What is the standard countable range?

The widely accepted countable range is 30–300 colonies per plate (some labs use 25–250). Plates with fewer than 30 colonies introduce statistical error from low sample size; plates over 300 are too crowded to count accurately (TNTC — Too Numerous To Count). This calculator flags plates outside the set range.

How is the dilution factor calculated for each step?

For a 1:10 (tenfold) serial dilution starting at 10⁻¹, successive plates are plated at 10⁻¹, 10⁻², 10⁻³, and so on. If your initial dilution factor is 0.1 and your dilution ratio is 10, step 2 becomes 0.01, step 3 becomes 0.001, etc.

Why average across multiple countable plates?

Averaging results from two or more countable plates reduces random error from pipetting variation and uneven colony distribution. This tool computes the standard arithmetic mean of countable plate concentrations; note that strict FDA BAM and ISO 4833-1 methods prescribe volume-weighted summation across consecutive dilution steps.

What volume should I enter for volume plated?

Enter the volume transferred to each plate in mL. Common values: pour plate = 1 mL; spread plate = 0.1 mL (100 µL). If different volumes were plated at different dilutions, note that this tool assumes a constant volume across all plates.

Can I change the countable range?

Yes — use the "Advanced Settings" to set custom minimum and maximum colony thresholds for your lab protocol or regulatory standard.

TNTC stands for "Too Numerous To Count." When colonies are too dense to distinguish individually (typically over 300), the plate is not used for calculation. Enter the plate count anyway so the tool can mark it and skip it from the average.

Dilution Plating Calculator

Enter colony counts for each dilution step in a serial dilution series and get CFU/mL with countable-plate filtering and averaged results.

Experiment Settings

Initial Dilution Factor

Dilution Ratio per Step

Volume Plated (mL)

100 µL = 0.1 mL

Number of Dilution Steps

Max 10 steps

Countable Range & Options

Min Countable (colonies)

FDA BAM standard: 30

Max Countable (colonies)

FDA BAM standard: 300

Include all plates in average (ignore range filter)

Final CFU/mL

Log₁₀ CFU/mL

Base-10 logarithm

Plates Used

Per-Plate Breakdown

Step	Dilution Factor	Colonies	CFU/mL	Status

Quick-Load Examples

Summary

The Dilution Plating Calculator computes CFU/mL from serial dilution plate count data. Unlike a basic single-plate calculator, this tool handles a full dilution series — you enter colony counts for each successive dilution step, and the tool identifies countable plates (30–300 colonies by default), applies the formula CFU/mL = Colony Count ÷ (Dilution Factor × Volume Plated), and averages results across valid plates for maximum accuracy. Ideal for standard plate count (SPC) procedures in food microbiology, environmental monitoring, and clinical lab work.

How it works

Set the initial dilution factor and volume plated (mL) for your experiment.
Choose a dilution ratio (commonly 1:10 or 1:100) used at each successive step.
Enter the number of colony-forming units counted on each plate.
The calculator flags plates outside the countable range (default 30–300 CFU).
CFU/mL is computed for each countable plate and averaged for a final result.
Review the per-plate breakdown table and copy the final CFU/mL value.

Use cases

Standard plate count (SPC) in food microbiology QC labs.
Bacterial load quantification in water quality testing.
Viable cell concentration in pharmaceutical bioburden testing.
Serial dilution verification for inoculum preparation in MIC assays.
Teaching microbiology lab technique with immediate feedback.
Environmental surface swab culture enumeration.
Probiotic or fermentation culture titration.
Clinical specimen culture interpretation support.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu