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Cell Confluence Calculator

Estimate how confluent a culture is from its cell number and the vessel's growth area, and how long it will take to reach a target confluence at a given doubling time.

Formula

N100%=Agrowth×ρconfluentN_{100\%} = A_{\text{growth}} \times \rho_{\text{confluent}}
confluence %=NnowN100%×100\text{confluence}\,\% = \dfrac{N_{\text{now}}}{N_{100\%}}\times 100
t=Tdlog⁡2NtargetNnowt = T_{\mathrm{d}} \log_2\dfrac{N_{\text{target}}}{N_{\text{now}}}
ρconfluent\rho_{\text{confluent}}
cells per cm² when the surface is fully covered; specific to the cell line
TdT_{\mathrm{d}}
doubling time

How it works

Confluence is the share of the growth surface covered by cells. If the number of cells per cm² at full coverage is known for a cell line, the current cell number converts directly to a percentage.

The time to reach a target follows from exponential growth. This is an estimate: the confluent density varies between cell lines and with cell size, and growth slows as cells run out of room. The default density is a placeholder, not a property of any particular line.

Worked example

A T-75 flask holds 7.5 × 10⁵ cells. The line reaches 10⁵ cells/cm² at confluence and doubles every 24 hours. The target is 80%.

  1. Cells at 100% = 75 cm² × 10⁵ = 7.5 × 10⁶.
  2. Confluence now = 7.5 × 10⁵ / 7.5 × 10⁶ = 10%.
  3. Time = 24 h × log₂(6 × 10⁶ / 7.5 × 10⁵) = 24 h × 3 = 72 h.

The flask is about 10% confluent and should reach 80% in roughly 72 hours, or 3 days.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • The confluent density entered is correct for your cell line and conditions.
  • Growth is exponential until the target is reached.
  • Cells are spread evenly across the surface.

Common mistakes

  • Using another cell line's confluent density. Large, flat cells cover a surface at far lower numbers than small ones.
  • Expecting the estimate to hold above about 90%, where contact inhibition slows growth.
  • Treating this as a measurement. Confluence judged by eye or by image analysis is the ground truth.

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Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.