Serial dilution calculator
Get the concentration, transfer volume, and diluent volume for every tube.
A 1:10 dilution is a factor of 10. Each step moves a measured volume from the previous tube into fresh diluent.
How a serial dilution works
In a serial dilution, you dilute a sample by the same factor several times in a row. Each tube’s concentration is the previous tube’s concentration divided by the dilution factor, so after n steps the total dilution is the factor raised to the power n.
For a final volume V and a dilution factor DF, transfer V ÷ DF of the previous tube into V − V ÷ DF of fresh diluent. A tenfold series with 1 mL tubes uses 0.1 mL transfers and 0.9 mL of diluent.
Worked examples
A 1:10 series. Starting from a 1.0 M stock, move 1.0 mL into 9.0 mL of diluent to make each next tube (10 mL total, a 1:10 dilution). After four tubes the concentrations are 0.1, 0.01, 0.001 and 0.0001 M, an overall dilution of 10⁴.
Counting bacteria. If 0.1 mL from the 10⁻⁵ tube grows 45 colonies, the original sample had 45 ÷ (0.1 mL × 10⁻⁵) = 4.5 × 10⁷ CFU/mL.
Frequently asked questions
What is the difference between a 1:10 dilution and a 10-fold dilution?
They mean the same thing: 1 part sample in 10 parts total (1 part sample + 9 parts diluent). Be careful, because some sources write 1:9 to mean 1 part sample plus 9 parts diluent.
Why do a serial dilution instead of one big dilution?
A 10⁶ dilution in one step would need a tiny, inaccurate volume of sample or a huge volume of diluent. Several 1:10 steps use easy, accurate volumes.
How do I get the total dilution factor?
Multiply the dilution factors of each step. Three 1:10 steps give 10 × 10 × 10 = 1000, a 10⁻³ dilution.
Which plates should I count?
In microbiology, count plates with roughly 30 to 300 colonies. Fewer is statistically unreliable, and more is too crowded to count accurately.