This free online MTT assay calculator converts absorbance readings from colorimetric cell viability assays into percentage viability, supporting MTT, MTS, WST-1, and resazurin formats. Researchers use it to quantify drug cytotoxicity, compare treatment conditions, and estimate IC50 values from dose-response curves — all with automatic background correction and replicate statistics.
Optional: if your plate reader recorded a reference wavelength (e.g. 630–690 nm for MTT) to subtract non-specific absorbance, enter those readings below. Leave blank to skip this correction.
Enter each treatment group name and its absorbance reading.
| Sample / Treatment | OD Reading | |
|---|---|---|
Enter drug concentrations and corresponding viability percentages (or absorbance values with a control). IC50 is estimated by linear interpolation between the two concentrations bracketing 50% viability.
Enter concentration and OD or % viability for each dose.
| Concentration | OD or Viability % | |
|---|---|---|
MTT Assay Results
Scenario: You treated HeLa cells in a 96-well plate with a candidate compound at 10 µM for 24 hours, then ran an MTT assay and read the plate at 570 nm.
Inputs used: Blank OD (media + MTT, no cells) = 0.048; Control OD (untreated cells) = 0.812; Sample OD (treated cells) = 0.395.
Result: Corrected control = 0.812 − 0.048 = 0.764. Corrected sample = 0.395 − 0.048 = 0.347. Viability = (0.347 ÷ 0.764) × 100 = 45.4%.
Why it matters: A viability of 45.4% places this compound in the "moderate cytotoxicity" range at 10 µM — a useful data point for building a dose-response curve and estimating an IC50 in a follow-up screen with additional concentrations spanning both above and below 50% viability.
| Assay | Read λ (nm) | Reference λ (nm) | Solubilization Needed |
|---|---|---|---|
| MTT | 570 | 630–690 | Yes (DMSO / acidic isopropanol) |
| MTS / CellTiter 96 | 490 | 630–690 | No |
| WST-1 | 450 | 600–690 | No |
| XTT | 450–475 | 630–690 | No |
| Resazurin / AlamarBlue | 570 (Abs) / 590 (Fl, Ex 560) | 600 | No |
| Crystal Violet | 595 | — | Yes (acetic acid / methanol) |
| Viability Range | Interpretation |
|---|---|
| ≥ 100% | No reduction in metabolic activity relative to control |
| 75–100% | Mild cytostatic / cytotoxic effect |
| 40–75% | Moderate cytotoxicity |
| 20–40% | Strong cytotoxicity |
| < 20% | Severe cell death or growth arrest |
How to Use the MTT Assay Calculator
This calculator supports three modes of analysis. For the Single Sample tab, enter the blank absorbance (media with reagent but no cells), the control absorbance (untreated cells at 100% viability), and the sample absorbance (treated cells). The calculator automatically subtracts the blank from both control and sample, then computes the percentage viability relative to the control. You can also select the number of replicates to calculate standard deviation across technical repeats.
For the Multi-Sample tab, enter one control and one blank value, then add up to 10 treatment groups with custom names and absorbance readings. The calculator generates a comparison bar chart, a data table with corrected OD values, and highlights the highest and lowest viability conditions. This is ideal for screening multiple drug concentrations or comparing different cell lines under the same treatment.
For the IC50 Estimator tab, enter a series of drug concentrations and their corresponding absorbance values (or viability percentages if you have already calculated them). The calculator uses linear interpolation between the two dose points that bracket 50% viability to estimate the half-maximal inhibitory concentration. For best results, include at least 5-7 data points spanning concentrations that produce both high and low viability.
MTT Assay Viability Formula
Where Sample OD is the absorbance reading from treated cells, Blank OD is the absorbance from media plus reagent without cells (background correction), and Control OD is the absorbance from untreated cells representing 100% metabolic activity. The blank correction is essential because culture medium, phenol red, and serum components can contribute significant background absorbance at the assay wavelength. Without this correction, viability percentages can be inflated by 5-15% depending on the medium composition.
How the MTT Assay Works
MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) is a yellow tetrazolium salt that is reduced by mitochondrial dehydrogenase enzymes in metabolically active cells to insoluble purple formazan crystals. The reduction reaction depends on NAD(P)H produced during cellular respiration and glycolysis. The amount of formazan produced is directly proportional to the number of viable cells and their metabolic activity. After incubation (typically 1-4 hours), the formazan crystals are solubilized in DMSO, isopropanol, or an SDS buffer solution, and the absorbance is measured spectrophotometrically. The intensity of the purple color correlates with cell viability — higher absorbance indicates more metabolically active cells.
Assay Variants and Wavelengths
- MTT: Read at 570 nm (reference 630–690 nm). Produces insoluble formazan crystals requiring DMSO or acidic isopropanol solubilization. The most widely used and cost-effective tetrazolium assay.
- MTS / CellTiter 96: Read at 490 nm. Produces a soluble formazan product in the presence of phenazine methosulfate (PMS) as an electron coupling reagent. No solubilization step required — faster workflow.
- WST-1: Read at 450 nm (reference 600–690 nm). Highly water-soluble formazan with higher sensitivity than MTT. Suitable for high-throughput screening and time-course measurements.
- Resazurin / AlamarBlue: Absorbance at 570 nm with reference at 600 nm, or fluorescence at 590 nm (excitation 560 nm). Non-toxic to cells — the same plate can be re-read over multiple time points to monitor kinetic responses.
When to Use This Calculator
This tool is essential for several common laboratory scenarios. Use it when screening novel compounds for cytotoxicity in 96-well or 384-well plate formats, where you need rapid viability percentages for dose-response curves. It is also valuable for comparing the relative potency of different drug candidates against the same cell line, or for assessing the cytotoxicity of nanoparticles, extracts, or environmental samples. The IC50 estimator is particularly useful during early drug discovery when you need a quick potency ranking before committing to more expensive assays. Additionally, the multi-sample mode helps normalize data across biological replicates or different experimental batches by ensuring consistent background correction.
Common Mistakes to Avoid
- Skipping the blank correction: Many researchers subtract only the blank from the sample but forget to subtract it from the control as well. Both control and sample OD values must be blank-corrected before the ratio is calculated. Failing to do this systematically underestimates viability at low OD values and overestimates it at high OD values.
- Using the wrong control: The control must represent untreated, healthy cells at the same seeding density and in the same medium as the treated samples. Using a vehicle-only control (e.g., DMSO at 0.1%) is acceptable, but using a different cell line or a different passage number introduces confounding variables that invalidate the viability percentage.
- Incomplete formazan solubilization: For MTT specifically, undissolved crystals will scatter light and produce artificially high or variable absorbance readings. Always verify under an inverted microscope that no purple crystals remain before reading the plate. For DMSO solubilization, incubate at 37°C with gentle shaking for 10-15 minutes.
- Ignoring edge effects: Wells at the perimeter of microplates often show higher evaporation and temperature gradients, leading to inconsistent cell growth. Always fill the outermost wells with sterile PBS or medium and use only the inner 60 wells of a 96-well plate for experimental samples.
- Reading too late after solubilization: Formazan can re-precipitate from DMSO over time, especially at room temperature. Read the plate within 30-60 minutes of adding the solubilization reagent. If delays are unavoidable, store the plate at 4°C in the dark and allow it to equilibrate to room temperature before reading.
- Colored compound interference: Test compounds that are strongly colored (e.g., natural product extracts, certain dyes, or metal nanoparticles) can absorb at the assay wavelength and produce false positives or negatives. Always run a compound-only control (no cells) to assess optical interference.
Interpreting Your Results
A viability percentage of 100% indicates that the treated cells have the same metabolic activity as the untreated control. Values between 75-100% suggest mild cytostatic or cytotoxic effects that may not be biologically significant depending on the experimental context. Values between 40-75% indicate moderate cytotoxicity — the treatment is affecting cell metabolism but a substantial population remains viable. Values below 40% represent strong cytotoxicity, and below 20% typically indicates severe cell death or growth arrest. However, remember that MTT measures metabolic activity, not direct cell viability. Treatments that induce mitochondrial biogenesis or stress responses can paradoxically increase metabolic activity per cell, leading to viability values above 100%. Conversely, treatments that cause cell cycle arrest without killing cells may show lower metabolic activity per cell than expected. Always confirm MTT results with complementary assays such as trypan blue exclusion, LDH release, or live/dead staining when making conclusions about cell death versus metabolic suppression.
Frequently Asked Questions
What is the correct formula for calculating cell viability from MTT assay absorbance?
The standard formula is: Viability (%) = [(Sample OD − Blank OD) / (Control OD − Blank OD)] × 100. The blank (media only, no cells) corrects for background absorbance from the medium and reagents. The control (untreated cells) represents 100% viability. Always subtract the blank from both sample and control readings before dividing. This normalization ensures that only formazan produced by metabolically active cells contributes to the final percentage.
How do I handle replicates and calculate standard deviation in MTT assays?
For each experimental condition, use at least triplicate wells (n=3). Calculate the mean absorbance for each group, then apply the viability formula to each replicate individually to obtain replicate viability percentages. Calculate the standard deviation (SD) from these replicate viability values. The calculator supports up to 6 replicates per sample. Report results as mean ± SD. If one replicate deviates by more than 20% from the others, investigate potential pipetting errors, edge effects, or incomplete solubilization before excluding it.
Why is my MTT assay viability reading above 100%?
Viability exceeding 100% usually indicates that the treated cells have higher metabolic activity than the control cells. This can occur when the treatment induces cellular stress responses that upregulate mitochondrial dehydrogenase activity, or when the control cells were not at optimal confluence. Other causes include incomplete blank correction, interference from colored compounds in the treatment, or using a control that was accidentally treated. Always verify that control wells represent truly untreated, healthy cells at the same seeding density as treated wells.
What is the difference between MTT, MTS, WST-1, and resazurin assays?
MTT produces insoluble purple formazan crystals requiring DMSO solubilization and is read at 570 nm. MTS (CellTiter 96) produces a soluble formazan product read at 490 nm without a solubilization step. WST-1 is highly water-soluble, read at 450 nm, and is more sensitive than MTT. Resazurin (AlamarBlue) is reduced to pink resorufin and can be measured by absorbance or fluorescence; it is non-toxic to cells, allowing the same plate to be re-read over time. All four measure metabolic activity via mitochondrial dehydrogenases, but their sensitivity, solubility, and readout wavelengths differ.
How accurate is the IC50 estimation from this calculator?
The calculator estimates IC50 using linear interpolation between the two dose points that bracket 50% viability. This method is fast and useful for preliminary screening, but it assumes a linear relationship between the bracketing points. For publication-quality data, use nonlinear regression (4-parameter logistic curve fitting) with at least 8-10 data points spanning a wide concentration range. The linear interpolation here requires at least one point above and one below 50% viability. If your data does not cross 50%, the calculator will report that IC50 cannot be interpolated.