Understand the relationship
The reasoning behind the result
Concentration times plated volume gives an expectation
E[count] = C₀Vplated/DF
The diluted concentration is original concentration divided by the cumulative dilution factor. Multiplying by plated volume in millilitres gives expected growth units per plate under the entered model.
An expected count of 0.5 is meaningful as a mean across comparable repetitions; it does not mean half a colony will be observed. Actual counts vary, and preparation, recovery or biological departures can change that distribution.
Use the cumulative factor from the original stock
A tube made by three successive tenfold dilutions has factor 1,000 relative to the original stock. Entering only the last step's factor of ten would overstate its expected count by a factor of 100.
This planner compares plated aliquots. The related serial-dilution ledger accounts for the transfers needed to prepare the tubes. Replicate aliquots and any further withdrawals must fit within the prepared amount.
Count comparisons are entered assumptions
The comparison interval labels expectations as below, inside or above the range you supplied. It does not certify a method's countable range or choose incubation, culture or organism-handling conditions.
CFU are growth units under the actual assay. Clusters and chains need not correspond to individual cells. This is a preparation estimate, not a back-calculation from measured plates or a conclusion about sample safety.
Follow the numbers
A 100 µL plate from a 1,000-fold dilution
- Original estimate: 1,000,000 CFU/mL. Divide by 1,000 to obtain 1,000 CFU/mL in the diluted sample.
- 100 µL = 0.1 mL, so the expected count is 1,000 × 0.1 = 100 CFU per plate.
- Two replicates require 200 µL from that dilution and have 200 expected CFU in aggregate.
The comparison is an expectation based on the supplied stock estimate; the observed plate counts remain to be measured.
Quick guide
How to use this calculator
- Enter an independently established original-stock estimate.
- Enter the comparison interval required for your own scenario; no universal plate-count interval is supplied.
- List cumulative dilution factors, plated volumes and replicate counts.
- Inspect expected counts, total plates and the volume needed from each prepared dilution.
Calculation method
Calculation and interpretation
See how dilution and plated volume affect expected count and the number of plates in an entered design.
Expected count per plate = original CFU/mL × plated µL/(1000 × cumulative dilution factor)
Worked example
A 100 µL plate from a 1,000-fold dilution
The comparison is an expectation based on the supplied stock estimate; the observed plate counts remain to be measured.
Expected count per plate = original CFU/mL × plated µL/(1000 × cumulative dilution factor)
Supported inputs
Precision and limits
An entered laboratory plan
This tool reconciles the measurements and choices you enter. It does not select a biological protocol, certify a preparation, assess sample suitability or establish an equipment operating limit.
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