Calculator
F-Value Calculator
Thermal lethality of a sterilisation cycle, and its equivalent minutes at 121 °C.
F₀ — equivalent minutes at 121 °C
9.00min
Meets the medical-device criterion (F₀ ≥ 8 min) but not overkill.
Lethality parameters
Minutes at the process temperature to kill 90% of the population.
6 log is typical; 12 log is the overkill and botulinum standard.
The temperature the load actually holds at.
121 °C is the reference that defines F₀.
Degrees of temperature change for a tenfold change in D.
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Working
Acceptance criteria
What a validation report is usually judged against.
Lethal rate per minute
Minutes of F₀ delivered by one minute at each temperature, assuming z = 10 °C.
F = D × log reduction
F₀ = F × 10^((T − T_ref) / z)
The D-value is the time needed to kill 90% of a population — one log. Killing six logs therefore takes six D-values, which is all the first line says.
The second line converts that time to its equivalent at the reference temperature. The z-value is how many degrees change D tenfold, so every z degrees above the reference multiplies the lethality delivered per minute by ten.
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References
- Aulton ME, Taylor KMG, editors. Aulton's Pharmaceutics: The Design and Manufacture of Medicines. 6th ed. Edinburgh: Elsevier; 2022.
Common questions
F is the lethality expressed at whatever reference temperature and z-value you choose. F₀ is the industry convention: reference 121 °C and z = 10 °C. Quoting F₀ lets cycles run at different temperatures be compared on one scale.
It delivers at least a 12-log reduction of an organism with D₁₂₁ of 1 minute. That is so far beyond any realistic bioburden that the cycle is sterilising regardless of what was on the load, which removes the need to characterise the bioburden itself.
Clostridium botulinum spores have a D₁₂₁ of about 0.21 minutes. Twelve logs of reduction is 12 × 0.21 = 2.52 minutes. It is the classic food-canning criterion and is far less stringent than the pharmaceutical overkill target.
Not with these defaults. Dry heat uses a reference of 170 °C and a z-value nearer 20 °C, and the equivalent term is F_H rather than F₀. The arithmetic is the same, but you must enter the correct reference temperature and z-value.
No. A real F₀ is the integral of the lethal rate across the entire recorded temperature profile, including ramp and cool-down, which usually adds lethality. This calculator gives the single-point estimate that a design calculation starts from.
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