Whether you're preparing an uneven concrete subfloor for luxury vinyl tile (LVT), engineered wood or large-format porcelain, ordering the right number of 20kg or 25kg self-levelling compound bags is critical. Running short mid-pour leaves a visible cold joint across the room, and pouring without proper priming can make the compound lift and crack.
Use the self-levelling compound calculator to get the exact number of bags for your room.
1. Self-levelling compound coverage per bag
Most UK trade levelling compounds (including Mapei Ultraplan, Bostik Screedmaster, Dunlop and Setcrete) use 1.6kg to 1.7kg of powder per m² per millimetre of depth.
| Pour depth | 20kg bag covers | 25kg bag covers | 20kg bags for 15m² |
|---|---|---|---|
| 3mm (feather skim) | ~3.9 m² | ~4.9 m² | 4 (+1 spare) |
| 5mm (standard LVT / tile) | ~2.35 m² | ~2.94 m² | 7 |
| 10mm (uneven concrete) | ~1.18 m² | ~1.47 m² | 13 |
| 20mm (deep fill / UFH) | ~0.59 m² | ~0.74 m² | 26 |
2. How to prime your subfloor (and why not PVA)
A common DIY mistake is priming concrete floors with woodworking PVA. PVA is water-soluble, so pouring wet compound onto it re-emulsifies the glue. That can stop the compound bonding, leaving it hollow-sounding and prone to cracking within weeks.
- Dusty concrete and sand/cement screeds: vacuum thoroughly, then apply a first coat of SBR or acrylic primer diluted about 3 parts water to 1 part primer. Once dry, apply a second coat diluted 1:1. This seals the pores so the concrete doesn't suck water out of the compound before it levels.
- Overboarded plywood and timber floors: always use a fibre-reinforced flexible compound at a minimum of 5mm. Prime with neat acrylic or SBR primer and screw boards down every 150mm.
- Existing ceramic tiles and old bitumen adhesive: degrease tiles with sugar soap and apply a gritted bonding primer (or a slurry of neat SBR and levelling powder) to create a mechanical key.
3. Preventing plasterboard joint and ceiling cracks
When drylining stud walls or ceilings with 2400 × 1200mm (12.5mm) tapered-edge plasterboard, hairline cracks along the joints are usually caused by timber shrinkage, movement, or weak self-adhesive mesh tape on boards that won't be skimmed.
If you're taping and jointing (rather than applying a full 2mm skim coat), bed pre-creased paper joint tape into wet bedding compound using a 6-inch taping knife, then feather out with 10-inch and 12-inch knives. Paper tape is much stronger across the joint than fibreglass mesh, so it resists cracking from movement.
Planning a drylining job? Work out sheets, screws and tape or calculate skim plaster bags.
4. Measuring depth and choosing a compound
Most people underestimate the depth. Lay a long straight edge across the floor in a grid, roughly every metre in both directions, and measure the gap under it at each point. Average the readings, then add the minimum the compound needs over the high spots (often 2–3mm).
- Standard compounds suit sound concrete and screed at about 2–10mm.
- Fibre-reinforced, flexible compounds are for plywood, timber and floors with underfloor heating, usually at 5mm or more.
- Deep-base compounds fill hollows of 20mm or more in one go, but for large deep areas a sand and cement screed is usually cheaper. Compare with the screed calculator.
5. Worked example: a 4.5m × 3.6m kitchen
The floor is 16.2m². The straight-edge readings average 4mm, so allow 5mm. With a fibre-reinforced compound at 1.7kg per m² per mm: 16.2 × 5 × 1.7 = 137.7kg, plus 5% = 145kg, so 8 × 20kg bags. You'll also need about 34 litres of water (measure it for each bag), roughly 3 litres of primer for two coats, and 16m of foam edge strip. That's eight mixes, so two people (one mixing, one pouring) make the job far easier.
6. Pouring, step by step
- Prepare: remove old adhesive ridges, vacuum, fill deep holes and cracks, and prime as above. Let the primer dry.
- Edge and dam: fit foam strip round the walls and seal doorways and gaps with a timber batten or tape so the compound can't run away.
- Set up: have every bag in the room, two buckets, measured water and a paddle mixer ready. The working time is short.
- Mix and pour: add powder to water, mix for 2–3 minutes and pour straight away, starting at the far wall and working towards the door.
- Spread and blend: ease it out with a trowel or gauge rake and run a spiked roller over it to release air and blend each mix into the last.
- Cure: keep draughts, direct sun and heating off while it sets. Don't walk on it until the bag says you can.
7. Fixing cracks in plasterboard you already have
Fine cracks along a joint can often be fixed by cutting out the old tape, re-bedding paper tape in fresh jointing compound and feathering it out. "Screw pops", round bumps where a screw has pushed through the surface, are fixed by driving a new screw just beside the old one, then filling both. Cracks that keep coming back across ceilings may point to movement in the floor above or loose boards. Add screws into the joists before you re-tape. Long cracks that grow wider, especially diagonal ones from the corners of doors and windows, can be a sign of structural movement and should be checked by a professional.
For flooring on top of the new level surface, see the flooring and tile calculators.