TradeCalculator.co.uk
Inhibitor Dosing Calculator — How Much Inhibitor Do I Need?
Calculate the correct amount of central heating inhibitor for your system. Covers Fernox F1, Sentinel X100, and generic inhibitors.
If you don't know your system volume, toggle this to estimate from the number of radiators
Use our System Volume Calculator if unsure
Enter your supplier price for a cost estimate
Leave blank to use the brand's standard rate (Fernox F1 0.5, Sentinel X100 1.0). Only set this if your product label states a different concentration.
Next up: related pages
How We Calculate This
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+A1:2024 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.
Frequently Asked Questions
Central heating inhibitor protects the system from internal corrosion, limescale build-up, and sludge formation. Without it, radiators can develop cold spots, the boiler can become noisy (kettling), and components can fail prematurely. BS 7593 (2019) requires inhibitor to be added to all new and refurbished central heating systems in the UK. You need to know your total system volume first — use our Central Heating Volume Calculator if unsure.
Inhibitor should be checked annually during your boiler service. If the system has been drained for any reason (radiator replacement, leak repair), inhibitor must be re-dosed. The inhibitor concentration can be tested using a Fernox or Sentinel test kit — your heating engineer should do this during servicing.
Both Fernox F1 and Sentinel X100 are excellent inhibitors widely used by UK heating engineers. They perform similarly in protecting against corrosion and scale. The choice often comes down to personal preference or what your boiler manufacturer recommends. Do not mix different brands in the same system.
For sealed (pressurised) systems, the easiest method is to inject inhibitor through a radiator bleed valve using a syringe or Sentinel/Fernox rapid-dose aerosol can. Alternatively, you can connect via a filling loop after partially draining the system. The system must be at zero pressure before adding inhibitor via the filling loop.
Mild over-dosing (up to 50% extra) is generally not harmful and provides extra protection. However, significant over-dosing is wasteful and unnecessary. Under-dosing is more problematic — it will not provide adequate protection against corrosion. Always aim for the correct dose rate.
Related Calculators
Was this calculator helpful?
Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.