CFU / Plating Efficiency Calculator
Estimate plating efficiency (PE), CFU/mL, and how many cells to seed for a target colony count.
PE% = colonies ÷ seeded cells × 100; CFU/mL = colonies ÷ volume(mL) × dilution.
Plating efficiency (PE)
24.00%
CFU/mL
12,000CFU/mL
| Target colonies | Cells to seed |
|---|---|
| 50 colonies counted | 208.33 |
| 100 colonies counted | 416.67 |
| 200 colonies counted | 833.33 |
| 500 colonies counted | 2,083.3 |
Cells needed to seed for a target number of colonies, using the current PE.
Learn more
What it does
Plating efficiency (PE) is the fraction of seeded cells that form a visible colony, and CFU/mL is the concentration of colony-forming units in the original sample. Together they describe how clonally competent a cell population is and how dense it was before plating. The calculator turns a simple colony count into PE, CFU/mL and a seeding table for common target colony numbers.
How it works
Plating efficiency is PE% = colonies ÷ seeded cells × 100. The colony-forming unit concentration is CFU/mL = colonies ÷ volume(mL) × dilution factor, where the dilution factor accounts for any pre-plating dilution of the sample. To plan an experiment you reverse the PE relation: cells to seed = target colonies ÷ (PE / 100).
Worked example
Seeding 500 cells that grow into 120 colonies gives PE = 24.00%. If 120 colonies grow from 0.1 mL of a 10-fold diluted sample, that is 12,000 CFU/mL. At PE 24%, obtaining 50, 100, 200 or 500 colonies requires seeding 208, 417, 833 or 2,083 cells respectively.
When to use it
Use this calculator when setting up a cloning experiment, validating a frozen cell stock, or reporting how many cells to plate for a desired number of colonies. PE is also a quality check: most mammalian cell lines fall in a 10–30% PE range, while primary and hard-to-clone lines are typically below 10%, and a drift in PE across passages or serum batches flags a change in culture health.
FAQ
- What is the difference between plating efficiency (PE) and cell viability?
- Viability measures the fraction of cells that are alive at a moment, often by trypan blue exclusion, and says nothing about whether a cell can divide. PE measures the fraction of seeded cells that form a full colony, so it captures clonal growth potential, not just survival. A population can be 95% viable yet have a PE of only 20% because most live cells cannot complete many divisions.
- Why are CFU samples diluted before plating?
- Undiluted samples usually give far too many colonies to count accurately and they merge into a lawn. Diluting brings the count into the 30–300 range where individual colonies are separable and the count is statistically reliable. The dilution factor is then multiplied back in to recover the true CFU/mL of the original, undiluted sample.
- What is a normal plating efficiency (PE)?
- Most well-established mammalian cell lines show a PE of about 10–30%. Primary cells and difficult or hard-to-clone lines are typically below 10%, and some suspension lines are much lower. PE also shifts with passage number, serum batch and plating density, so compare a value against the same line under the same conditions rather than against an absolute standard.
- Why re-plate if I get fewer than 30 or more than 300 colonies?
- Counts below 30 colonies have wide sampling error, so a small extra colony swings the percentage a lot. Counts above 300 colonies start to merge and compete for space and nutrients, so the true number is underestimated. The 30–300 window is the range where a colony count is both precise and unbiased, which is why plates outside it should be repeated at a different dilution.