What this calculator works out
This calculator divides patio area by the coverage rate on the product and adds an allowance, giving the litres of cleaner needed.
Coverage varies enormously between products — some are ready to use, some dilute at 1:10 — so the figure on the bottle matters far more than the arithmetic.
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Before you buy
- Check whether the coverage figure is for concentrate or diluted solution. A 1:10 concentrate covers ten times the area per litre of bottle.
- Porous surfaces absorb more. Sandstone and concrete take considerably more than dense porcelain.
- The extra allowance covers uneven application and re-treating stubborn areas.
- Check the product is suitable for your surface. Some cleaners damage limestone, and some stain resin-bound surfaces.
A pressure washer removes surface growth quickly and also erodes soft stone, blasts out jointing sand and can force water under slabs. For sandstone and older concrete a chemical clean and a stiff brush is gentler and lasts as long. Where a pressure washer is used, keep the nozzle moving and use the widest fan setting that works.
How this calculator works
Area divided by coverage, plus an allowance:
Litres = patio area ÷ coverage in m² per litre × (1 + allowance ÷ 100)The coverage figure must match the form you are buying. Entering the diluted coverage while buying concentrate overestimates the quantity by the dilution ratio, which for a 1:10 product means buying ten times too much.
Worked example: a 30 m² patio
Using the default figures — 30 m² at 10 m² per litre with a 10% allowance:
- Base quantity: 3 litres
- With allowance: 3.3 litres
3.3 litres of ready-to-use solution. If the product is a 1:10 concentrate covering 10 m² per litre of diluted solution, you need only 330 ml of concentrate — a single small bottle rather than three large ones. That distinction is worth checking on the label before ordering.
Common mistakes
- Confusing concentrate and diluted coverage. The difference can be a factor of ten.
- Using an unsuitable product on soft stone. Some cleaners etch limestone and sandstone.
- Applying to a dry, hot surface. It evaporates before it works; treat in cool damp conditions.
- Letting run-off reach planting or a pond. Many cleaners harm plants and aquatic life.
- Pressure washing soft stone hard. It erodes the surface permanently.
Frequently asked questions
What causes the green growth on patios?
Algae, lichen and moss, all of which thrive on damp shaded surfaces. It is more a symptom than a problem in itself — improving drainage, cutting back overhanging planting to let light and air in, and clearing leaves in autumn all reduce how quickly it returns.
Are patio cleaners safe around plants and pets?
It varies by product and many are not. Sodium hypochlorite based cleaners damage planting and are harmful to aquatic life. Read the label, keep pets off until dry, and prevent run-off reaching beds, ponds and drains. Some products are marketed as plant-safe and are worth seeking out where run-off is unavoidable.
Should I seal a patio after cleaning?
It depends on the material. Sealing reduces staining and slows regrowth on porous stone and concrete, and needs redoing every few years. It can also trap moisture and change the appearance, sometimes permanently. Test on a hidden area first, and do not seal a surface that has not fully dried.
How often should a patio be cleaned?
Once a year is typical, ideally in spring before growth accelerates or in autumn after leaf fall. A shaded north-facing patio may need it more often; a sunny open one considerably less. Regular sweeping and clearing leaves reduces the need substantially.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — chemical safety, run-off and pollution prevention guidance
- Royal Horticultural Society — hard surface maintenance and plant-safe practice
Sources are checked at publication and can change — how I choose and check references.