Serial Dilution Calculator

The Serial Dilution Calculator computes the concentration at each step of a sequential dilution series, given the initial concentration, the dilution factor applied at each step, and the total number of steps. It returns the concentration after every step (with a bar chart of the series), the total dilution factor, and the final concentration after all steps are complete.

A serial dilution is a chain of equal dilution steps where each step uses the output of the previous step as its input. If a stock solution at 1 mol/L is diluted 10-fold at each step, the concentrations form a geometric sequence: 10⁻¹, 10⁻², 10⁻³, 10⁻⁴ mol/L, and so on. The defining feature of a serial dilution is this geometric progression — each step reduces concentration by the same factor, making it straightforward to calculate any intermediate or final concentration using the formula Cₙ = C₀ ÷ DF^n.

Serial dilutions are preferred over single large dilutions for two practical reasons. First, pipetting accuracy: a 10⁶-fold dilution requires measuring 1 µL of sample into 999,999 µL of diluent — a practically impossible single step. Six sequential 10-fold dilutions achieve the same result with each step pipetting a manageable 1:10 ratio. Second, coverage: plating three consecutive dilutions from a serial series guarantees that at least one will fall within the countable range (30–300 colonies) even when the original sample concentration is unknown.

In Indian microbiological food and water testing, 10-fold serial dilutions (log dilutions) are the standard procedure. The Food Safety and Standards Authority of India (FSSAI) and the Bureau of Indian Standards (BIS) specify serial dilution protocols in IS 5403 (Total Plate Count), IS 1622 (Bacteriological Examination of Water), and IS 1479 (Methods of Test for Dairy Products). Every accredited testing lab in India performs serial dilutions as part of daily testing workflow.

For a single dilution step, use the Solution Dilution Calculator. For the dilution factor between two concentrations, see the Dilution Factor Calculator.

1. Determine your starting concentration — measure or calculate C₀ in mol/L, CFU/mL, mg/mL, or any linear unit. Use the Concentration Calculator if starting from a mass and volume.
2. Enter Initial Concentration (C₀) — type the starting concentration into the Initial Concentration (C₀) field (unit: mol/L).
3. Enter Dilution Factor per Step — type the dilution factor applied at each step into the Dilution Factor per Step field. For a 10-fold dilution series, enter 10. For a 2-fold series, enter 2.
4. Enter Number of Dilution Steps — type the total number of sequential dilution steps into the Number of Dilution Steps field (range 1–12).
5. Read Final Concentration — the highlighted output gives the concentration after all steps. Confirm this is in the expected range for your assay.
6. Check step 1–3 concentrations — verify intermediate concentrations and use the bar chart to visualise the series. For back-calculation: original concentration = measured value × Total Dilution Factor.