| Tube | Total dilution | Concentration |
|---|
How to make a serial dilution
A serial dilution is a stepwise dilution where each tube is diluted from the one before it by the same factor, letting you span many orders of magnitude with small, accurate steps. For a 10-fold series you move 1 part sample into 9 parts diluent at each step. The concentration in tube n is the stock divided by the dilution factor raised to the n, so a 10-fold series starting at 1,000,000 gives 100,000 then 10,000 then 1,000, and so on. To prepare each tube, add the transfer volume of sample to the diluent volume: for a chosen final volume V and factor F, transfer V/F of sample into V − V/F of diluent, mixing well before moving to the next tube.
Concentration units carry through unchanged. Use cells/mL, M, µg/mL, whatever your stock is in.
Related tools: Single dilution (C₁V₁) · Antibody dilution calculator · all biochem tools.
Worked example 1: the default 6-tube series
Stock = 1,000,000 (any unit), 10-fold dilution, 6 tubes, 1000 µL final volume per tube, the tool's defaults.
| Tube | Total dilution | Concentration |
|---|---|---|
| 1 | 1 : 10 | 100,000 |
| 2 | 1 : 100 | 10,000 |
| 3 | 1 : 1,000 | 1,000 |
| 4 | 1 : 10,000 | 100 |
| 5 | 1 : 100,000 | 10 |
| 6 | 1 : 1,000,000 | 1 |
Prep per tube: transfer = V/F = 1000/10 = 100 µL of sample from the previous tube into 1000 − 100 = 900 µL of fresh diluent, mix, then repeat for the next tube.
Worked example 2: diluting bacteria to a countable plate
A liquid culture is at 1×10⁹ CFU/mL (colony-forming units per mL), far too concentrated to count directly. The lab wants a plate with 30-300 colonies, the conventionally "countable" range, by plating 100 µL (0.1 mL) from some tube in a 10-fold dilution series.
100 colonies sits right in the middle of the 30-300 countable range, so tube 6 is the one worth plating. In practice a lab usually plates several neighboring tubes (5, 6, and 7) since the exact starting concentration isn't always known ahead of time.
FAQ
Why use a serial dilution instead of one big dilution?
Diluting a million-fold in one step means pipetting an immeasurably small volume of stock. Repeated, larger, accurate transfers reach the same overall dilution using volumes a standard pipette can actually measure.
What's the countable range for a bacterial plate count?
By convention, 30-300 colonies is considered statistically reliable, fewer has too much random error, more tend to overlap and become impossible to distinguish. Serial dilution finds which tube, once plated, lands there.
Does the order matter if I use different folds per step?
No, total dilution after several steps is just the product of each step's factor, regardless of order. This tool assumes the same factor every step (the most common setup), but the underlying math works either way.
Why mix between each dilution step?
Each transfer needs to represent the true, uniform concentration of the previous tube. Without thorough mixing, the sample can stay stratified, so the small volume pipetted forward doesn't reflect that tube's real concentration, and the error compounds down the series.
Practice problems
1. A stock at 1×10⁵ is serially diluted 5-fold across 4 tubes. What's the concentration in each tube?
Show answer
2. Why would a lab choose a 5-fold dilution series instead of the more common 10-fold?