Serial Dilution Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The concentration after a serial dilution is calculated with initial concentration ÷ (dilution factor)ⁿ, where n is the number of steps. For 10⁶ CFU/mL diluted 1/10 over 3 steps, the final concentration is 1000 CFU/mL.
Explanation
A serial dilution consists of repeating the same basic dilution several times in a row, with each tube serving as the source for the next: it's the standard microbiology technique for obtaining a range of decreasing concentrations from a single concentrated sample, for example before a bacterial count on a Petri dish, or before reasoning about the result with our molar concentration calculator once the count is converted to moles per liter. At each step, a fixed volume of the previous solution is drawn and topped up with diluent to the tube's final volume; the dilution factor (DF) is the ratio between the tube's final volume and the volume drawn (a DF of 10 means one-tenth of the final volume is drawn as sample, topped up with nine-tenths diluent). After n identical steps, the final concentration is divided by DF raised to the power n — an exponential decrease, not a linear one, which explains why just a few steps are enough to reach very low concentrations from a highly concentrated sample. This calculator gives both the expected concentration after the last step and the exact sample volume to draw as well as the diluent volume to add to prepare each individual step, at a given tube final volume — a preparation that often precedes measuring a nucleic acid concentration brought back into a spectrophotometer's reliable reading range.
Example: 10⁶ CFU/mL, 1/10 dilution, 3 steps
Inputs
Initial concentration: 10⁶ CFU/mL. Dilution factor: 10. Number of steps: 3. Final volume per tube: 10 mL.
Calculation
Final concentration = 10⁶ ÷ 10³ = 1000 CFU/mL. Volume to draw per step = 10 ÷ 10 = 1 mL. Diluent volume per step = 10 − 1 = 9 mL.
Result
After 3 dilutions of 1/10, the concentration is 1000 CFU/mL; each step is prepared with 1 mL of sample and 9 mL of diluent.
Frequently asked questions
Why does the concentration drop so fast after just a few steps?
Because the reduction is exponential, not linear: each step multiplies the reduction of the previous step instead of adding to it. With a dilution factor of 10, three steps already reduce the concentration by 1000 (10³), and six steps reduce it by a million (10⁶) — a far faster progression than linear intuition would suggest.
Does the volume to draw change from one step to the next?
No, if the dilution factor and the tube's final volume stay the same at each step (the standard case for a serial dilution), the volume to draw and the diluent volume stay identical at every step: only the concentration changes, decreasing at each new step.
What happens if I change the dilution factor partway through the series?
This calculator assumes a constant dilution factor at each step, the most common case in practice. If the factor changes from one step to another, each step must be calculated separately and the individual reduction factors multiplied together to get the final concentration, rather than using a single factor raised to the power n.