Calculation Excel — Spline

Given (n) points ((x_i, y_i)) with (x_1 < x_2 < ... < x_n), a cubic spline is a set of cubic polynomials (S_i(x)) between each pair of points. For natural splines:

With this method, you can perform professional‑grade spline interpolation directly inside Excel, without macros or external tools. Once set up, changing input points automatically updates the spline – a practical, reusable solution for smooth data fitting. spline calculation excel

For repetitive tasks or large datasets, is the gold standard. You can create a custom function =CubicSpline(x_known, y_known, x_new) . Given (n) points ((x_i, y_i)) with (x_1 &lt; x_2 &lt;

CubicSpline = a * yData(i) + b * yData(i + 1) + c * (yData(i + 1) - yData(i)) + d * (yData(i) - yData(i + 1)) Exit Function End If Next i End Function Once set up, changing input points automatically updates

Set up the tridiagonal matrix A (size (n-2 \times n-2)) and vector B.

' Step 5: Compute coefficients b, c, d For i = 1 To n - 1 b(i) = (a(i + 1) - a(i)) / h(i) - h(i) * (c(i + 1) + 2 * c(i)) / 3 d(i) = (c(i + 1) - c(i)) / (3 * h(i)) Next i