Screed Calculator UK | Sand & Cement Bags, m³ & Weight

How to use the screed calculator

Choose how to give the floor size: Length and width of the room, or I already know the area.
Pick the Type of screed: bonded, unbonded, floating (normal or heavy loads) or over underfloor heating. This sets the minimum thickness we check.
Type in the Screed thickness. Count the screed only, not the insulation under it.
Under What will you use? pick a 1:3, 1:4 or 1:4.5 mix to make yourself, or bagged ready-mixed dry screed.
Leave Extra for waste and uneven slab at 10%, or raise it if the slab underneath is rough or out of level.
Read the cement, sand and weight, then tap Add to shopping list to keep them or Share link to send them to your builder.

Why use this calculator

It checks your thickness. Each type of screed has a minimum, and you get a warning if yours is too thin. Thin sand and cement screed cracks and can come loose.

It gives you both ways to buy. Mix it yourself and you get bags of cement and tonnes of sharp sand with bulk bag counts. Buy it bagged and you get the number of 25 kg bags.

It tells you the weight. Screed is heavy, which matters on upper floors, so you get the total tonnes and the kg a m².

It shows its working. Under How we worked it out you can see every step with your own numbers.

How we worked it out

Your numbers, step by step. This updates as you change the calculator.

Floor area = length × width = 5 m × 4 m = 20 m²
Volume = area × thickness = 20 m² × 0.065 m = 1.3 m³
Add 10% for waste: 1.3 × 1.1 = 1.43 m³
At 1:4, 1 m³ of screed needs about 13.8 bags of cement, so 1.43 m³ needs about 19.73, round up to 20 × 25 kg bags
Sharp sand = 1.43 m³ × 0.96 m³ per m³ of screed = 1.373 m³, × 1.7 t/m³ = about 2.33 tonnes (3 bulk bags of 850 kg)
Weight on the floor = 1.3 m³ × 2000 kg/m³ = about 2.6 tonnes (130 kg per m²)

The formula

The volume of screed is floor area × thickness, in metres. A 5 m by 4 m floor is 20 m², and at 65 mm (0.065 m) that is 1.3 m³. With 10% waste it is 1.43 m³.

Cement: volume × bags of cement per m³ for your mix, rounded up. 1:3 takes 17.3 bags, 1:4 takes 13.8 and 1:4.5 takes 12.6. For 1.43 m³ at 1:4 that is 19.7, so 20 bags.
Sharp sand: volume × m³ of sand per m³ of screed (0.9 for 1:3, 0.96 for 1:4, 0.98 for 1:4.5) × 1.7 tonnes a m³. For 1.43 m³ at 1:4 that is about 2.3 tonnes, or 3 bulk bags.
Bagged screed: volume × 2,000 kg a m³ ÷ 25 kg a bag, rounded up. For 1.43 m³ that is 115 bags.
Weight: volume before waste × 2,000 kg a m³. For 1.3 m³ that is 2.6 tonnes, or about 130 kg a m².

What we assume

The mixes are cement to sharp sand by volume. The range of 1:3 to 1:4.5 comes from the Concrete Centre.
The cement and sand quantities are worked out from standard densities, not from a maker’s table. Treat them as good to about 8% either way. Water adds no volume.
Laid screed weighs about 2,000 kg a m³ (the range is 1,800 to 2,200). We use the same figure for bagged screed.
A bag of ready-mixed screed is taken as 25 kg, a common retail size. Check the bag you buy.
Minimum thicknesses are 25 mm bonded, 50 mm unbonded, 65 mm floating, 75 mm floating for heavy loads and 65 mm over underfloor heating. For unbonded screed we also say when you are under the recommended 70 mm.
Liquid screed, calcium-sulphate screed and self-levelling compounds are different products and are not covered.

Tips

Don't skimp on thickness. The minimum depends on the type: a floating screed on insulation needs 65 mm, much more than a bonded screed on a slab.
If the slab underneath is uneven, measure the depth in several places and raise the waste to 15% or more. Low spots swallow a lot of screed.
Over underfloor heating, check the heating maker's manual for how deep they want the screed over the pipes. Liquid and calcium-sulphate screeds can be thinner, but they are different products and are not worked out here.
For a floor of about 1 m³ or more, get a quote for delivered screed as well as pricing the bags. Mixing a big floor by hand is hard work.
Ask your supplier how long the screed needs to dry before you lay flooring. It depends on the thickness and the type of screed.
Laying tiles on top? Work out the tiles, adhesive and grout with the tile calculator. Need the same mix for another job? Use the sand and cement calculator.

FAQs

How much sand and cement do I need for floor screed per m²?

At 1:4 and 50 mm thick, about 0.69 of a 25 kg bag of cement and 82 kg of sharp sand per m², before waste. At 65 mm it is about 0.9 bags and 106 kg of sand per m².

How thick should a sand and cement screed be?

At least 25 mm for bonded, 50 mm for unbonded (70 mm or more is recommended), 65 mm for floating on insulation (75 mm for heavy loads) and 65 mm over underfloor heating. These are minimums, so the calculator warns you if you go below them.

What is the best screed mix ratio: 4:1 or 3:1?

1:4 (cement to sharp sand) is the most common. 1:3 is stronger and uses more cement, about 17.3 bags a m³ against 13.8. Guidance allows anything from 1:3 to 1:4.5.

How many bags of cement do I need for 1 m³ of screed?

About 13.8 bags of 25 kg at 1:4, 17.3 at 1:3 and 12.6 at 1:4.5. A 1:4 mix also needs about 0.96 m³ of sharp sand, which is roughly 1.6 tonnes.

How much screed do I need over underfloor heating?

At least 65 mm of sand and cement screed, with 65–75 mm being ideal, or 50 mm if it is fibre-reinforced. Check the heating maker's manual for how much cover they want over the pipes.

How much does a screed floor weigh?

About 20 kg per m² for every 10 mm, so a 65 mm screed is about 130 kg per m². Ask a builder or structural engineer before you put a thick screed on an upper floor.

How many bags of ready-mixed screed do I need?

A 20 m² floor at 65 mm with 10% waste needs about 115 bags of 25 kg, or 4 bulk bags of 850 kg. Check the weight printed on the bag you are buying, because bag sizes vary.