Floor Levelling Compound Calculator

Floor Levelling Guide

What's the key thing to understand about Coverage Rates?

Self-levelling compound coverage at 1mm depth: standard SLC (Ardex F6, Mapei Ultraplan): approximately 1.7-1.8 kg/m²/mm. Feather-edge compound: approximately 1.5-1.6 kg/m²/mm. Heavy-duty screed: approximately 1.6 kg/m²/mm. A 25kg bag at 3mm depth: 25/(1.7×3) = approximately 4.9 m². For 20m² at 3mm depth: 20 × 1.7 × 3 = 102 kg = 4.1 bags → buy 5 bags. Add 10% waste: 4.5 bags → buy 5 bags. Always check the manufacturer datasheet — coverage rates vary significantly between products.

What's the key thing to understand about Preparation is Critical?

Self-levelling compound requires meticulous preparation — failures are almost always caused by poor prep. Steps: remove all dust, grease, and contamination. Repair cracks and holes in the substrate (use floor repair compound, not filler). Vacuum thoroughly. Apply primer: SLC primer is essential for porous substrates (concrete, sand/cement screed, plywood) — prevents the substrate absorbing water from the SLC before it can set properly. Allow primer to dry completely (usually 1-3 hours). Prime low-absorbency substrates like existing tiles or sealed concrete more lightly, since over-priming can leave a slippery film that prevents the compound from bonding properly.

What do I need to know about Mixing and Pouring?

Mix to a fluid, self-smoothing consistency: add powder to water (not water to powder) to minimise lumps. Use a drill with paddle attachment — not a trowel. Mix for 3-4 minutes until completely smooth. Typical water ratio: 5.5-6 litres per 25kg bag. Pour immediately — most SLC has only 20-30 minutes working time. Pour along one wall and allow to flow. Assist flow with a spiked roller or smoother on thicker pours. Do not walk on it until it has set (typically 3-6 hours for foot traffic, 24 hours for heavier furniture or fitting flooring on top, though full cure for maximum strength can take considerably longer).

When to Use Levelling Compound?

SLC is appropriate for: creating a smooth, level surface for vinyl, LVT, or carpet. Covering old tile adhesive or minor undulations (up to 20mm in most cases). Re-levelling after removing old floor coverings. Feathering out slight humps or hollows. SLC is not appropriate for: structural repairs (use concrete repair mortar). Wet areas without waterproofing membrane (SLC is not waterproof). Areas with rising damp (treat damp first, then level). Depths over 50mm in one pour (apply in layers). If the floor has significant structural movement or cracking, address the underlying cause first, since levelling compound will simply crack again if applied over an unstable substrate.

What should I know about Self-Levelling Compound Thickness Limits?

Standard self-levelling compounds are typically rated for application between about 2mm and 10mm in a single pour; beyond that (commonly up to 50mm with 'deep fill' or fibre-reinforced products specifically formulated for thicker applications), a standard thin compound risks cracking as it dries and shrinks, because thicker pours trap more water that must slowly migrate out, creating internal stresses a thin-pour formula isn't designed to handle. For floors needing more than about 50mm of levelling in one area, it's usually more appropriate (and cheaper) to build up the level with a sand:cement screed first, then finish the top few millimetres with self-levelling compound for a smooth final surface, rather than trying to pour an extreme depth of pure self-levelling compound, which also gets significantly more expensive per square metre as depth increases since the material itself costs considerably more than screed sand and cement.

What do I need to know about Primer and Substrate Preparation?

Nearly all self-levelling compounds require the substrate to be primed first — typically a diluted PVA or a purpose-made acrylic primer — which seals a porous substrate (like old screed or concrete) to prevent it drawing water out of the compound too fast, which would cause poor curing, cracking, and weak adhesion. Skipping primer is one of the most common causes of levelling compound failure, alongside pouring over a substrate that's dusty, greasy, or still damp from a recent concrete pour (concrete needs proper curing time — often 28 days for a full structural cure, though levelling compound can sometimes go down sooner per manufacturer guidance for surface-only work). On timber subfloors, flexible or latex-modified compounds designed specifically for that purpose are needed, since standard rigid compounds can crack as timber flexes under load in a way solid concrete doesn't.

What do I need to know about Levelling Compound Over Underfloor Heating?

Self-levelling compound is commonly used both to encase electric UFH heating mats and to level a screed above water-based UFH pipework, but the product and application method differ from a standard levelling job. Compounds used over UFH need to be specifically rated for the purpose — look for 'suitable for underfloor heating' on the datasheet — because they must tolerate repeated heating and cooling cycles without cracking, which a standard compound isn't formulated to do. The heating system should be run for a 'commissioning' cycle and then turned off, allowing the compound to be installed at a stable, moderate temperature rather than onto either a cold slab or a hot one, since pouring onto an actively heated surface can cause the compound to dry unevenly or crack. After installation, the heating must stay off for the compound's full cure period (commonly at least 7 days, check the specific product) before being brought back up gradually in small temperature increments over several days, never switched straight to full output, which can cause rapid moisture loss and cracking. Depth over heating cables or pipes also needs checking against both the compound manufacturer's and UFH manufacturer's minimum cover requirements, which don't always match.

Worked Example: Calculating Bags Needed for a Room

Take a living room measuring 4.5m × 3.6m where the floor needs a 5mm average levelling pour before new flooring goes down.

This room needs roughly 8 bags of standard self-levelling compound for a 5mm average pour, though it's worth buying at least one extra bag beyond the calculated figure since real floors are rarely perfectly flat and low spots will consume more compound than high spots, meaning the "average" depth figure always understates the true quantity needed by a small margin. If the floor has more significant dips of 15-20mm in places rather than a consistent 5mm gap, those areas typically need a separate deeper fill first using a trowel-grade compound rather than trying to pour self-levelling compound thick enough to fill them in one go, since standard self-levelling products have a maximum single-pour depth (often around 10mm) beyond which they're prone to cracking or slumping.

Fibre-Reinforced vs Standard Compound

Standard self-levelling compound works well for straightforward sub-floor preparation ahead of flooring like carpet, laminate, or vinyl, but fibre-reinforced compounds have become the preferred choice for a growing number of jobs because the added fibres resist cracking under flex and impact considerably better than standard mixes. This matters most on timber sub-floors, which naturally flex slightly underfoot in a way solid concrete sub-floors don't — a standard compound poured over timber boards is more likely to develop hairline cracks over time as the boards move, while a fibre-reinforced compound bridges that movement far better. Fibre-reinforced products also tend to achieve a stronger final surface, making them a sensible choice ahead of hard flooring like tile or engineered wood where any flex or cracking in the sub-floor would transmit through to the finished surface. The trade-off is cost: fibre-reinforced compounds typically cost noticeably more per bag than standard formulations, which is why many tradespeople reserve them specifically for timber sub-floors or areas with known movement, while using cheaper standard compound on solid concrete slabs that don't flex. Checking the manufacturer's data sheet for maximum pour depth and recommended use over timber versus concrete before buying is worth the few minutes it takes, since using the wrong type on a flexing timber floor is one of the most common causes of levelling compound failure reported by flooring fitters.

Latex vs Cement-Based Levelling Compound

Two broad families of self-levelling compound dominate the DIY and trade market, and choosing between them affects both cost and suitability. Latex-based (or latex-modified) compounds use a polymer additive that improves flexibility and adhesion, making them a reliable all-rounder for most domestic sub-floors including timber, and they're generally the easier product for a DIY user to mix and pour successfully since they're more forgiving of minor technique errors. Standard cement-based compounds without the latex additive are typically cheaper and perform well on solid concrete sub-floors where flexibility isn't a major concern, but are more prone to cracking if used over timber or any sub-floor with movement, which is why many manufacturers specifically restrict standard cement-only formulations to solid, rigid sub-floors in their technical data sheets. Rapid-setting formulations within either family allow flooring to be laid sooner (sometimes within 2-4 hours rather than the 24-72 hours standard compounds need) at a price premium, which is useful on time-pressured renovation jobs but generally unnecessary extra cost on a straightforward DIY project with no tight deadline.