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Serial Dilution Calculator
In short: Plan an n-step serial dilution: the concentration in every tube and how much to transfer and how much diluent to add, as a printable table. The reverse calculator finds the factor or number of steps to reach a target. It is free, works in your web browser and needs no download.
For a single dilution from a stock, the C1V1 = C2V2 dilution calculator is quicker. This page is for a series of tubes: dose–response curves, standard curves, MIC plates and cell or colony counts.
Reverse: what factor or how many steps?
Serial dilution formulas
C_n = C_0 / F^n total dilution after n steps = F^n transfer = V / F diluent = V − V / F
factor for n steps: F = (C_0 / C_target)^(1/n) steps for factor F: n = log(C_0 / C_target) / log F (round up)
equal volume left in every tube: diluent = V, transfer = V / (F − 1), discard V / (F − 1) from the last tube
F is the dilution factor of one step: 1 part sample in F parts total. 1:10 means F = 10 (1 part + 9 parts diluent), and a two-fold series has F = 2. A half-log series uses F = √10 ≈ 3.162, so every second tube is ten times weaker.
| Series | Factor | Transfer + diluent for 1 mL | Typical use |
|---|---|---|---|
| Ten-fold | 10 | 100 µL + 900 µL | Bacterial and cell counts, wide concentration ranges |
| Two-fold | 2 | 500 µL + 500 µL | MIC tests, antibody titres, ELISA standards |
| Half-log | 3.162 | 316.2 µL + 683.8 µL | Dose–response (IC₅₀/EC₅₀) curves |
| Five-fold | 5 | 200 µL + 800 µL | Standard curves |
How to do a serial dilution
- Enter the stock concentration, the factor per step, the number of tubes and the final volume per tube.
- Label the tubes and add the diluent volume from the table to each one.
- Transfer the stated volume of stock into tube 1 and mix well (pipette up and down or vortex).
- With a fresh tip, move the same volume from tube 1 to tube 2, mix, and carry on down the series. Print the table and tick each tube as you go.
Limitations
- The calculation assumes perfect pipetting and mixing. Errors multiply down the series: a 2% error per step is about 10% after five steps.
- Transfer volumes under about 2 µL are hard to pipette accurately; the calculator warns when you go below that.
- Volumes are assumed additive, which is right for dilute aqueous solutions but not for mixing, for example, ethanol and water.
- Molar ↔ mass conversion needs the molar mass of the solute. Cells, CFU, % and U/mL are scaled only in their own unit.
Making the stock first? Work out the grams for a buffer of any pH, or the mass for a solution of given molarity.
Buffer preparation calculatorFrequently asked questions
How do you calculate a serial dilution?
The concentration in tube n is C0 / F^n, where F is the dilution factor per step. For each tube, transfer V/F from the previous tube into V − V/F of diluent, where V is the final volume. 1 mM diluted 1:10 five times gives 1 mM / 100,000 = 10 nM.
What does 1:10 mean in a dilution?
Here 1:10 means 1 part sample in 10 parts total (1 part sample plus 9 parts diluent), a dilution factor of 10. Some fields use 1:9 for the same thing. Check which convention your protocol uses.
How do I find the dilution factor I need?
For n equal steps from C0 to a target C, the factor is (C0/C)^(1/n). Going from 1 mM to 1 µM in 3 steps needs a factor of 10. The reverse calculator also gives the number of steps for a fixed factor.
Why is the last tube's volume larger than the others?
In a standard series every tube except the last loses a transfer volume to the next tube. Tick Equal volume left in every tube: each tube then gets V of diluent, you transfer V/(F−1), and you discard that volume from the last tube.
Can I use mg/mL or cells/mL?
Yes. The calculator works with M, mM, µM, nM, pM, mg/mL, µg/mL, ng/mL, pg/mL, cells/mL, CFU/mL, copies/µL, % and U/mL. With a molar mass it also converts between molar and mass units.
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