Floor Screed Calculator

How the floor screed calculator works

A screed is the sand and cement layer laid over a concrete base to give a flat, level surface for the floor finish. This calculator is for bagged screed mix. It works out the volume from the area and depth, adds your waste allowance, converts the volume to dry mix by weight and rounds up to whole bags:

N = ⌈A × d × (1 + w) × 1930 ÷ b⌉

A is the area, d the depth, w the waste as a decimal, b the bag weight in kg, and 1930 the kilograms of dry mix that make one cubic metre of laid screed. It also checks the depth against the minimum for the kind of screed you are laying.

Where 1930 kg per cubic metre comes from

Bag makers print a yield on the bag. The Quikrete Sand/Topping Mix data sheet gives about 0.66 ft³ (18.7 L) from an 80 lb (36.2 kg) bag and 13 L from a 25 kg bag. Both work out at close to 1930 kg of dry mix per cubic metre of screed. If your bag states a different yield, divide the volume by it instead; the difference between products is usually a few percent.

Worked through

With the metric defaults, 20 m² at 50 mm is 1.0 m³. Add 10% and it is 1.1 m³, which needs 1.1 × 1930 = 2123 kg of mix, or 85 bags of 25 kg. At £6 a bag that is £510. In US units, 215 ft² at 2 in is 35.8 ft³, or 39.4 ft³ with 10% added; at 0.66 ft³ per 80 lb bag that is 60 bags. A floating screed over underfloor heating insulation, 30 m² at 65 mm, comes to 2.15 m³ with waste, 4.1 tonnes of mix and 166 bags.

Numbers like that are why larger floors are usually laid with ready-mixed or pumped screed delivered by the tonne. Eighty-odd bags is roughly two tonnes to carry, mix and place in one working window.

How thick the screed has to be

Thickness depends on what the screed sits on. According to The Concrete Society's guidance on screeds, a sand and cement screed bonded to the concrete base works best at 25 to 40 mm (1 to 1 9/16 in). An unbonded screed, laid on a membrane, should be at least 50 mm (2 in). A floating screed, laid on insulation such as the boards under wet underfloor heating, should be more than 65 mm (2 9/16 in) for lightly loaded floors and 75 mm (3 in) for more heavily loaded ones, and 65 mm only works if the insulation is firm enough to compact the screed against. Pick the screed type and the calculator flags a depth below the minimum, or a bonded screed thicker than the 25 to 40 mm range.

Measure the depth at several points across the room before ordering. Bases are rarely flat, and a floor that is 50 mm at one wall and 70 mm at the other needs the average, not the thinnest reading. The simplest way is to mark a level line around the walls at the height of the finished screed, then measure down from it to the base on a rough grid, every metre or so, and at the doorways where the screed has to meet the next floor. Average those readings for the depth, and note the thinnest one: it is the reading to check against the minimum for the screed type. If the thinnest point falls short, the answer is usually to lower the finished level or deal with the high spot in the base, not to screed thinner there.

Before the floor finish goes down

A screed is not ready for flooring when it can be walked on. The water has to leave, and the accepted test is the hygrometer box: a sealed, insulated box on the surface that measures the humidity of the trapped air once it settles. Contract Flooring Journal notes that a maximum of 75% relative humidity by this method is acceptable for most floor finishes, while the flooring maker's own figure still applies, and Floorscape's moisture testing guide lists the British Standards that reference the method. Wood and moisture-sensitive finishes often ask for a lower reading.

What this tool does not do

It does not design mixes for site-mixed screed, size the screed for heavy or industrial loads, or check the base for movement. Self-levelling and flowing screeds use different bags and depths; follow their own data sheets. Underfloor heating, insulation and membranes follow the heating and insulation makers' details.

Using this calculator with other BuildMetricLab tools

The concrete slab calculator sizes the base beneath, the concrete mix ratio calculator helps if you batch the base yourself, and the tile calculator counts the finish. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.