What is EC50 and how is it different from IC50?
EC50 is the concentration that produces 50% of the maximum effect, while IC50 is the concentration that produces 50% inhibition. EC50 is used for activation studies, IC50 for inhibition studies. See also our Confidence Interval Calculator (Biology).
What data format should I use for the calculator?
Enter concentration and response values separated by commas, one pair per line. You need at least 4-6 data points spanning above and below the expected EC50/IC50 value for accurate curve fitting.
What does the Hill slope tell me about my dose-response curve?
The Hill slope indicates the steepness of the curve. A slope of 1 indicates a standard sigmoidal curve, while slopes >1 indicate cooperative binding and slopes <1 indicate negative cooperativity.
How is the R² value calculated and what does it mean?
R² measures how well the 4-parameter logistic model fits your data, ranging from 0 to 1. Values above 0.95 indicate excellent fit, while values below 0.8 suggest poor curve fitting.
Should I use logarithmic or linear concentration scale?
Logarithmic scale is recommended for most dose-response studies as it better displays the sigmoidal nature of the curve and is standard in pharmacology. Linear scale may be used for narrow concentration ranges.
What is the 4-parameter logistic equation used in this calculator?
The equation is: Y = Bottom + (Top - Bottom) / (1 + (X/EC50)^HillSlope). This models the sigmoidal dose-response relationship with four key parameters: top, bottom, EC50, and hill slope.
How many data points do I need for reliable EC50 calculation?
You need at least 4-6 concentration points, but 8-12 points provide better accuracy. Include concentrations both above and below your expected EC50 value, ideally spanning 2-3 orders of magnitude.