Plan a serial dilution or work back from one: the concentration in every tube, the dilution factor, the number of steps, and how much sample and diluent to pipette. A second calculator turns a plate count into CFU/mL for microbiology.
Serial dilution formulas
Worked example: tenfold dilution series
A culture has 1 × 10⁶ cells/mL. Six 1:10 steps give 10⁶ / 10⁶ = 1 cell/mL in the last tube. For 10 mL per tube, move 1 mL into 9 mL of diluent each time.
If 0.1 mL from the 10⁻⁵ tube grows 150 colonies, the original sample had 150 × 10⁵ / 0.1 = 1.5 × 10⁸ CFU/mL.
Common dilution steps
| Dilution | Factor | Sample + diluent (10 mL) |
|---|---|---|
| 1:2 | 2 | 5 mL + 5 mL |
| 1:5 | 5 | 2 mL + 8 mL |
| 1:10 | 10 | 1 mL + 9 mL |
| 1:20 | 20 | 0.5 mL + 9.5 mL |
| 1:100 | 100 | 0.1 mL + 9.9 mL |
Frequently asked questions
What is a dilution factor?
The final volume divided by the sample volume. Adding 1 mL of sample to 9 mL of diluent is a 1:10 dilution, a factor of 10.
How do I calculate the total dilution of a serial dilution?
Multiply the dilution factors of every step. Three 1:10 steps give 10 × 10 × 10 = 1:1,000.
How do I calculate CFU/mL?
Multiply the colony count by the total dilution factor of that plate and divide by the volume plated in mL.
Which plates should I count?
Plates with 30 to 300 colonies. Fewer is statistically unreliable and more is usually too crowded to count.
How many steps do I need?
Leave the steps box empty and enter the starting and target concentrations. The calculator uses n = log(C₀/Cₙ) / log(DF).
Related tools
Free educational tool by HawkInc. Results are calculated in your browser and rounded for display; check critical work by hand or with a second method.