IC50 / EC50 4PL Calculator
4PL dose-response · IC50/EC50 · Hill slope · R²
IC50 / EC50 4PL Calculator
4PL dose-response · IC50/EC50 · Hill slope · R²
Calculated locally in this browserColumn order: concentration, response
Editable data table
| concentration | response | |
|---|---|---|
Paste CSV / TSV
Conversion: 1 µM = 1 µM
Analysis result
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PRACTICAL GUIDE
Use the result with confidence.
Use this 4PL dose-response calculator to estimate an IC50 or EC50 from concentration-response observations. It reports the fitted midpoint, Hill slope, top and bottom plateaus, R² and curve figure while keeping the raw data and fit-quality warnings visible.
Open the calculatorCheck the calculation in context.
Midpoint bracketed by measured concentrations
What makes an IC50 interpolation more defensible?
- Measure low-response and high-response plateaus.
- Include several concentrations through the transition region.
- Confirm the fitted midpoint falls inside the tested concentration range.
Result
A midpoint supported on both sides by observations is interpolated rather than projected beyond the experiment.
Interpretation: Still review replicates, residual pattern, normalization and parameter plausibility; R² alone is not validation.
Curve never reaches 50% effect
Can the model still return an IC50?
- A nonlinear optimizer may estimate a midpoint outside the measured range.
- The estimate then relies strongly on model shape and fitted plateaus.
- Expand the concentration range or report that the midpoint was not reached.
Result
A precise-looking extrapolated IC50 can be weakly supported even when the fit converges.
Interpretation: Do not present an extrapolated midpoint as equivalent to a bracketed experimental estimate.
Correct numbers can still lead to a poor experiment.
Using concentration zero on a log scale
Log-concentration models cannot directly transform zero.
What to do
Treat vehicle control separately and fit positive concentrations according to the predefined analysis.
Forcing plateaus without justification
Fixed top or bottom values can shift midpoint and slope.
What to do
Use constraints only when the assay definition and analysis plan support them.
Judging the curve by R² alone
A high R² can coexist with sparse transition data, systematic residuals or implausible parameters.
What to do
Inspect coverage, replicates, residuals, parameter uncertainty and whether the midpoint is bracketed.
REPORTING NOTES
Document the calculation clearly.
Copy a methods-ready sentence or preparation checklist, then adapt it to your actual protocol, instrument and acceptance criteria.
Methods sentence
Concentration-response data were fitted with a four-parameter logistic model; concentrations were positive and analyzed on the stated concentration scale.
Result sentence
Report IC50/EC50 with units, Hill slope, fitted top and bottom, concentration range, replicate summary, fitting constraints and whether the midpoint was bracketed.
Questions researchers often ask.
What is the difference between IC50 and EC50?
IC50 describes the concentration associated with 50% inhibition under the assay definition; EC50 describes the concentration associated with half-maximal effect. Both are model- and assay-dependent.
How many concentrations are needed for a 4PL curve?
Four valid points are a mathematical minimum, not a robust design. Use enough concentrations and replicates to define both plateaus and the transition region.
Should concentration zero be included in the 4PL fit?
Zero cannot be log-transformed. Preserve vehicle controls for normalization and QC, and follow a predefined fitting method for positive concentrations.
Is an IC50 valid when 50% inhibition was not observed?
The software may return an extrapolated value, but support is weak. Report the limitation and consider expanding the tested range.
Data requirements
Enter at least 4 valid rows. Concentrations must be greater than 0 and responses must be numeric.
concentration, responseReview before reporting
Research use only. Validate raw data, exclusions and interpretation against the approved analysis plan.
Check raw observations, excluded rows, model assumptions and study-specific acceptance criteria before using the result.
Transparent analysis
Inputs are processed locally in your browser. Review the calculation policy, limitations and verification approach before reporting a result.
Read the methodology →EVIDENCE & REVIEW
A result you can audit.
Reviewed 12 August 20264-parameter logistic workflow review complete
Implementation check
The workflow preserves raw rows, shows the fitted curve and reports model parameters so users can judge whether the observed concentrations bracket the response midpoint.
Interpretation boundary
An estimated IC50 is model- and assay-dependent. Sparse plateaus, constrained parameters, outliers or an unbracketed midpoint can produce a precise-looking but weak estimate.
Method references
Assay Operations for SAR SupportNCBI Assay Guidance ManualDose-response curve fitting, 4PL use and IC50 interpretation context.
REFERENCE VALIDATION CASE
Bracketed synthetic 4PL midpoint
Noise-free values generated from the same four-parameter logistic form used by the calculator.
Inputs
- Bottom 0, top 100
- Hill slope 1
- Synthetic midpoint at 1 µM
- Concentrations on both sides of 1 µM
Expected result
- Fitted midpoint approximately 1 µM
- Top and bottom approach 100 and 0
- R² greater than 0.999 for noise-free data
Scope: A software regression check, not biological or assay validation.
Review and correction history
12 August 2026 · Internal implementation review
4-parameter logistic workflow review complete. Formula behavior, unit handling, limitations and linked references were checked internally.
Independent reviewer: Not yet published. A name, relevant qualification, date and exact scope will appear here only after a real review is completed.
Source-informed and internally checked; independent scientific or regulatory validation is not claimed. Research use only.