Why a flat average gets it wrong
On a level slab, screed is simply area times thickness. On a floor that slopes 40 mm from one side to the other, the screed has to be at least the minimum thickness at the highest point and much thicker at the lowest. Guess the average wrongly and you either run out halfway through the pour or pay for a pallet you do not need.
The finished screed surface will be flat, so the question is how much material fills the space between that flat top and the uneven base. This calculator offers two ways to answer it, depending on how carefully you have measured.
Method 1: level difference
Find the highest point of the subfloor and decide the minimum screed thickness there — the product minimum for your construction, such as 25 mm for a bonded sand-cement screed. Then measure the level difference Δh down to the lowest point. The average thickness is the minimum plus a fraction k of the difference.
The fraction depends on the shape of the floor. Pick the profile that matches what you see:
- Even slope or tilted floor: k = 0.5, the average sits halfway.
- Mostly flat with one low spot: k = 0.33, most of the floor is near the high point.
- Mostly low with one high spot: k = 0.67, most of the floor is near the low point.
- Custom: any value between 0 and 1 if you know better.
The uneven floor formula
t_avg = t_min + k × Δh (level difference) | t_ij = (max h + t_min) − h_ij (grid)
With the grid method, the average is the weighted mean of all t_ij with weight w_i × w_j, where w = 1 at the edges and 2 inside. The volume is V = A × t_avg ÷ 1000 × (1 + waste).
Worked example
- Level difference: 5.0 × 4.0 m room, t_min 30 mm, Δh 40 mm, even slope (k = 0.5): t_avg = 30 + 0.5 × 40 = 50 mm → 20 × 50 × 2.0 × 1.05 = 2,100 kg = 84 bags.
- Height grid: 4.0 × 4.0 m, 3 × 3 readings 0, −5, −10 / −5, −10, −20 / −10, −20, −30 mm, t_min 35 mm.
- Point thicknesses 35, 40, 45 / 40, 45, 55 / 45, 55, 65 mm; weighted average 750 ÷ 16 = 46.875 mm; maximum 65 mm.
- Liquid screed with 5% waste: 16 × 0.046875 × 1.05 = 0.7875 m³ → order 0.8 m³ (below a 1 m³ minimum order, so check with the supplier).
Method 2: height grid
For a more accurate answer, set up a rotating laser or optical level and take readings on a regular grid across the room, edges included — 3 × 3 points for a small room, up to 10 × 10 for a large one. Enter the relative heights in millimetres; positive means higher and any reference point works. The finished top is set at the highest reading plus the minimum thickness, and each point’s screed thickness follows from it.
The average uses the two-dimensional trapezoidal rule: corner points count once, edge points twice and inner points four times, because each represents a larger share of the floor. A colour heatmap shows where the screed is thickest, and readings that differ from their neighbours by more than 50 mm are flagged as a possible typo.
Check the thickest point
The maximum thickness matters as much as the average. Sand-cement screed is usually limited to about 100 mm, liquid anhydrite to about 80 mm and self-levelling compound to about 30 mm. If the deepest point exceeds the product maximum you will see a warning. For a large level difference under a self-leveller, a better build-up is a sand-cement screed first, then a thin self-levelling layer on top — the calculator suggests it when Δh exceeds 50 mm.
Frequently asked questions
How do I measure an uneven floor?
Use a rotating laser level or a water level and a measuring staff. Take readings at evenly spaced points including the walls, and note the height of each relative to any fixed reference.
What value of k should I use?
Use 0.5 for a floor that slopes evenly. If most of the floor is close to the highest point with one dip, use 0.33; if most of it is low with one hump, use 0.67. A height grid removes the guesswork.
Can I use self-levelling compound on a very uneven floor?
Only within the product’s maximum thickness, typically about 30 mm. For bigger differences, lay a screed first and finish with a thin self-levelling layer.
Does the uneven floor need more waste?
If you have measured with a grid, 5% is usually enough. With a rough level difference estimate, consider 10%.