A colony-forming unit (CFU) represents a single viable microbial cell or a clump of cells capable of forming one visible colony on an agar plate. CFU/mL is used to express the concentration of living microorganisms in a liquid sample.

How do I express the dilution factor?

Enter the dilution factor as a decimal (e.g. 0.001 for 10⁻³) or use the exponent field (e.g. -3). A 1:1000 dilution = dilution factor of 0.001. For a series of 1:10 steps, a three-step series gives 10⁻³.

What colony count range is statistically valid?

Standard microbiological practice uses plates with 30–300 colonies for calculation. Fewer than 30 is too few for statistical reliability; more than 300 is too dense to count accurately (TNTC — too numerous to count).

Can I calculate from multiple plates?

Yes. Use the multi-plate mode to enter counts from several dilutions. The calculator computes CFU/mL for each valid plate (30–300 colonies) and reports their arithmetic mean.

What volume should I plate?

Spread plates typically use 0.1 mL; pour plates use 1 mL. Enter the exact volume you plated. The calculator accounts for it so results are correct regardless of technique.

Dilution CFU Calculator

Enter colony count, dilution factor, and plated volume to calculate the CFU/mL of your original sample.

Single Plate Calculation

Calculate CFU/mL from one plate count.

Colony count (integer)

Dilution factor entry mode

Decimal (e.g. 0.001) Exponent (e.g. −3)

Volume plated (mL)

Spread plate = 0.1 mL | Pour plate = 1 mL

Multi-Plate Average

Average 2–4 plates from different dilutions (30–300 colony rule).

Multi-Plate Average Result

Average CFU / mL

Average CFU / L

Colonies	Dilution	Volume (mL)	CFU/mL	Valid?

Fill in the fields on the left and click Calculate.

Serial Dilution Reference

Dilution steps (1:10 each)	Dilution factor	Decimal
1	10⁻¹	0.1
2	10⁻²	0.01
3	10⁻³	0.001
4	10⁻⁴	0.0001
5	10⁻⁵	0.00001
6	10⁻⁶	0.000001

Summary

The Dilution CFU Calculator computes colony-forming units per mL (CFU/mL) from standard plate count data. It applies the formula CFU/mL = colonies ÷ (dilution factor × volume plated) and supports both single-plate and multi-plate averaging. Use it to quickly convert raw plate counts from serial dilution experiments into original sample concentrations.

How it works

Enter the number of colonies counted on your agar plate.
Enter the dilution factor used (e.g. 0.001 for a 10⁻³ dilution, or enter the exponent directly).
Enter the volume plated in mL (typically 0.1 mL for spread plates or 1 mL for pour plates).
The calculator applies the formula: CFU/mL = colonies ÷ (dilution factor × volume plated).
Results are shown in CFU/mL and CFU/L, with scientific notation for large values.

Use cases

Determine bacterial load in a broth culture after serial dilution plating.
Quantify microbial contamination in food or water samples.
Calculate viable cell counts for inoculation in fermentation experiments.
Verify starter culture concentrations in dairy or beverage production.
Monitor growth curves by tracking CFU/mL over time during batch culture.
Assess antibiotic efficacy by comparing pre- and post-treatment plate counts.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu