Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Education
Input types: number
Output type: json
Sample coverage: 4
API ready: Yes

Overview

Calculate CFU/mL from a colony count, dilution exponent, and plated volume. The calculator applies CFU/mL = colonies × 10ⁿ ÷ plated volume in mL and interprets counts against the 30–300 countable range. Educational use only.

When to use

Convert colonies counted on a dilution plate into the estimated CFU/mL of the original culture.
Check whether a plate count falls within the standard 30–300 countable range.
Estimate total CFU when the original sample volume is also known.

How it works

1Enter the number of colonies counted on the incubated plate.
2Enter the dilution exponent n for the 10⁻ⁿ dilution used to seed the plate.
3Enter the plated volume in milliliters, such as 0.1 mL for 100 µL.
4The calculator returns CFU/mL, optional total CFU from the original sample volume, and count-quality guidance.

Use cases

Microbiology students can practice dilution-plate calculations with explicit colony, dilution, and volume inputs.
Laboratory planners can compare plate counts with the 30–300 countable range before selecting a dilution for repeat plating.
Researchers can estimate original culture concentration and optionally calculate total CFU for a stated sample volume.

Examples

1. Countable 10⁻⁶ dilution plate

Microbiology student

Background

A student spread 0.1 mL from a 10⁻⁶ dilution and counted 142 colonies after incubation.

Problem

Calculate the original culture concentration and confirm whether the plate count is usable.

How to use

Enter 142 for Colonies Counted, 6 for Dilution Exponent, 0.1 mL for Plated Volume, and leave Original Sample Volume at 0.

colonies: 142; dilutionExponent: 6; platedVolumeMl: 0.1; sampleVolumeMl: 0; decimalPlaces: 2

Outcome

The result is approximately 1.42 × 10⁹ CFU/mL, and the 142-colony plate is within the 30–300 countable range.

2. Crowded 10⁻⁴ plate flagged TNTC

Laboratory technician

Background

A 0.1 mL plate made from a 10⁻⁴ dilution contains 380 crowded colonies with some neighboring colonies merging.

Problem

Calculate the arithmetic estimate while identifying that the plate is outside the preferred counting range.

How to use

Enter 380 for Colonies Counted, 4 for Dilution Exponent, and 0.1 mL for Plated Volume.

FAQ

What is the CFU/mL formula?

CFU/mL = colonies counted × 10ⁿ ÷ plated volume in mL.

What does the dilution exponent mean?

It is n in 10⁻ⁿ, representing the dilution tube used to seed the plate. For a 10⁻⁶ plate, enter 6.

What plated volume should I enter for 100 µL?

Enter 0.1 mL because 100 µL equals 0.1 mL.

What does TNTC mean?

TNTC means too numerous to count. Counts above 300 are flagged and generally call for plating a higher dilution.

What is the recommended countable range?

The calculator uses 30–300 colonies per plate as the countable range. Counts below 30 may be flagged as too few to count.

colonies: 380; dilutionExponent: 4; platedVolumeMl: 0.1; sampleVolumeMl: 0; decimalPlaces: 2

CFU/mL Calculator (Colony Forming Units)

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Countable 10⁻⁶ dilution plate

2. Crowded 10⁻⁴ plate flagged TNTC

FAQ

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