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Trypan Blue Cell Viability Calculator

Cell viability is the percentage of cells in a sample that are alive, and the trypan blue dye-exclusion assay is the standard way to measure it manually. Live cells have intact membranes that exclude the blue dye and stay bright; dead cells have leaky membranes that take up the dye and turn blue. Count each population under a hemocytometer and compute viability = live / (live + dead) × 100. Enter your counts below and the calculator returns the viability percentage plus, if you also give the squares counted and dilution, the viable cell density (VCD) in cells/mL. Below about 90% for a healthy log-phase culture is a warning sign.

Before you seed cells, transfect them, or run an assay on them, you need to know how many are actually alive. Viability answers that, and the trypan blue exclusion test is the workhorse manual method: it is fast, cheap and needs only a hemocytometer and a microscope. This page is part of the Cell Culture calculator set and computes viability from your live and dead counts, optionally alongside viable cell density.

The principle is membrane integrity. Trypan blue is a diazo dye that cannot cross an intact plasma membrane, so a healthy cell excludes it and looks bright and unstained. A dead or dying cell has a compromised membrane that lets the dye in, staining the cytoplasm blue. Counting stained versus unstained cells therefore partitions the sample into dead and live, and viability is the live fraction expressed as a percentage. The method measures whether the membrane is intact, not whether the cell is metabolically healthy or functionally normal — an important caveat covered below.

The timing is the part people get wrong. Dye exclusion is time-dependent: given enough time, even live cells begin to take up trypan blue, so the viability you read falls the longer the sample sits. Load the hemocytometer and count promptly — ideally within about three minutes of mixing cells with dye — and keep the count consistent between samples so the comparison is fair. Mixing 1:1 with dye also means a dilution factor of 2 when you convert counts to a concentration.

Viable cell density (VCD) combines the two numbers you usually want at once: how many cells there are per millilitre and what fraction are alive. It uses the same hemocytometer geometry as the cell counting calculator — one large square holds 1e-4 mL, so live cells per square × dilution × 10⁴ gives viable cells/mL. VCD is the number you seed from: it tells you the volume to add to reach a target density of living cells, which total-cell concentration alone cannot. If you only need total cells regardless of viability, use the cell counting calculator instead.

Cells that exclude trypan blue — bright and unstained.

Cells that take up the dye — stained blue.

Number of large squares the live cells were counted over, for viable cell density.

Dilution applied when loading, e.g. 2 for a 1:1 trypan-blue mix.

Cell viability95 %
Live cells190 cells
Dead cells10 cells
Total counted200 cells
Viable cell density—

How to use the cell viability calculator

Prepare a single-cell suspension and mix it 1:1 with 0.4% trypan blue. Note the dilution factor this creates (2 for a 1:1 mix).

Load the hemocytometer and count promptly — within about 3 minutes — under the microscope.

Count live (bright, unstained) cells and dead (blue) cells separately, using the standard top/left border rule so shared grid lines are counted once.

Enter the Live and Dead totals. That alone gives the viability percentage.

For viable cell density too, enter the number of Large squares counted and the Dilution factor.

Press Calculate. The primary card is viability %; supporting cards show live, dead and total counts and, when geometry is given, the VCD. A viability below 90% triggers a healthy-culture warning.

Trypan Blue Cell Viability formula

total cells = live + dead
viability % = live / total × 100
VCD = (live / large squares) × dilution factor × 10⁴ cells/mL
Healthy log-phase culture ≳ 90% viability
Viability is the live fraction of the total counted. VCD reuses the hemocytometer volume factor — one 1 mm² large square under a 0.1 mm coverslip holds 1e-4 mL, so 10⁴ converts a per-square count to cells/mL — and counts only the live cells. Read within ~3 minutes of staining, because dye uptake by live cells rises with time.

Variables and units

SymbolMeaningUnit
liveDye-excluding (unstained) cells countedcells
deadTrypan-blue-positive (stained) cells countedcells
totallive + deadcells
viabilityLive fraction of the total%
VCDViable cell density (live cells per mL)cells/mL
dilution factorDilution applied when loading (2 for 1:1 dye)×

How cell viability is calculated

  1. Add the live and dead counts to get the total cells counted.
  2. Divide live by total and multiply by 100 for the viability percentage.
  3. If squares counted and dilution factor are given, compute VCD = (live / squares) × dilution × 10⁴ cells/mL.
  4. Compare viability with the ~90% healthy-culture threshold and warn if it falls below.

Worked laboratory examples

Example 1 — A healthy culture

Given: live 190 · dead 10

  1. Total = 190 + 10 = 200.
  2. Viability = 190 / 200 × 100 = 95%.

Result: 95% viable — healthy

Example 2 — Viability with viable cell density

Given: live 190, dead 10 · counted over 4 large squares · dilution factor 2

  1. Viability = 190 / 200 × 100 = 95%.
  2. VCD = (190 / 4) × 2 × 10⁴ = 47.5 × 2 × 10⁴ = 9.5 × 10⁵ viable cells/mL.

Result: 95% viable; VCD 9.5 × 10⁵ cells/mL

Example 3 — A stressed culture

Given: live 70 · dead 30

  1. Total = 100; viability = 70 / 100 × 100 = 70%.
  2. Well below the ~90% threshold, so the tool warns.

Result: 70% viable — flagged: check confluence, trypsin time, contamination

Example 4 — Seeding from VCD

Given: VCD 9.5 × 10⁵ cells/mL · you need 1 × 10⁵ live cells in a well

  1. Volume = target cells / VCD = 1 × 10⁵ / 9.5 × 10⁵ ≈ 0.105 mL.
  2. Add about 105 µL of the suspension to the well.

Result: ≈ 105 µL delivers 1 × 10⁵ viable cells

Want the reasoning behind the numbers? Read How to Calculate Cell Viability with Trypan Blue and How to Calculate Cell Seeding Density.

How to interpret the result

Viability is a quality gate, not just a number. A log-phase mammalian culture should sit above roughly 90%; a value below that signals a problem — over-confluence, prolonged trypsin exposure, contamination, a rough passaging technique, or cells that were already declining.

VCD is the figure to seed from, because it counts only living cells. Two suspensions with the same total concentration but different viability deliver different numbers of viable cells, and only VCD captures that.

Always read the assay promptly. If viability looks low, consider whether the sample sat too long in dye before counting — time-dependent uptake by live cells depresses the reading artificially.

Remember what the assay measures. Dye exclusion reports membrane integrity; a cell can exclude trypan blue yet be metabolically stressed or early-apoptotic. For functional or proliferative readouts, pair this with an assay such as MTT, resazurin or a colony-formation test.

Assumptions

  • The dye is trypan blue and the sample was read within a few minutes of mixing.
  • Live cells are counted as unstained and dead cells as blue-stained, with a consistent border rule.
  • The suspension is single-cell and homogeneous; clumps are counted conservatively or dispersed first.
  • When VCD is requested, the squares counted and dilution factor reflect the actual chamber loading.
  • Membrane integrity is an acceptable proxy for viability for the purpose at hand.

Limitations

  • Measures membrane integrity only — not metabolism, proliferation capacity or apoptosis stage.
  • Time-sensitive: viability drifts downward the longer cells sit in dye.
  • VCD needs the geometry inputs; without them only the percentage is returned.
  • Manual counting carries operator and sampling error, and dead cells can be missed if debris obscures them.

Common mistakes

  • Letting the stained sample sit too long before counting, so live cells take up dye and viability reads artificially low.
  • Counting only total cells and reporting viability without distinguishing live from dead.
  • Using total-cell concentration instead of viable cell density when seeding, so the intended number of living cells is missed.
  • Forgetting the 1:1 dye dilution factor when converting to VCD.
  • Treating a sub-90% viability as acceptable without investigating the cause.

FAQ

How is cell viability calculated?

Viability % = live cells / (live cells + dead cells) × 100. Count dye-excluding (live) and dye-positive (dead) cells separately with a hemocytometer, add them for the total, then divide the live count by the total and multiply by 100.

Why does trypan blue only stain dead cells?

Because it cannot cross an intact plasma membrane. Live cells exclude the dye and stay bright; cells with compromised membranes — dead or dying — take it up and turn blue. That membrane-integrity difference is what makes the exclusion assay work.

What is a good cell viability percentage?

For a healthy log-phase mammalian culture, generally above 90%. Viability below that suggests stress from over-confluence, prolonged trypsinisation, contamination or rough handling, and is worth investigating before using the cells downstream.

What is viable cell density (VCD)?

VCD is the concentration of living cells, in cells/mL. It equals (live cells counted / large squares) × dilution factor × 10⁴, reusing the hemocytometer volume factor. Seed from VCD rather than total concentration so you deliver the intended number of living cells.

How long can cells sit in trypan blue before counting?

Keep it short — ideally within about 3 minutes of mixing. Live cells progressively take up the dye over time, so a delayed count reads lower viability than the sample truly has. Count promptly and keep the timing consistent between samples.

Does trypan blue tell me if cells are healthy or just alive?

It reports membrane integrity, so it distinguishes live from dead but not healthy from stressed. A cell can exclude the dye yet be metabolically impaired or early in apoptosis. For functional health, pair it with a metabolic or proliferation assay such as MTT or resazurin.

Scientific Sources

External links open in a new tab and carry rel="nofollow". We cite primary references (NIST, NIH/NCBI, peer-reviewed papers) and established manufacturer technical documentation (NEB, Thermo Fisher, Promega, IDT). Calculated results are estimates; your protocol and manufacturer guidance take precedence.

Last reviewed: 2026-09 · Method version: 1.0