How the serial dilution calculator works
A serial dilution repeats the same small dilution several times in a row. You move a fixed volume from one tube into fresh diluent, mix, and then move the same volume from that tube into the next. Each step divides the concentration by the dilution factor, so a few tubes cover several orders of magnitude.
The serial dilution calculator needs four inputs:
- Starting concentration of the stock or sample, in any unit (M, mM, mg/mL, %, cells are fine as long as the unit is consistent).
- Dilution factor per step, for example 2 for a twofold series or 10 for a tenfold series.
- Number of tubes you want to make.
- Final volume in each tube after mixing.
From these it calculates the transfer volume (final volume ÷ dilution factor), the diluent already in each tube (final volume − transfer volume) and the concentration in every tube. The table under the result is generated as you type, so you can try a 1:2 series against a 1:10 series and see which one brackets your target range.
Worked examples from lab manuals
Tenfold series in cell culture. The Irvine Valley College cell culture text on LibreTexts starts from 1.0 mg/mL and makes three tenfold dilutions, giving 0.10, 0.010 and 0.0010 mg/mL. The overall dilution is 10 × 10 × 10 = 1,000. Enter 1 mg/mL, factor 10 and 3 tubes above to reproduce it.
1:100 steps for plate counts. The Hartline microbiology laboratory manual transfers 10 µL into 990 µL, which is a 1:100 (10⁻²) dilution, and repeats it four times to reach 10⁻², 10⁻⁴, 10⁻⁶ and 10⁻⁸. Set the factor to 100, four tubes and 1,000 µL per tube, and the calculator returns the same 10 µL into 990 µL step.
Two big steps. The University of Maryland MathBench module on serial dilution puts 1 mL into 99 mL twice, for a total dilution of 1:10,000; 300,000 cells become 3,000 after the first step and 30 after the second.
Total dilution factor of a series
To find the total dilution, multiply the factors of all steps. Five tenfold steps give 10⁵ (1:100,000); three twofold steps give 2 × 2 × 2 = 8. To get back to the original concentration from a count in a later tube, multiply by that total factor. In a plate count, for example, the colonies per mL measured in a diluted tube are multiplied by the total dilution to estimate the original sample.
Tips for accurate serial dilutions
- Mix thoroughly before each transfer, otherwise the next step starts from an uneven solution.
- Mind the final tube volume. In this layout every tube holds the full volume until you take the transfer out of it, so all tubes except the last end up slightly smaller. Make a little extra if you need the full volume in each tube.
- Choose the factor from the range you need. A twofold series halves the concentration at each tube, giving small steps; a tenfold series covers a wide range in few tubes.
If you only need a single dilution, the main dilution calculator is faster. To convert between ratio notation and a factor, see the dilution factor calculator.