How to Make a Serial Dilution

A serial dilution makes a series of step-wise dilutions, each one using the previous tube as its stock. Instead of one huge, hard-to-pipette step, you repeat a small comfortable dilution several times, and the factors multiply. This guide shows how to choose the dilution factor and the number of tubes, gives a full tube-by-tube concentration table for a common series, states the transfer and diluent volumes precisely, and covers the pipetting mistakes that turn a clean geometric series into noise. The serial dilution calculator prints every tube’s concentration and the overall factor.

What does a 1/10 serial dilution mean?

A 1/10 (or 1:10) serial dilution reduces the concentration tenfold at each step. Tube 1 is 1/10 of the stock, tube 2 is 1/100, tube 3 is 1/1000, and so on. Each step has the same dilution factor of 10, and the overall factor after n steps is 10ⁿ:

• After 1 step: DF = 10 (10⁻¹)
• After 2 steps: DF = 100 (10⁻²)
• After 3 steps: DF = 1000 (10⁻³)

This exponential growth is the whole point — three small steps reach a thousand-fold dilution that would otherwise need you to pipette 1 µL into 999 µL accurately.

Why use a serial dilution instead of one big step?

Two reasons. First, accuracy: a single 1:1000 dilution of 100 mL means pipetting 0.1 mL, which is fine, but a 1:1000 dilution of 1 mL means 1 µL — at the edge of what a pipette can deliver reliably. Serial steps keep every transfer in a comfortable range. Second, coverage: a series produces a range of concentrations in one go, which is exactly what a standard curve, a minimum inhibitory concentration (MIC) assay, or a titre needs. You get many data points, not one.

How do I plan a serial dilution step by step?

1. Choose the step factor. 1:10 (or 1:2, 1:5) are standard; pick one that spans your range in a practical number of tubes.
2. Decide the number of tubes. n tubes reach an overall factor of (step factor)ⁿ.
3. Fix the volumes. Each tube holds diluent volume V_d, and you transfer volume V_t into it. The step factor is (V_t + V_d) / V_t.
4. Make the series. Add diluent to every tube, transfer stock into tube 1, mix, transfer from tube 1 to tube 2, mix, and repeat — always discarding or holding back the final excess.
5. Calculate each concentration. C_tube_k = C_stock ÷ (step factor)ᵏ.

For a 1:10 step, V_t / (V_t + V_d) = 1/10, so if you transfer 1 mL you need 9 mL of diluent in each tube (1 + 9 = 10). For a 1 mL final volume in a 1:10 step, transfer 100 µL into 900 µL.

Tube-by-tube table: a 1:10 series from 1 M

Here is a complete 1:10 series starting from a 1 M stock, transferring 100 µL into 900 µL at each step:

Tube	Transfer	Diluent	Step factor	Cumulative factor	Concentration
1	100 µL stock	900 µL	1:10	10	100 mM
2	100 µL of tube 1	900 µL	1:10	100	10 mM
3	100 µL of tube 2	900 µL	1:10	1000	1 mM
4	100 µL of tube 3	900 µL	1:10	10,000	100 µM
5	100 µL of tube 4	900 µL	1:10	100,000	10 µM

Each row is the previous concentration ÷ 10. The cumulative factor is what you divide the original stock by to get that tube’s value. The serial dilution calculator generates this table for any factor and starting concentration.

How much do I transfer and how much diluent do I add?

The transfer-to-diluent split is set by the step factor. For a factor of n (1:n), the transfer is one part and the diluent is n − 1 parts:

Step factor	Transfer : diluent	Example for 1 mL final
1:2	1 : 1	500 µL + 500 µL
1:5	1 : 4	200 µL + 800 µL
1:10	1 : 9	100 µL + 900 µL
1:100	1 : 99	10 µL + 990 µL

A frequent slip is adding the transfer to the full final volume of diluent (100 µL into 1000 µL = 1:11, not 1:10). The diluent is final volume minus transfer.

How many steps do I need to reach a target?

If you need an overall dilution factor DF with a step factor s, the number of steps is:

n = log(DF) / log(s)

To reach a 1:1,000,000 dilution with 1:10 steps: n = log(10⁶)/log(10) = 6 tubes. With 1:100 steps: n = log(10⁶)/log(100) = 3 tubes. Fewer, larger steps save tubes but demand more precise pipetting at each; more, smaller steps are gentler on accuracy but accumulate error. The calculator solves this in reverse — give it a target factor and step, and it tells you how many tubes.

Common mistakes and troubleshooting

• Not mixing between steps. Each transfer must come from a homogenous tube; skipping the mix makes every downstream concentration wrong and erratic.
• Reusing the same tip. Carry-over of a concentrated solution on a used tip inflates the next tube. Change tips at every transfer.
• Adding transfer to full diluent volume. 100 µL into 1000 µL is 1:11. Diluent is final minus transfer.
• Pipetting below the accurate range. Transfers under ~2 µL are unreliable; increase the final volume or use a smaller step factor.
• Cumulative error from many steps. Each step adds a little imprecision, so a very long series drifts; prefer fewer, larger steps where accuracy allows.
• Miscounting the starting tube. Tube 1 is already one step from the stock, not the stock itself.

If the measured concentrations do not fall on a clean geometric progression, suspect mixing or tip carry-over before blaming the calculation.

Worked example: an MIC or standard curve series

You need eight doubling dilutions from 256 µg/mL (a 1:2 series): 256, 128, 64, 32, 16, 8, 4, 2 µg/mL. Transfer 500 µL into 500 µL of diluent at each step. The overall factor across the series is 2⁷ = 128 from first to last tube. This is exactly the geometric spacing a dose-response or MIC readout wants, and it comes out of repeated 1:2 steps rather than any single large dilution.

Where does this fit in the wider workflow?

A serial dilution is a chain of the single-step math in how to calculate a dilution and the C1V1 equation. You prepare the starting stock with the stock solution calculator, confirm single-step factors with the dilution calculator, and plan the whole series with the serial dilution calculator. These tools sit together in the laboratory calculators collection.