Fit your data to your own custom equation in seconds. This is the most powerful custom equation curve fitting application online. Updated:3/6/2026
Fit Residuals (Error Distribution)
R-Squared -
Adj. R-Squared -
SSR -
RMSE -
AIC -
BIC -
Status -
Parameter Confidence Diagnostics
| Parameter | Estimate | Std Error | 95% CI | t-Stat |
|---|
Optimization Command Center
Run controls, status, and readiness are always visible while you work.
Data Pending Equation Pending Run Pending Results Waiting Mode Desktop
Optimizing with robust multi-start strategy...
Engine: Idle
Status: Waiting for input.
About
Tip: press Ctrl+Enter to run.
What This Tool Does
This is a custom equation curve fitting tool for nonlinear regression online. Enter your own equation, paste your X/Y data, and click run to get best-fit parameter values.
You can set parameter guesses and bounds to guide the fit. The page is built to handle difficult models and still stay responsive.
What makes it unique: you are not limited to preset equations. You can fit your own model, review fit quality with charts and diagnostics, and copy an Excel or Google Sheets formula in one step.
Data Readiness Pending
Need at least 3 valid (x, y) rows.
Equation Readiness Pending
Type or edit any equation with at least one parameter.
Run Readiness Pending
Configure bounds and solver settings, then run.
About and How to Use
This custom equation curve fitting application performs nonlinear regression online using robust least-squares optimization with multi-start seeding, optional CMA-ES pre-search, and diagnostics for reliability.
You can fit user-defined equations directly, control initial guesses and bounds for each parameter, and inspect convergence quality using R-squared, adjusted R-squared, SSR, RMSE, AIC, BIC, residual distributions, and parameter confidence diagnostics.
For best outcomes on difficult equations, constrain bounds to physically realistic ranges, keep units consistent, and verify residual structure rather than relying only on R-squared.
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Need fixed model families first? Start with Polynomial, Exponential, or Logarithmic regression.
Need broader model exploration? Use CurveFit Pro or Curve Fitting Online.