Self-Leveling Compound Calculator

How the self-leveling compound calculator works

Self-levelling compound, also called floor leveller or self-leveling underlayment, is poured over a floor to flatten it before the finish goes down. The amount you need is a volume: floor area times the average depth. Each bag fills a certain area at a certain depth, so the bag count is:

N = ⌈A × d × (1 + w) ÷ C⌉

A is the floor area, d the average pour depth, w the waste allowance and C what one bag covers, as an area times a depth. The one figure that varies from product to product is C, and makers state it in two different ways, so the calculator takes it the way the bag or data sheet gives it.

Reading the coverage off the bag

Data sheets outside the US usually give a consumption in kilograms per square metre per millimetre. Mapei's Ultraplan Eco 20 data sheet gives 1.6 kg/m² per mm, from a 23 kg bag. Divide the bag weight by the consumption and you have its coverage: a 25 kg bag at 1.6 covers 25 ÷ 1.6 = 15.6 m² at 1 mm, or 3.1 m² at 5 mm. The metric side takes the bag weight and the consumption for that reason.

US bags usually state an area at a given thickness instead. The ARDEX K 15 product overview gives 30 ft² per 55 lb bag at 1/4 in (2.8 m² at 6 mm). Coverage scales with depth, so the same bag covers 60 ft² at 1/8 in and 15 ft² at 1/2 in. The US side takes the coverage and the thickness it is quoted at, whatever thickness your bag uses, and the result shows what one bag covers at your depth.

Put on the same basis, the two differ. K 15's 2.8 m² at 6 mm is 16.8 m²·mm per 25 kg bag, which works out at 1.49 kg/m² per mm, against Mapei's 1.6. That is about 7% between two mainstream products, enough to change the bag count on a large floor, so use the figure for the product you are buying rather than a generic one. Even one maker's two versions can differ: K 15's imperial figure, 30 ft² at 1/4 in, works out at 17.7 m²·mm because 1/4 in is 6.35 mm, not the rounded 6 mm of its metric figure. Use the figure printed for your market.

Worked through

With the metric defaults, 15 m² at an average of 5 mm is 75 m²·mm. At 15.6 m²·mm per 25 kg bag that is 4.8 bags, or 5.3 with 10% waste, so six bags. In US units, 160 ft² at 1/4 in with K 15's 30 ft² per bag is 5.3 bags, or 5.9 with 10% waste, so six bags as well.

Finding the average depth

Floors that need levelling are rarely off by the same amount everywhere, and the depth you enter should be the average, not the deepest point. Lay a long straightedge across the floor in several directions, measure the gap under it at regular spacings, and average the readings. A floor that is flat over most of the room but dips at one side can be averaged by area: most of the room at the minimum depth, and the dip at its own depth. If the deepest point is well beyond the product's range, fill it first with a patching or deep-fill product and then level over the whole floor.

Thickness range and priming

Each product has a working range, and the calculator does not know yours. The K 15 overview gives 1/8 in to 1 1/2 in (3 mm to 4 cm) neat and up to 5 in (13 cm) with aggregate; the Mapei sheet gives 1 to 10 mm per coat, with more only after adding graded sand or gravel on the maker's advice. Both makers call for a primer on the substrate first, and the right primer depends on what the floor is made of. Check the bag you are buying against your deepest and shallowest points before you mix.

Choosing the waste allowance

The default is 10%. Leveller creeps into gaps, joints and low spots you have not measured, some is left in the bucket or pump, and running short partway through a pour leaves a joint in the finished floor. It is better to have a bag left over than to stop mid-pour. On the default metric floor, the 10% adds about half a bag to the 4.8 needed.

What this tool does not do

It does not pick the product, work out priming, mixing water or drying times, or check the floor structure. Timber floors in particular have their own requirements for stiffness and reinforcement before any leveller goes on; follow the maker's instructions for your substrate.

Using this calculator with other BuildMetricLab tools

The floor screed calculator covers thicker sand and cement layers, and the tile calculator and tile layout planner cover the finish that goes on top. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.