Serial Dilution Calculator
Plan a serial dilution: the volume to carry from tube to tube, the diluent in each tube, and the concentration at every step.
Formula
- fold dilution at each step (10 for a ten-fold series)
- starting concentration
- concentration in tube n
How it works
A serial dilution repeats the same dilution several times, each tube serving as the stock for the next. Because the factors multiply, six ten-fold steps reach a one-in-a-million dilution using only small, easily pipetted volumes.
Each tube is prepared with diluent first. A fixed volume is carried in from the previous tube and mixed, and the same volume is then carried on to the next.
Worked example
Dilute a culture at 10⁸ cells/mL through six ten-fold steps, 1 mL per tube.
- Transfer = 1 mL / 10 = 100 µL, into 900 µL of diluent.
- Tube 1 is 10⁷, tube 2 is 10⁶, and so on down to 100 cells/mL in tube 6.
Carry 100 µL into 900 µL at every step. Tube 6 is a 1 : 1,000,000 dilution.
These are the values the calculator opens with, so you can check its output against this example.
Assumptions
- Every tube is mixed thoroughly before the next transfer.
- The same factor is used at every step.
- Nothing is carried over on the outside of the pipette tip.
Common mistakes
- Reusing one pipette tip down the series. Carry-over on the tip makes later tubes more concentrated than calculated; change tips at each step.
- Transferring before mixing, which carries an unrepresentative sample forward.
- Forgetting that errors compound. A 5% pipetting error at each of six steps is far more than 5% at the end.