, a 4.5m × 3.6m kitchen (16.2m²) at 5mm with a fibre-reinforced compound needs 16.2 × 5 × 1.7 = 137.7kg, plus 5% wastage = 144.6kg, or 8 × 20kg bags.
Coverage per bag at common depths
| Depth | 20kg bag covers | 25kg bag covers | Bags for 15m² (20kg) |
|---|---|---|---|
| 3mm | 3.9 m² | 4.9 m² | 4 |
| 5mm | 2.35 m² | 2.9 m² | 7 |
| 10mm | 1.18 m² | 1.47 m² | 13 |
| 20mm | 0.59 m² | 0.74 m² | 26 |
These figures are based on 1.7kg/m²/mm with no wastage. Always add 5%–10% for low spots you didn't measure. Running out mid-pour leaves a visible joint that you'll need to sand or skim over.
Priming the subfloor
- Concrete and sand/cement screed: vacuum thoroughly, then apply SBR or acrylic primer diluted about 3:1 with water, followed by a second coat diluted 1:1. This stops the floor sucking water out of the compound before it levels.
- Plywood or timber: screw boards down every 150mm, prime with neat primer and use a fibre-reinforced compound at a minimum of 5mm.
- Ceramic tiles: degrease thoroughly and use a gritted bonding primer so the compound has a mechanical key.
- Bitumen or old adhesive: scrape off loose ridges and use a primer approved for bitumen before pouring.
Fix a foam perimeter strip around the walls before pouring. It lets the floor move and stops the compound bonding to skirting or plaster. Seal any gaps and doorways with tape or a timber batten so the liquid compound can't escape.
Mixing and pouring tips
- Have all bags in the room and two buckets ready. Work fast: most compounds have 15–30 minutes of working time.
- Add powder to water (not water to powder), mix with a paddle mixer for 2–3 minutes and pour immediately.
- Work in strips away from the door and use a spiked roller to release air and blend each mix into the last.
- Keep the room free of draughts and direct heat (including underfloor heating) while it cures.
Measuring the average depth
The depth you enter makes the biggest difference to the number of bags. Lay a long straight edge (2m if you have one) across the floor in a grid, every metre or so in both directions, and measure the gap under it at each point. Add up the gaps and divide by the number of readings for the average, then add the minimum depth the compound needs over the high spots (often 2–3mm). A floor that "looks flat" can easily average 5–8mm once measured this way.
When not to use levelling compound
- Bouncy floors: compound cracks on floors that flex. Fix loose boards and overboard with plywood first.
- Damp floors: rising damp or a missing damp-proof membrane needs sorting first. Compound won't stop moisture.
- Very deep fills: for more than about 20–30mm over large areas, a sand and cement screed is usually cheaper. Compare with the screed calculator.
Common mistakes
- Running out mid-pour. Have every bag in the room before you start. The 5% wastage allowance is there for a reason.
- Too much water. It flows nicely but leaves a weak, dusty surface. Measure the water every time.
- Skipping the primer. Unprimed concrete sucks the water out and causes pinholes and poor bonding.
Our self-levelling compound guide goes into priming and coverage in more detail. Once it's down, plan the finish with the flooring or tile calculator.