Online splines generator

This free web app generates a cardinal spline1, cubic spline2, and monotone cubic spline3 - curves that run exactly through your data points - and returns interpolated points.
If the graph of your data points is convex (or concave), then spline-based data interpolation will outperform linear interpolation, which will systematically overestimate (or underestimate) values between two consecutive data points.

  1. The cardinal spline has parameter tension, with a value between 0 and 1 (default 0.5). If set to 0, the spline reduces to straight lines between data points.
  2. The cubic spline typically yields a smoother overall curve than a cardinal spline, which may change direction more sharply near the data points, partly depending on its tension parameter.
  3. The monotone cubic spline’s advantage is that it avoids “wobbling” in the interpolated curve as it preserves monotonicity (see wikipedia). Monotonicity means a function moves in only one direction: it never increases then decreases (or never decreases then increases). Example below is clearly not monotone.
For an Excel VBA equivalent - including demo file - see Excel splines based interpolation.
For interpolation within a 2-dimensional table, see Bilinear and spline based 2D-interpolation.

Enter semicolon or tab separated.
Points get sorted by acending X,
points with duplicate X deleted.
 X                           Y

Input: parameters

 Create same X-values as both other splines
       (Of course, spline remains the same.)
Generate splines

Output: interpolated points


 X                           Y

 X                           Y

 X                           Y

 X                           Y

 X                           Y

 X                           Y

Output: splines graph