Serial dilution: what is in each tube?
Type the starting concentration, the dilution factor per step, the number of steps and the volume of each tube. The serial dilution calculator shows the concentration in every tube and how much sample and diluent to pipette.
- Concentration in the last tube
- 1
After 6 steps of 1:10, the last tube holds 1, a total dilution of 1,000,000.
- Total dilution factor
- 1,000,000
- Sample to move each step
- 1 mL
- Diluent in each tube
- 9 mL
- Each tube
- Tube 1: 1:10 of the stock, 100000; Tube 2: 1:100 of the stock, 10000; Tube 3: 1:1000 of the stock, 1000; Tube 4: 1:10000 of the stock, 100; Tube 5: 1:100000 of the stock, 10; Tube 6: 1:1000000 of the stock, 1
Concentration in the last tube: 1. After 6 steps of 1:10, the last tube holds 1, a total dilution of 1,000,000.
What is in each tube?
How to calculate
Plans a serial dilution: the concentration in every tube, the total dilution factor, and how much sample and diluent go in each tube for a chosen factor per step and final volume.
Example with the default inputs (Starting concentration 1,000,000, Dilution factor per step 10, Number of dilution steps 6, Final volume in each tube 10 mL): After 6 steps of 1:10, the last tube holds 1, a total dilution of 1,000,000.
Method: Tube k = C₀ ÷ Fᵏ; total dilution factor = Fⁿ; sample moved each step = V ÷ F; diluent in each tube = V − V ÷ F.
- Every step uses the same dilution factor F and the same final volume V.
- Each tube is mixed well before its sample is moved on, and volumes add up.
- The concentration keeps the unit of the starting concentration (M, mg/mL, cells/mL).
Worked examples
Each example is checked against the calculator on every build.
- Starting concentration 1,000,000, Dilution factor per step 10, Number of dilution steps 6, Final volume in each tube 10 mL gives Concentration in the last tube 1, Total dilution factor 1,000,000, Sample to move each step 1 mL, Diluent in each tube 9 mL.Source: OpenStax, Microbiology, section 9.1 How Microbes Grow (1.0 mL of culture into 9.0 mL of broth is a dilution factor of 10, or 1:10; a second 1.0 mL into 9.0 mL makes 1:100 of the original culture), CC BY 4.0, https://openstax.org/books/microbiology/pages/9-1-how-microbes-grow (retrieved 2026-10-03)
- Starting concentration 1, Dilution factor per step 10, Number of dilution steps 2, Final volume in each tube 10 mL gives Concentration in the last tube 0.01, Total dilution factor 100, Sample to move each step 1 mL, Diluent in each tube 9 mL, Each tube Tube 1: 1:10 of the stock, 0.1; Tube 2: 1:100 of the stock, 0.01.Source: OpenStax, Microbiology, section 9.1 How Microbes Grow (1.0 mL of culture into 9.0 mL of broth is a dilution factor of 10, or 1:10; a second 1.0 mL into 9.0 mL makes 1:100 of the original culture), CC BY 4.0, https://openstax.org/books/microbiology/pages/9-1-how-microbes-grow (retrieved 2026-10-03)
- Starting concentration 2, Dilution factor per step 2, Number of dilution steps 5, Final volume in each tube 1 mL gives Concentration in the last tube 0.0625, Total dilution factor 32, Sample to move each step 0.5 mL, Diluent in each tube 0.5 mL.Source: OpenStax, Microbiology, section 9.1 How Microbes Grow (1.0 mL of culture into 9.0 mL of broth is a dilution factor of 10, or 1:10; a second 1.0 mL into 9.0 mL makes 1:100 of the original culture), CC BY 4.0, https://openstax.org/books/microbiology/pages/9-1-how-microbes-grow (retrieved 2026-10-03)
How it works
Each step takes a volume of the tube before it and makes it up to the same final volume with diluent. With a starting concentration C₀, a dilution factor F per step, n steps, and a final volume V in each tube:
- Concentration in tube k = C₀ ÷ Fᵏ, for k = 1 to n. The last tube holds C₀ ÷ Fⁿ.
- Total dilution factor = Fⁿ. Tube k is 1:Fᵏ of the stock.
- Sample to move each step = V ÷ F.
- Diluent in each tube = V − V ÷ F.
Rules. The starting concentration is above 0 and at most 10³⁰, in any unit; the answer keeps that unit. The factor is above 1 and at most 1,000,000. Steps are whole numbers from 1 to 30. The final volume is above 0 and at most 1,000 L, in mL, L or cm³. When a concentration or factor is too small or too large to show as a number (below about 2.2 × 10⁻³⁰⁸ or above about 1.8 × 10³⁰⁸), there is no answer and the page says so.
Exact arithmetic. Typed numbers are exact decimals. The calculator divides and multiplies them exactly and rounds once, for display.
Output format. Concentrations and the total factor show 6 significant figures. Volumes show in mL (or the unit you pick) with 6 significant figures. The list of tubes writes each tube as Tube k: 1:Fᵏ of the stock, C₀ ÷ Fᵏ, numbers with up to 6 significant figures, tubes separated by ; .
Worked examples by hand
A ten-fold series of 6 tubes, 10 mL each, from 1,000,000 cells per mL. Total factor = 10⁶ = 1,000,000. Last tube = 1,000,000 ÷ 1,000,000 = 1 cell per mL. Sample = 10 ÷ 10 = 1 mL; diluent = 10 − 1 = 9 mL.
OpenStax’s two tubes. 1 mL of culture into 9 mL of broth is 1:10; 1 mL of that into 9 mL more is 1:100. From a stock of 1, tube 1 holds 0.1 and tube 2 holds 0.01; total factor 100.
A two-fold series of 5 tubes, 1 mL each, from 2 mg/mL. Total factor = 2⁵ = 32. Last tube = 2 ÷ 32 = 0.0625 mg/mL. Sample = 1 ÷ 2 = 0.5 mL; diluent = 0.5 mL.
Other questions people ask
What is a serial dilution?
A series of dilutions in which each tube is made from the tube before it, not from the stock. A 1:10 series takes 1 mL of the stock into 9 mL of diluent, then 1 mL of that tube into 9 mL more, and so on. Each tube is ten times weaker than the last.
How do I work out the total dilution factor?
Multiply the factors of the steps. Six 1:10 steps give 10 × 10 × 10 × 10 × 10 × 10 = 10⁶, so the last tube is 1:1,000,000 of the stock. With the same factor F in each of n steps, the total is Fⁿ.
How much sample and diluent go in each tube?
Sample = final volume ÷ factor; diluent = final volume − sample. For 10 mL tubes in a 1:10 series that is 1 mL of sample and 9 mL of diluent. For a two-fold series it is half and half.
What is the difference between 1:10 and 1 to 10?
In this calculator 1:10 means 1 part sample in 10 parts total (1 mL in a final 10 mL). Some labs write 1:9 for 1 part sample to 9 parts diluent. Both describe the same tube; check which one your protocol uses.
Why use a serial dilution instead of one big dilution?
A single 1:1,000,000 dilution would need 1 µL in a litre, which is hard to measure well. Small steps keep each pipetting volume easy to measure, and the series gives a range of concentrations, such as plates of 30 to 300 colonies in a plate count.
Does the unit of the concentration matter?
No. The result keeps the unit you type the stock in: molar, mg/mL, cells/mL or colony-forming units per mL. Only the volumes need a unit.