Cell Viability Calculator (Trypan Blue and MTT)

Background.

A cell viability calculator turns either a live/dead count or a plate-reader absorbance into a percentage — and, just as importantly, tells you what that percentage does and does not mean. This page covers the two assay families that dominate the bench: dye exclusion, where you count stained and unstained cells under a microscope, and tetrazolium or neutral-red absorbance assays such as MTT, XTT and NRU, where you read a colour change against an untreated control.

They are not the same measurement, and the difference is not academic. Trypan blue and its relatives are excluded by cells with an intact plasma membrane and taken up by cells whose membrane has failed, so dye exclusion is a **membrane-integrity** assay. A cell that has committed to apoptosis but has not yet lost membrane integrity scores as viable. MTT, XTT and neutral red instead measure **metabolic activity** — formazan formation or lysosomal dye uptake by living cells — and report it relative to untreated wells. Those two numbers can disagree substantially on the same population, and when they do, the disagreement is information rather than error. This calculator keeps them in separate modes and labels every result with which one you ran.

The absorbance mode implements the equation from ISO 10993-5:2009, the international standard for in vitro cytotoxicity testing of medical devices. Annex C, clause C.2.5, Equation (C.1) states that Viab.% = 100 × OD570e ÷ OD570b, where OD570e is the mean optical density of the 100% test-sample extracts and OD570b is the mean optical density of the untreated wells. Annex D gives the identical relation for XTT at 450 nm. With the optional background field left at zero — its default — this calculator reproduces that equation literally. The background field exists because subtracting a cell-free medium-plus-reagent well is near-universal bench practice, but setting it to a non-zero value is a departure from the letter of the standard, and the page says so rather than doing it silently.

One threshold from that standard is worth understanding precisely, because its direction matters. The normative body of ISO 10993-5:2009, clause 8.5.1, says 'Reduction of cell viability by more than 30 % is considered a cytotoxic effect'; Annex A says a material is non-cytotoxic when relative viability is 'greater than or equal to 70 % of the control group'; Annexes C and D say cytotoxic potential exists 'if viability is reduced to < 70 % of the blank'. All four agree: **strictly below 70% indicates cytotoxic potential, and exactly 70% does not**. This calculator implements that inequality without rounding first, so a result of 69.9999% classifies as cytotoxic and 70.0000% does not. It also reports the signed margin against 70 in percentage points, so you can see how much headroom you have.

That threshold governs medical-device extract testing under a specific protocol — L929 cells, 1 × 10⁴ per well, 24 hours of attachment then 24 hours of exposure at 37 °C in 5% CO₂ — and it is not a general verdict on whether a culture is healthy. A routine passage sitting at 85% viability is not 'nearly cytotoxic'; it is just a slightly rough passage. The calculator states this beside the number rather than hiding it in an accordion.

Deliberate behaviour worth knowing before you start: the calculator refuses a total count of zero rather than reporting 0% viability, because 0 ÷ 0 is undefined and 'no cells found' is a detection-limit statement, not a viability. It also refuses a control absorbance at or below the background, because without signal above background there is no denominator — and ISO 10993-5 clause C.2.3.3 independently requires the mean control OD570 to reach at least 0.2 before the assay is acceptable at all. A treated absorbance below the background is refused too: that is a negative signal, which means a problem with the blank or the wavelength, not a very dead plate.

Finally, the density outputs. Viable and dead cells per millilitre are computed from a total density you supply, measured however you like — a hemocytometer, a Coulter counter, an automated counter. This page deliberately does not re-derive a density from counting-chamber geometry; the hemocytometer calculator does that, and the two are linked so you can carry a number from one to the other without either page duplicating the other's job.

What is cell viability calculator?

Cell viability is the fraction of cells in a population that are alive by some stated operational definition. The phrase 'by some stated operational definition' is doing real work: there is no assay that measures 'alive' directly, so every viability figure is a proxy, and the proxy has to be reported with the number.

Dye-exclusion assays use membrane integrity as the proxy. Trypan blue is a large, charged, membrane-impermeant dye; cells with an intact plasma membrane keep it out and appear bright and clear under brightfield, while cells with a compromised membrane stain deep blue. You count both populations on a counting chamber and express the unstained fraction as a percentage. The method is fast, cheap, needs no instrument beyond a microscope, and is described in the peer-reviewed Current Protocols series. Its weaknesses are equally well known: trypan blue is itself cytotoxic on prolonged exposure so the count must be read within a few minutes, scoring is subjective for faintly stained cells, early apoptotic cells score as viable, and cells that have already lysed have vanished from both counts entirely — which inflates the apparent viability of a badly treated sample.

Tetrazolium assays use metabolic activity as the proxy. MTT is a yellow, water-soluble tetrazolium salt that living cells reduce to an insoluble blue-violet formazan, which is then dissolved and read photometrically; XTT produces a water-soluble formazan read at 450 nm; the neutral red uptake assay instead measures accumulation of a dye in functional lysosomes. In each case the signal is compared with untreated control wells and expressed as a percentage. These assays are quantitative, high-throughput and well standardised — ISO 10993-5 specifies them in full — but the proxy can fail: a test item that changes metabolic rate per cell shifts the signal without killing anything, a reducing chemical in the extract can reduce the tetrazolium directly and produce a false high, and a coloured compound absorbing at the read wavelength contaminates the reading. ISO 10993-5 Annex C explicitly warns to remove culture medium carefully before adding MTT for exactly this reason.

Because the assays answer different questions, the honest way to read a viability number is always as a pair: the percentage, and the assay that produced it. A calculator that presented them as interchangeable would be hiding the most important part.

How to use this calculator.

1. 01Choose the assay you actually ran. Do not convert between them — a trypan blue percentage and an MTT percentage are different quantities and neither can be derived from the other.
2. 02For dye exclusion: mix your suspension with trypan blue (typically equal volumes of suspension and 0.4% dye), load a counting chamber, and read within a few minutes — prolonged dye exposure kills cells and inflates the dead count.
3. 03Count both populations and enter them. Aim for at least 200 cells in total; the statistical counting error is 100 ÷ √N, so 200 cells gives roughly 7% and 50 cells gives roughly 14%.
4. 04Enter your measured total cell density in cells per mL if you want viable and dead densities as well. That number comes from your counting method, not from this page — use the hemocytometer calculator if you counted on a chamber. Enter 0 if you only want the percentage.
5. 05For absorbance: enter the mean OD of the treated wells and the mean OD of the untreated control wells. Use the same wavelength for both — 570 nm with a 650 nm reference for MTT, 450 nm for XTT, 540 nm for neutral red.
6. 06Leave the background field at 0 to reproduce ISO 10993-5 Equation (C.1) exactly. Set it only if your SOP subtracts a cell-free medium-plus-reagent well, and record that you did.
7. 07Read the margin against the 70% ISO reference in percentage points, and read the interpretation line — it states which proxy the number rests on and how far the ISO threshold actually reaches.
8. 08Report viability to one decimal place at most, and always report it together with the assay, the cell line, the exposure time and the read wavelength. A viability percentage without its assay conditions is not reproducible.

The formula.

Both modes are ratios, but they divide different things.

Dye exclusion divides a count by a count. The unstained cells are the numerator, the sum of unstained and stained is the denominator, and multiplying by 100 gives a percentage. For the worked example, 184 unstained out of 184 + 16 = 200 counted gives 184 ÷ 200 × 100 = 92%, so 8% of the cells took up the dye. If you also supply the total density of the original suspension — 1.0 × 10⁶ cells/mL in the example — those percentages split it: 1.0 × 10⁶ × 0.92 = 920,000 viable cells/mL and 1.0 × 10⁶ × 0.08 = 80,000 dead cells/mL. The two densities add back to the total exactly, which the test suite asserts as a dimensional check.

The absorbance mode divides a signal by a signal. ISO 10993-5:2009 Annex C, clause C.2.5, Equation (C.1) gives Viab.% = 100 × OD570e ÷ OD570b, with OD570e the mean optical density of the 100% test extracts and OD570b that of the untreated wells. This calculator computes 100 × (A_t − A_b) ÷ (A_c − A_b), which reduces to the standard's equation exactly when the optional background A_b is 0. For the worked example, 100 × 0.612 ÷ 0.845 = 72.426…%, so the treated wells retain about 72.4% of the control's metabolic signal. Because both terms are absorbances, the ratio is dimensionless and scaling both readings by any factor leaves it unchanged — another dimensional check the test suite makes.

The ISO margin is simply viability minus 70, reported in percentage points. For the dye example that is 92 − 70 = +22 pp; for the absorbance example, 72.426… − 70 = +2.426 pp. Percentage points, not per cent: the treated wells are 2.4 percentage points above the threshold, which is a different statement from being 2.4% above it.

Rounding stage: everything is carried at full working precision and rounded only at the return boundary, to twelve significant figures. Critically, the viability percentage is **not** rounded before it is compared with the 70% threshold. That matters at the boundary: 69.9999% classifies as having cytotoxic potential and 70.0000% does not, exactly as ISO 10993-5 clause 8.5.1 and Annex A state the inequality. A version that rounded to one decimal place first would call 69.9999% 'exactly 70%' and flip the classification — the class of boundary defect that is invisible to any test checking only the middle of a range, so this page is tested at 69.9%, 70.0% and 70.1% in both modes.

A worked example.

Frequently asked questions.

References& sources.

1. [1]ISO 10993-5:2009(E), Biological evaluation of medical devices — Part 5: Tests for in vitro cytotoxicity, third edition. Clause 8.5.1 ('Reduction of cell viability by more than 30 % is considered a cytotoxic effect'); Annex A (3T3 NRU protocol; '≥ 70 % of the control group … shall be considered non-cytotoxic'); Annex C clause C.2.5 Equation (C.1), Viab.% = 100 × OD570e / OD570b, for MTT at 570 nm; Annex D clause D.2.5 Equation (D.1) for XTT at 450 nm; clause C.2.3.3 (blank acceptance criterion, mean OD570 ≥ 0.2). Standard text consulted directly 2026-07-29. PAYWALLED for purchase — cited by clause so every figure above is verifiable against the document.
2. [2]Strober, W. (2015). 'Trypan Blue Exclusion Test of Cell Viability.' Current Protocols in Immunology, 111, A3.B.1–A3.B.3. DOI 10.1002/0471142735.ima03bs111, PMID 26529666. The peer-reviewed reference protocol for dye exclusion: cells are suspended in PBS containing trypan blue and scored as clear (viable) or blue (non-viable) cytoplasm, giving the percentage this calculator's dye mode computes. Publisher paywall on the full text; the PubMed record is open.
3. [3]World Health Organization, WHO Laboratory Manual for the Examination and Processing of Human Semen. §2.6 covers vitality assessment by dye exclusion (eosin–nigrosin) and its membrane-integrity basis; §2.8.3 gives the ≥200-cells-per-replicate counting target and the two-significant-figure reporting rule used throughout this page. Public PDF, retrieved 2026-07-29.
4. [4]ISO 20391-1:2018, Biotechnology — Cell counting — Part 1: General guidance on cell counting methods. Establishes the distinction between total and differential counting — which is precisely the distinction between a total cell density and the viable fraction of it that this page computes. PAYWALLED — cited bibliographically; scope confirmed from the ISO catalogue entry, retrieved 2026-07-29.