F0 Value, D Value and Z Value Calculations : Pharmaguideline

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F0 Value, D Value and Z Value Calculations

Calculation of D - Value, Z - Value and F0 - Value for sterilization in autoclave used for microbiology analysis as per USP and BP.


F0-Value at a particular temperature other than 121ºC is the time in minutes required to provide the lethality equivalent to that provided at 121ºC for a stated time. (As per USP 2008)
F0-Value of a saturated steam sterilization process is the lethality expressed in terms of equivalent time in minutes at a temperature of 121ºC delivered by the process to the product in its final container with reference to microorganisms possessing a Z-Value of 10. (As per BP 2009)  
In general, for aqueous preparations, a microbiologically – Validated steam sterilization process that delivers, in total ( including the heating up & cooling down phases of the sterilization cycle ), an Fo-Value of Not less than 8 to every container in the autoclave load is considered satisfactory. (IP 2007)
Related: Importance and Determination of F0 Value in Sterilization

Determination of D Value and Z Value in Microbiology

D-Value (Decimal Reduction Value)

D value, F0 value and Z Value CalculationIt is the time required at temperature T to reduce a specific microbial population by 90% or by a factor of 10.


Z-Value is the number of degrees of temperature change necessary to change the D-Value by a Factor of 10.
For example, if D-Value at 121ºC is 1.5 min & Z-Value is 10ºC.
Then D-Value at 131ºC will be 0.15 minute.
Related: Determination of Z-Value for Biological Indicators

Sterility Assurance Level (SAL)

SAL of a Sterilization process is the degree of assurance with which the process in question renders a population of items sterile.
Required SAL for steam sterilization process = 10-6 or better
Characteristics of Biological indicator (IP 2007)

Sterilization Mode
Examples of Typical D-Value
Minimum D-Values
for selecting a suitable biological indicator (minutes)

Minimum Survival Time

Kill Time
Moist Heat 121º
Min 1.5
Max 3.0

Min 4.5
Max 14.0
Min 13.5
Max 32.0

Formulas Related to Autoclave Validation

Fo = D121 ( logNo – log N )
Where No= Initial spore population (B. stearothermophilus) N = Spore population after sterilization cycle.
For example, if D121 = 1.5 min, No=106, N=1(for SAL of 10-6)
Fo = 1.5 (log106 – log 1) = 9 min
So to achieve 6 log reduction (6 D) 9 minute sterilization cycle will have to be run.
Fo = Δt Σ 10(T-121)/z
Where Δt = Time interval between two following temperature measurements (1 minutes), T = Observed Temperature at that particular time, z = Z-Value
Also see: Log reduction and SAL value

Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
.moc.enilediugamrahp@ofni :liamENeed Help: Ask Question

10 comments: Post Yours! Read Comment Policy ▼

  1. if at 121C sterilization for 15min growth of microorganism comes in my media and i want to run the cycle at set point of 122.5C for 15-16min, is it possible to justify this cycle?. how can i calculate the F0 value for the same and what is the miniumum and maximum range of F0 value.

  2. You should increase time for sterilization instead of temperature.

    1. Thank for your article, i want to know if you are familiar with heat penetration study because I want to conduct one for canned tuna sterilization.

  3. THANK YOU SO MUCH!!!!!!! I finally got this!

  4. Dear sir how to calculate fo value at 107°c

  5. depyrogenating temperature of dry heat 250 for 30min while depyrogenating temperature of tunnel of moist heat 300for 3min..why ..could you please tell me this

    1. If temperature increases it will decrease the D value. Every 46.4 degree centigrade(z-value) increases exposure time will decrease.
      Temperature will increases log reduction will decreased.

    2. in the question it is written moist heat no moist heat goes to 300 degrees. and second 300 degrees for 30 min or 3 min is based on Fh value 46.4 if you tremove 46,4 degrees from 300 degree it will be 254 degrees and the factor of 30 minutes will get reduced to a factor of 10 that is 3 minutes

  6. How the Z value for Depyrogenation i.e. 46.4 is derived.

  7. Sir,what is the minimum time of sterilization in sterilization zone in tunnel with respect to Dvalue.


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