What is a serial dilution?

A serial dilution is a stepwise series where each tube is diluted by the same factor from the previous one. Repeating a 1:10 dilution five times gives concentrations spanning five orders of magnitude, which is why the technique is used to count bacteria and titre antibodies.

How is the concentration at each step calculated?

The concentration after step n equals the stock concentration divided by the dilution factor raised to the power n. So a 1000 unit stock at a 1:10 dilution gives 100, 10, 1, and so on. The stock itself is step 0.

How much do I transfer at each step?

The transfer volume is the total tube volume divided by the dilution factor. For a 1:10 series with 1 mL tubes you move 0.1 mL forward into 0.9 mL of fresh diluent. Mix well before the next transfer.

Why change pipette tips between steps?

Residual liquid clinging to a reused tip carries concentrated sample forward and inflates the lower dilutions, which corrupts colony counts. Always use a fresh tip and vortex or pipette-mix each tube before the next transfer.

Can I use this for CFU plate counts?

Yes. Pick the tube whose plated dilution lands in the 30 to 300 colony range, multiply the count by the cumulative dilution factor for that tube, and divide by the plated volume to get CFU per mL of the original sample.

Serial Dilution Calculator

A serial dilution spreads a single stock across many tubes, each diluted by the same factor from the one before it. This calculator shows the concentration at every tube, the cumulative dilution factor, and the exact transfer and diluent volumes so your series is reproducible.

How it works

Each step divides the previous concentration by the same dilution factor, so the concentrations fall geometrically:

concentration[n] = stock / DF^n        (n = 0 is the undiluted stock)
cumulative DF[n]  = DF^n
transfer volume   = total volume / DF
diluent volume    = total volume − transfer volume

A 1:10 (DF = 10) series therefore produces concentrations of stock, stock/10, stock/100, stock/1000, and so on. For 1 mL tubes you move 1 / 10 = 0.1 mL forward into 0.9 mL of fresh diluent at every step.

Example and tips

Starting from a 1000 µg/mL stock with a dilution factor of 10 over five steps, the tubes read 1000, 100, 10, 1, 0.1, and 0.01 µg/mL. Mix each tube thoroughly and use a fresh pipette tip per step — carried-over residue from a reused tip is the most common cause of distorted dilution curves and over-counted plates.