Floor Screed Calculator
Calculate exactly how much sharp sand and Portland cement you need for floor screed. Choose your mix ratio, enter room dimensions or floor area, and get instant quantities for bonded, unbonded or floating screed.
Cement Bag Size
Select the size your supplier sells.
Metric Bags
In Australia, cement is sold as GP cement (General Purpose) under AS 3972 — this is the local equivalent of Portland cement.
Default: 94 lb (imperial) · 20 kg (metric)
Mix ratio (cement : sand)
Standard floor screed
How to Use This Screed Calculator
- Select your unit system — Metric (recommended for screed) or Imperial.
- Choose Room Dimensions to enter length, width and depth, or Floor Area if you already know the m².
- Select your mix ratio — 1:4 is standard for most floor screed.
- Adjust the cement bag size if needed using the bag selector.
- Results show volume, cement bags, sand quantity, and a 10% waste allowance.
Screed Mix Ratios Explained
Floor screed uses Portland cement and sharp sand only — no coarse aggregate. The ratio is expressed as cement : sand by volume:
- 1:3 — Strong mix for underfloor heating or areas subject to heavy loads. Higher cement content, better crack resistance.
- 1:4 — Standard domestic floor screed. Good balance of strength and workability.
- 1:4.5 — General purpose. Slightly weaker but easier to lay flat over large areas.
- 1:5 — Weak/levelling screed. For thin top-up layers or low-traffic areas only.
The dry factor of 1.3 is applied to account for the fact that dry materials compact to approximately 77% of their original volume when mixed. This is less than concrete (1.54) as screed contains no coarse aggregate.
How Thick Should Floor Screed Be?
What Sand to Use for Screed
Floor screed must use sharp sand (also called grit sand, concreting sand, or coarse sand). Sharp sand has angular, irregular particles that interlock under load and provide structural integrity.
Do not use builders sand, soft sand, or brickies sand. These contain finer, rounder particles with higher clay content that prevent proper bonding with cement. Screed made with soft sand will be weak, powdery, and prone to cracking or delamination.
Sharp sand is known by different names internationally:
- UK: sharp sand, grit sand, concreting sand
- US: concrete sand, coarse sand
- Australia/NZ: coarse sand, washed sand
- India: river sand, M-sand (manufactured sand)
- South Africa: crusher sand, pit sand (coarse grade)
Frequently Asked Questions
What is the mix ratio for floor screed? +
The standard mix for floor screed is 1 part cement to 4 parts sharp sand (1:4). For stronger screed over underfloor heating use 1:3. A 1:4.5 or 1:5 mix is suitable for non-structural levelling work. Always use Portland cement and sharp sand — never soft or builders sand.
How much sand and cement do I need for 1 m² of screed? +
At 65 mm depth using a 1:4 mix, 1 m² of floor screed requires approximately 14 kg of cement (roughly 0.6 of a 25 kg bag) and 56 kg of sharp sand (about 0.034 m³). Use the calculator above for exact quantities based on your room dimensions and chosen mix ratio.
Can I use builders sand for screed? +
No. Builders sand (soft sand, brickies sand) has fine rounded particles and higher clay content which significantly weakens screed. Floor screed must use sharp sand (also called grit sand or concreting sand) with angular particles that interlock and provide structural strength. Using the wrong sand is one of the most common causes of screed failure.
How long does floor screed take to dry? +
Traditional sand and cement screed dries at roughly 1 day per millimetre for the first 50 mm, then slower. A 65 mm screed typically takes 6–8 weeks before it is dry enough to tile over or lay wood flooring. Forced drying with heat or dehumidifiers can shorten this but risks cracking. Anhydrite (flowing) screed dries faster but is a different product.
What is the difference between bonded and unbonded screed? +
Bonded screed is mechanically keyed to the concrete subfloor — the substrate is primed and the screed is applied at 25–40 mm thickness. Unbonded screed sits on a polythene damp-proof membrane and is not fixed to the base — minimum 50 mm thickness is required to prevent cracking. Floating screed goes over rigid insulation or underfloor heating pipes — minimum 65 mm, or 75 mm over UFH to allow enough cover.
How thick should floor screed be? +
Recommended minimum screed thickness depends on the application: bonded screed 25–40 mm, unbonded screed 50–75 mm, floating screed (over insulation) 65–100 mm, screed over underfloor heating 65–75 mm minimum. Thinner screed over UFH risks cracking when the pipes heat and cool.
What is the difference between screed and concrete? +
Concrete contains coarse aggregate (gravel or crushed stone) giving it structural strength for slabs, foundations, and columns. Screed is a finer mix of cement and sharp sand only — no aggregate — making it smooth enough to receive floor finishes directly. Screed is not structural and cannot replace concrete as a base. Concrete is typically 100–200 mm thick; screed is typically 25–75 mm.