Inhibitor Dosing Calculator — How Much Inhibitor Do I Need?

Our inhibitor dosing calculator uses a simple formula based on your total system water volume and the manufacturer's recommended dose rate.

The formula

Inhibitor required (litres) = System volume (litres) ÷ 100 × Dose rate

Manufacturer dose rates

• Fernox Protector F1: one 500ml bottle treats up to ~130 litres / 16 single radiators / 250 m² underfloor (≈0.5 litre per 100 litres, ≈0.5% in-use)
• Sentinel X100: 1 litre per 100 litres of system water (1% in-use; one 1-litre bottle treats ~8-10 single radiators)
• Generic inhibitors: typically 1 litre per 100 litres (1% — always check the product label)

BS 7593:2019 requires the system to be dosed to the manufacturer's recommended concentration, then tested annually (and re-dosed at drain-down or at 5-year intervals). Confirm the in-use level with a Fernox or Sentinel test kit rather than relying on the dose alone.

If you do not know your system volume, use our Central Heating Volume Calculator to work it out first, or enable the volume estimation option, which uses the trade rule of thumb of roughly 10 litres per single radiator (count a double-panel radiator as two). This is an estimate, not a precise figure — use a measured volume where you can.