Calculate viral titer using Reed-Muench, Spearman-Kärber, and Improved Kärber methods. Step-by-step audit trail with 95% confidence intervals. Data never leaves your browser.
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Load example TCID50 data to see the full workflow
| Dilution | Positive Wells | Total Wells | % Positive |
|---|---|---|---|
| 10-1 | 0.0% | ||
| 10-2 | 0.0% | ||
| 10-3 | 0.0% | ||
| 10-4 | 0.0% | ||
| 10-5 | 0.0% | ||
| 10-6 | 0.0% | ||
| 10-7 | 0.0% | ||
| 10-8 | 0.0% |
When to use
Do not use for
Four wells per dilution is the practical minimum; 6–8 wells substantially narrow the confidence interval. With only 2–3 wells, the proportional distance calculation is coarse and small pipetting errors dominate the result.
Cytopathic effect (CPE) scoring is subjective. Define CPE criteria before reading plates and have the same person read all plates in a study. Ambiguous wells at borderline dilutions can shift the TCID50 by a full log.
Spearman-Kärber requires at least one dilution at 100% positive and one at 0% positive. If your highest dilution still shows 100% CPE, extend the series — otherwise only Reed-Muench is available, and even that result is an extrapolation.
Infection rates should decrease monotonically with increasing dilution. If a higher dilution shows more positives than a lower one, investigate: well contamination, cross-talk between wells, or a transcription error. Non-monotonic data produces unreliable titer estimates.
The TCID50-to-PFU conversion (PFU 0.693 TCID50) assumes one infectious unit per cytopathic event and Poisson-distributed infection. This holds well for lytic viruses but can overestimate PFU for viruses that spread cell-to-cell or cause bystander CPE.
Implements Reed-Muench cumulative method, Spearman-Kärber proportional distance, and Kärber improved estimator with 95% confidence intervals via the Hamilton method. TCID50-to-PFU conversion uses the Poisson relationship: PFU = TCID50 ln(2).
Last validated 2026-03-16. Calculations are designed for planning and documentation support; verify procurement decisions against manufacturer specifications or institutional SOPs.
ConductScience TCID50 Calculator (v1.0). ConductScience, Inc. 2026. Available at: https://conductscience.com/tools/tcid50-calculator
Reed LJ, Muench H. A simple method of estimating fifty per cent endpoints. Am J Epidemiol. 1938;27(3):493-497.
Spearman C. The method of "right and wrong cases" (constant stimuli) without Gauss's formulae. Br J Psychol. 1908;2:227-242.
Kärber G. Beitrag zur kollektiven Behandlung pharmakologischer Reihenversuche. Arch Exp Pathol Pharmakol. 1931;162:480-483.
Lei C, et al. On the Calculation of TCID50 for Quantitation of Virus Infectivity. Virologica Sinica. 2021;36:141-144.
TCID50 (Tissue Culture Infectious Dose 50%) is the gold standard metric for quantifying viral infectivity in cell culture. It represents the dilution of a virus stock at which 50% of inoculated cell culture wells show cytopathic effect (CPE).
Unlike plaque assays (which measure PFU), TCID50 assays work for viruses that do not form distinct plaques — including many respiratory viruses, retroviruses, and some enteroviruses. This makes TCID50 the preferred method across virology, vaccinology, and gene therapy.
This calculator implements three established methods for TCID50 estimation:
