Serial dilution GUIDE
How to plan a 10-fold serial dilution
Transfer and diluent volumes for a 10-fold serial dilution at any final tube volume, with a worked four-tube example and cumulative dilution tracking.
A 10-fold (1:10) series divides the concentration by ten at every step. It is the workhorse dilution scheme for plate counts, covering five or six orders of magnitude in four to six tubes. The planning arithmetic is the same at any final tube volume.
The volume relationship
For a step factor F and a chosen final tube volume V:
transfer volume = V ÷ F
diluent volume = V − transfer volumeWith F = 10 and V = 10 mL, that is 1 mL transferred into 9 mL of diluent. With F = 10 and V = 5 mL, it is 0.5 mL into 4.5 mL. Pick the final volume your tubes and pipettes handle comfortably — larger volumes reduce relative pipetting error, but waste diluent and sample.
Worked example: four tubes
Four 1:10 steps at 10 mL final volume each:
| Tube | Transfer | Into diluent | Cumulative factor | Fraction of original |
|---|---|---|---|---|
| 1 | 1 mL sample | 9 mL | 10 | 10⁻¹ |
| 2 | 1 mL from tube 1 | 9 mL | 100 | 10⁻² |
| 3 | 1 mL from tube 2 | 9 mL | 1,000 | 10⁻³ |
| 4 | 1 mL from tube 3 | 9 mL | 10,000 | 10⁻⁴ |
Plating 0.1 mL of tube 4 places 10⁻⁵ of the original sample on the plate. A count of 86 colonies there corresponds to 8.6 × 10⁵ CFU/mL — the calculation shown in how to calculate CFU/mL from colony counts. The serial dilution planner generates this table for any number of tubes.
Procedure notes
- Prepare all tubes before starting. Rack and label every tube, dispense all diluent, then begin transfers. Mid-series preparation invites timing and contamination errors.
- Mix every tube before the next transfer. Vortex or pipette up and down until the suspension is homogeneous. Poor mixing at tube 2 corrupts every tube after it, and no later step can fix it.
- Change tips between tubes. Residue on the outside of a tip carries concentrated sample forward and biases the series high.
- Watch the first transfer especially. The first transfer has the largest relative error impact — it scales everything downstream. Pre-wet the tip, touch it against the tube wall above the liquid line, and wait for full drainage before withdrawing it.
Choosing the final volume
Common final volumes are 9 mL (classic tube method) and 1 mL or 0.9 mL (microdilution plates). Smaller volumes save sample and reagents but demand smaller, less accurate pipetting; if your pipette’s reliable range starts at 10 µL, a series built on 100 µL transfers will carry visible volume scatter. Match the scheme to the equipment actually on the bench — and remember that a 9 mL tube receiving 1 mL and a 0.9 mL well receiving 0.1 mL are the same 1:10 step at different scale; the cumulative arithmetic does not care which you use.
Planning backwards from an expected concentration
The useful planning question is not “how many tubes?” but “what cumulative fraction puts me in the countable window?” The window holds N colonies when:
cumulative fraction ≈ target colonies ÷ (expected concentration × plated volume)Suppose you expect 10⁶ CFU/mL and plan to plate 0.1 mL. For a target of roughly 80 colonies: fraction ≈ 80 ÷ (10⁶ × 0.1) = 8 × 10⁻⁴ ≈ 10⁻³. That is three 10-fold steps, so plate from tube 3 — and plate tube 4 as well, since it costs one extra spread and covers an underestimate of the starting concentration. If the count on tube 3 lands above 300, tube 4 rescues the experiment; if both are TNTC, the series needs two more steps, which is cheaper to discover at the bench than after incubation.
The serial dilution planner performs this arithmetic in reverse too: enter the expected concentration and plated volume, and it reports the cumulative fraction to aim for alongside the tube-by-tube plan.
Mixed series
When the expected concentration is known roughly, combine schemes: three 10-fold steps to cross the bulk of the range, then two or three 2-fold steps to bracket the countable window finely. The planner supports per-step factors entered as a list, and tracks the running cumulative product across mixed steps exactly as for a uniform series — see dilution factor vs cumulative dilution for how the mixed product is formed.
Limitations
Each transfer carries a volumetric uncertainty that accumulates down the series. Larger transfer volumes reduce relative pipetting error; replicate series reveal whether your technique, rather than the biology, is driving the spread. The plan assumes perfect mixing and transfer accuracy — neither is guaranteed, so when precision matters, run the series in duplicate and compare the countable plates.
Frequently asked questions
How do I calculate transfer volumes for a 10-fold dilution?
Divide the final tube volume by 10 for the transfer, and subtract that from the final volume for the diluent. At 10 mL final volume: 1 mL transfer into 9 mL diluent. At 5 mL final volume: 0.5 mL into 4.5 mL. The ratio stays 1:9 regardless of scale.
How many 10-fold tubes do I need?
Enough that one tube falls in the countable window when plated. As a rough guide, a 10⁶ CFU/mL culture plated at 0.1 mL needs a cumulative fraction near 10⁻⁴ — four 10-fold steps — to land around 60–100 colonies. When unsure, plan one extra tube and plate two adjacent dilutions.
How do I track the cumulative dilution through the series?
Multiply the step factors as you go: 10, 100, 1,000, 10,000 after four 1:10 steps. The tube's fraction of the original is the reciprocal. The serial dilution planner prints this table for your exact setup, including mixed step factors.
Open the serial dilution planner ↗
More serial dilution guides
- How to plan a 2-fold serial dilutionTransfer and diluent volumes for a 2-fold (1:2) serial dilution, how cumulative factors grow more slowly than 10-fold series, and when to choose finer steps.
- Serial dilution factor vs cumulative dilutionThe difference between a single-step dilution factor and the cumulative dilution of a series, how to compute each, and how both enter the CFU calculation.
- Serial dilution: definition, principle, and purposeWhat a serial dilution is, the principle behind stepwise dilution, and why laboratories dilute in a series instead of one large dilution.