What is a serial dilution?

A serial dilution is a stepwise process of diluting a sample by a constant factor at each step. For example, a 1:10 serial dilution takes 1 part of the previous dilution and adds 9 parts diluent, reducing the concentration by 10-fold at every step.

What dilution factor should I use?

The most common dilution factor in microbiology is 10 (1:10). Factors of 2 (1:2) and 100 (1:100) are also used depending on the expected concentration range. Choose a factor that will bring your sample into the countable range of 30–300 colonies per plate.

How do I calculate CFU/mL from plate counts?

Use the formula: CFU/mL = colony count / (dilution factor × volume plated in mL). For example, 150 colonies on a plate inoculated with 0.1 mL of a 10⁻⁶ dilution = 150 / (0.000001 × 0.1) = 1.5 × 10⁹ CFU/mL.

What is the countable range for a plate?

The standard acceptable range for viable plate counts is 30–300 colonies per plate (some guidelines use 25–250). Below 30, counts are statistically unreliable; above 300, colonies merge and are uncountable.

Why do concentrations drop so fast?

Exponential math: with a 1:10 factor, each step reduces concentration by 10×. After 6 steps you are at 10⁻⁶ of the original. This is intentional — it spreads bacteria far enough apart to form distinct, countable colonies.

Can I use this for non-bacterial samples?

Yes. The same math applies to viruses (PFU/mL), yeast, mold spores, or any particle that requires enumeration. Simply substitute your concentration unit.

Serial Dilution Calculator (Microbiology)

Calculate concentration, dilution factor, and estimated CFU counts at each step of a serial dilution series for microbiology lab work.

Dilution Series Inputs

Stock Concentration (CFU/mL)

Enter the initial sample concentration (e.g. 1e9)

Dilution Factor per Step

Number of Dilution Steps

Maximum 12 steps

Concentration Unit Label

Dilution Series Results

Configure inputs and click Calculate.

Step	Notation	Cumulative Factor	Concentration (CFU/mL)	Scientific
Enter values and click Calculate to see the dilution table.

CFU Back-Calculator

Count colonies on a plate, enter the dilution step used and volume plated to recover original concentration.

Colony Count

Dilution Step Used

From table above (step #)

Volume Plated (mL)

Quick Reference: Countable Range

< 30

Too few — statistically unreliable

30 – 300

Ideal countable range

> 300

Too many — colonies overlap

Summary

Calculate concentration, dilution factor, and estimated CFU counts at each step of a serial dilution series for microbiology lab work.

How it works

Enter the initial stock concentration in CFU/mL (or any consistent unit).
Set the dilution factor for each step (e.g., 10 for a 1:10 dilution).
Choose the number of dilution steps (up to 12).
Optionally enter colony counts from plates to back-calculate original concentration.
Click Calculate to see the full dilution table with concentrations and dilution factors.
Use the CFU back-calculator at the bottom to estimate original sample concentration from plate counts.

Use cases

Determine microbial load in food safety testing.
Plan dilution series before plating bacterial cultures.
Calculate CFU/mL from colony counts on agar plates.
Teaching serial dilution concepts in microbiology lab courses.
Verify dilution math before preparing antibiotic susceptibility assays.
Estimate viable cell counts in fermentation monitoring.
Prepare standards for calibration curves in enumeration assays.
Calculate dilutions for environmental water quality testing.

Frequently Asked Questions

Last updated: 2026-09-26 · Reviewed by Nham Vu