Serial Dilution Calculator

What a Serial Dilution Is

A serial dilution is a chain of dilutions, each one made from the tube before it, using the same dilution factor at every step so the concentration drops predictably and repeatedly. Wikipedia's serial dilution entry distinguishes this from a single dilution: diluting a stock solution once, in one step, gets you to one target concentration, while a serial dilution builds an entire ladder of concentrations in one pass, exactly what a microbiology lab needs to find a countable colony range, or what a pharmacology lab needs for a dose-response curve.

This calculator covers both halves of that workflow: building the dilution series itself, tube by tube, and working backward from a colony count on one of those plates to the concentration in the original, undiluted sample.

How to Calculate Each Step's Concentration

The concentration after n steps is C_n = C₀ ÷ Dⁿ, where C₀ is the starting concentration and D is the per-step dilution factor. Omni Calculator's serial dilution tool walks through the same exponential relationship: a 10-fold series starting at 1,000,000 CFU/mL gives 100,000 at tube 1, 10,000 at tube 2, 1,000 at tube 3, and so on, each tube exactly one factor of ten below the last. Because the exponent compounds, small changes to the dilution factor produce very different concentrations by the fourth or fifth tube, which is why getting that factor right at every transfer matters more than it might first appear.

Choosing a Dilution Factor: Two-Fold, Ten-Fold, Half-Log

Ten-fold (1:10) dilutions are the standard for microbiology plate counts because each step moves a full order of magnitude, quickly spanning a wide concentration range in just a few tubes. Microbe Notes' serial dilution guide notes that two-fold (1:2) and half-log (1:3.162) series are more common in pharmacology and enzyme assays, where a gentler step size gives finer resolution across a dose-response curve rather than jumping a full decade at once. Our calculator includes all three as presets, plus a custom factor for anything else, such as a 1:5 or 1:3 series some protocols call for.

Preparing Each Tube: Transfer Volume and Diluent

In practice, a dilution factor is set by how much sample you transfer relative to how much diluent it lands in, transferring 1 mL into 9 mL of diluent gives a 1:10 dilution, since the 1 mL sample now sits in a 10 mL total volume. AAT Bioquest's serial dilution planner generates exactly this kind of transfer-and-diluent protocol table for a full series, and our calculator does the same, showing the diluent volume needed for every tube once you set a transfer volume and dilution factor.

Serial Dilution in Microbiology: Counting Colonies

The reason microbiologists run a whole dilution series rather than a single dilution is that they don't know the sample's starting concentration in advance, plating an undiluted or lightly diluted sample usually produces a lawn of overlapping colonies too dense to count. A frequently asked Quora question on back-calculating bacterial concentration reflects how often this trips people up: you need a plate landing in the countable range, typically 30 to 300 colonies, before the count is statistically reliable enough to work backward from.

Back-Calculating CFU/mL from a Plate Count

Once you have a countable plate, CFU/mL = colonies counted × the plate's dilution factor ÷ the volume plated in mL. Dilutions Calculator's serial dilution calculation table guide works through the same example our calculator uses by default: 150 colonies on a plate from the 1-in-10,000 dilution, with 0.1 mL plated, works out to 150 × 10,000 ÷ 0.1, or 15,000,000 CFU/mL in the original sample. If you only need to prepare one single dilution step rather than a full series, our Dilution Calculator handles that C1V1=C2V2 case directly.

Common Mistakes in Serial Dilution

The most common error is failing to mix each tube thoroughly before transferring from it to the next, an unmixed sample carries an inconsistent concentration at the point you draw from it, throwing off every downstream dilution in the chain. ConductScience's serial dilution calculator guidance also flags reusing the same pipette tip across dilutions without changing it as a carryover risk, transferring trace concentrated sample into what should be a much more dilute tube. Using a fresh tip per transfer, and vortexing or inverting each tube several times before drawing the next sample, are the two habits that fix most inconsistent dilution series. Once you have a final concentration figure, our Concentration Calculator converts it between molarity, percent, and ppm if your protocol needs a different unit.