What is the CFU per mL formula?

CFU/mL = Colony Count ÷ (Dilution Factor × Volume Plated in mL). This is the universally accepted formula for calculating microbial concentration from serial dilution plating.

What dilution factor should I enter?

Enter the actual numeric value of the dilution. For a 10⁻³ dilution, enter 0.001. For a 10⁻⁶ dilution, enter 0.000001. The dilution factor equals the reciprocal of the fold-dilution.

Which plates give the most accurate count?

Plates with 30 to 300 colonies are considered statistically reliable (TNTC-free range). Plates outside this range may skew results. This calculator accepts any count, so verify plate validity before recording.

Why does my result appear in scientific notation?

Microbial concentrations can span many orders of magnitude, often reaching billions per mL. Scientific notation (e.g., 2.5 × 10⁸) makes these large numbers easier to read and compare.

Does the volume need to be in mL?

Yes. Enter the volume plated in milliliters. If you plated 100 µL, enter 0.1. If you plated 1 mL, enter 1.0.

Can I calculate CFU/g instead of CFU/mL?

Yes — the math is identical. If your original sample was weighed (e.g., 1 g diluted in 9 mL for a 0.1 dilution), apply the same formula. The result will be CFU per gram of the original sample weight.

How many bacteria are in 1 mL?

It depends on the sample. Drinking water may hold under 100 CFU/mL, while a dense overnight broth culture commonly reaches 1 × 10⁹ CFU/mL (about a billion cells). Plate a countable dilution (30–300 colonies) and use this calculator to get the exact figure.

CFU per mL Calculator

Calculate CFU/mL from plate colony count, dilution factor, and volume plated — standard microbiology formula.

Enter Plate Data

Fill in the three values from your dilution plating experiment.

Colony Count (colonies on plate)

Number of visible colonies counted on the agar plate.

Dilution Factor

Decimal value of dilution (10⁻³ = 0.001, 10⁻⁶ = 0.000001).

Volume Plated (mL)

Volume spread onto the plate (100 µL = 0.1 mL).

Result

Enter plate data and click Calculate.

Formula Reference

CFU/mL = Colony Count ÷ (Dilution Factor × Volume Plated)

Colony Count: Colonies visible on the agar plate (30–300 for TNTC-free accuracy)

Dilution Factor: Decimal form of sample dilution (10⁻³ = 0.001)

Volume Plated: mL spread on plate (100 µL = 0.1 mL)

Quick Examples

Summary

Calculate CFU/mL from plate colony count, dilution factor, and volume plated — standard microbiology formula.

How it works

Enter the number of colonies counted on your agar plate.
Enter the dilution factor (e.g., 0.001 for a 10⁻³ dilution).
Enter the volume plated in milliliters (typically 0.1 mL or 1 mL).
Click Calculate to compute CFU/mL using the standard formula.
Review the result in both standard and scientific notation.
Use the Reset button to clear all fields for a new calculation.

Use cases

Quantify bacterial concentration in food safety testing.
Determine microbial load in water quality analysis.
Calculate cell density for pharmaceutical microbiology QC.
Verify culture concentrations in academic microbiology labs.
Assess contamination levels in environmental sampling.
Calculate viable cell counts for probiotic research.
Standardize inocula for minimum inhibitory concentration (MIC) assays.
Document plate counts in clinical microbiology workflows.

Frequently Asked Questions

Last updated: 2026-09-22 · Reviewed by Nham Vu