Stone and sealing
Water absorption of a tile or stone sample (porosity class)
- where
- water absorbed (g)
- mass after soaking, surface wiped (g)
- dry mass (g)
- water absorption (%)
Explain this equation
Use this to judge how porous a stone or tile is before choosing a sealer, grout or adhesive, using an offcut, a bucket of water and kitchen scales.
md is the weight of the offcut fully dried, ms its weight after soaking with the surface water wiped off. The difference is the water that has gone into the pores. Ab expresses it as a percentage of the dry weight.
Absorption is measured against dry mass because that does not change with how wet the sample is. Multiplying by 100 turns the fraction into a percentage, the form used by tile and stone standards.
Porcelain is 0.5% or less; a 1 kg piece would take up under 5 g of water. Soft limestones and terracotta are often above 10%, taking up over 100 g per kg, and drink several times as much sealer.
Watch out: if the sample is not properly dry first the result is far too low. A site test is a guide, not a lab result.
Measures how porous a tile or stone is by weighing a dry offcut, soaking it and weighing again, then places it in the ceramic absorption groups.
Enter your values to see the result.
Good to know
Dry the offcut in a warm place until its weight stops changing, weigh it, soak it in clean water (24–48 h for stone), wipe the surface with a damp cloth and weigh straight away. ISO 13006 / BS EN 14411 group dry-pressed tiles as BIa ≤0.5% (porcelain), BIb 0.5–3%, BIIa 3–6%, BIIb 6–10%, BIII over 10%. The figure helps choose a sealer and estimate how much it will take: LTP quote about 1–2 litres per 10 m² for lower-porosity floors and 3–4 litres for higher porosity.
Common mistake
Weighing a sample that was never properly dried, which hides most of the absorption.
Limits
A site version of the EN 13755 / ISO 10545-3 method, which use controlled drying, timed immersion and several specimens. Absorption is not the same as open porosity by volume. Polished faces seal differently from the body.
Sources
- EN 13755 Natural stone test methods — determination of water absorption at atmospheric pressure (md dry mass, ms saturated mass, Ab in %), sample
- ISO 13006:2012 Ceramic tiles — definitions, classification (water absorption groups BIa ≤0.5%, BIb 0.5–3%, BIIa 3–6%, BIIb 6–10%, BIII >10%), preview
- LTP Mattstone technical data sheet (1–2 L per 10 m² lower porosity, 3–4 L per 10 m² higher porosity)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.