CFU per Gram Calculator

Calculate microbial loads per gram from dilution data. Compare replicates, log values, and dry-basis estimates. Built for cleaner lab reviews, exports, charts, and records.

Result preview area

Submit the calculator to show CFU per gram, log values, exports, and the chart here.

Calculator inputs

Use the grid below. It shows three columns on large screens, two on smaller screens, and one on mobile.

Sample ID

Sample weight (g)

Suspension volume (mL)

Plated dilution exponent

Enter 4 for a 10^-4 plated dilution.

Plated volume (mL)

Moisture percent (optional)

Colony count 1

Colony count 2

Colony count 3

Colony count 4

Countable minimum

Countable maximum

Example data table

Sample	Weight (g)	Suspension (mL)	Dilution	Plated Volume (mL)	Replicate Counts	Average	CFU/g
Food A	10	90	10^-4	0.1	112, 108, 115, 110	111.25	1.0013E+08
Soil B	5	45	10^-5	0.1	74, 79, 81	78.00	7.0200E+08
Feed C	25	225	10^-3	1.0	41, 39, 43	41.00	3.6900E+05

Formula used

CFU/g = (Average colony count × Dilution factor × Suspension volume) ÷ (Plated volume × Sample weight)

For a plate made from a 10^-d dilution, the dilution factor equals 10^d.

Average colony count comes from the entered replicate plates.

Suspension volume is the liquid holding the sample after homogenization.

Plated volume is the amount transferred onto the agar.

When moisture is known, a dry-basis value is also estimated.

Dry-basis CFU/g = Wet-basis CFU/g ÷ (1 − moisture fraction)

How to use this calculator

Enter a sample name for traceability.
Provide sample weight in grams.
Enter the homogenized suspension volume in milliliters.
Type the dilution exponent used for the plated aliquot.
Enter the plated volume in milliliters.
Add at least two colony counts from replicate plates.
Adjust the preferred countable range if your method differs.
Optionally enter moisture percent for dry-basis reporting.
Click calculate to show the result above the form.
Download CSV or PDF after reviewing the output.

FAQs

1. What does CFU per gram mean?

CFU per gram estimates viable microorganisms in one gram of sample. It reflects colonies able to grow under the chosen culture conditions, not total cells.

2. Why do I enter a dilution exponent?

Microbiology plates are usually labeled as 10^-1, 10^-2, and so on. Entering the exponent makes the dilution factor easy to apply during the final CFU per gram calculation.

3. Why are multiple replicate counts helpful?

Replicates reveal plating consistency and reduce random error. Their average gives a steadier estimate than relying on a single plate.

4. What is the preferred countable range?

Many methods prefer roughly 30 to 300 colonies per plate. Plates below that range may be less stable, while crowded plates may underestimate true counts.

5. When should I use the dry-basis value?

Use dry-basis reporting when comparing samples with different moisture contents. It normalizes counts to dry material and improves cross-sample interpretation.

6. Does this calculator replace lab method validation?

No. It is a reporting aid. Your laboratory method, dilution scheme, media choice, incubation conditions, and SOP remain the controlling factors.

7. What if my counts are all too high?

Use a higher dilution on the next run. Overcrowded plates can merge colonies and reduce count accuracy.

8. What if my counts are all too low?

Use a lower dilution or plate a larger volume, if your method allows it. Very low counts increase uncertainty.