How do you calculate a serial dilution?
Each point is the previous one divided by the dilution factor F, so point n has concentration top ÷ F^(n - 1). To end with the same volume V in every tube, put V of diluent in tubes 2 onwards, start tube 1 with V plus one transfer of the top-concentration solution, and move transfer = V ÷ (F - 1) down the series, mixing before each transfer.
Worked example: 8 points, threefold, from 10 µM, 100 µL per tube. Transfer = 100 ÷ (3 - 1) = 50 µL; tube 1 holds 150 µL at 10 µM; the series runs 10, 3.333, 1.111, 0.3704, 0.1235, 0.04115, 0.01372 and 0.004572 µM.
How do you choose the dilution factor?
If you know the range, let the calculator find it: F = (top ÷ bottom)^(1 ÷ (points - 1)). Going from 10 µM to 1 nM in 5 points is a 10-fold series. For dose-response curves, the range has to bracket the response: Sebaugh (2011) recommends at least two concentrations on each plateau for a reliable relative IC50 or EC50. Half-log steps (a factor of about 3.162) are a common compromise between range and resolution.
Why use a serial dilution instead of one large dilution?
A single 10,000-fold dilution needs a very small stock volume that is hard to pipette accurately. A series of smaller steps keeps every transfer in a range a pipette handles well, and it produces all the intermediate concentrations a dose-response experiment needs. The calculator warns if the transfer falls under 1 µL. To place the series on a plate, use the Plate Layout Planner.
Frequently asked questions
What is a 10-fold serial dilution?
A series where each tube is one tenth the concentration of the one before, made by moving one part into nine parts of diluent at each step.
What is a 2-fold serial dilution?
Each step halves the concentration: equal volumes of the previous solution and diluent are mixed at each step.
What is a half-log dilution?
A series with a factor of about 3.162 (the square root of 10), so two steps make a 10-fold change. It gives more points per decade than a 10-fold series.
How many points should a dose-response series have?
Enough to define the top and bottom of the curve. Sebaugh (2011) recommends at least two concentrations beyond each bend of the curve. Eight fits a 96-well column and twelve a row.
Why does the last tube have extra volume?
Because it receives a transfer but does not pass one on. Remove one transfer volume from it if every well must hold the same volume.
Is anything I enter saved or sent?
No. The calculator runs in your browser. Nothing you type is saved, sent to BioBridgeX or added to the page address, and analytics never see your numbers.
Related free tools
- Dilution Calculator: C1V1 = C2V2 for any unknown, with stock and diluent volumes and the dilution factor.
- 96 and 384 Well Plate Layout Planner: Lay out compounds, concentrations, replicates and controls on 96- or 384-well plates and count the plates.
- In Vitro Assay and Screening Planner: Assay type, cell models, compounds, concentration series, controls, replicates and readout as an organized assay plan.
- All lab calculators and every free tool on BioBridgeX.
Sources
- Sebaugh JL. Guidelines for accurate EC50/IC50 estimation. Pharmaceutical Statistics 2011;10(2):128-134
- IUPAC Compendium of Chemical Terminology (Gold Book): amount concentration
Check every result before use. Formulas assume ideal conditions; follow your validated lab procedures and the reagent supplier's instructions. Facts on this page last checked 27 October 2026.
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