Interpolation blog

Short write-ups on interpolation, each with numbers you can check by hand. If you just need an answer, the calculator is on the homepage.

Tutorials

• Linear interpolation tutorial Your table has 20 and 30 but you need 24. Formula, examples, and what goes wrong.
• Cubic spline interpolation, solved by hand When straight lines between your points look too crude. Solved by hand for four points.
• How to interpolate in Excel Three formulas that work, and the one that quietly returns the wrong answer.

Articles

• How to interpolate between two numbers Work out how far across the gap you are, then apply that fraction. Four worked examples.
• The linear interpolation formula explained What each part of the formula means, why it works, and how wrong it gets.
• Interpolation vs extrapolation Filling a gap inside your data is safe. Guessing past the end of it is not.
• Bilinear interpolation explained When your table has values across the top and down the side. Worked both ways.
• Interpolating steam tables in engineering The table has rows at 100 and 125 kPa but you need 115. Worked on real data.
• Lagrange interpolation explained Write the curve through your points straight from the data, no equations to solve.
• Newton's divided difference interpolation Build the same curve one term at a time, so a new data point costs one line.
• Polynomial interpolation and Runge's phenomenon Adding more data points can make a polynomial fit worse. Watch it happen.
• How interpolation is used in data science Missing values, percentiles, SMOTE and warmup are all the same arithmetic.
• Linear interpolation in Python with numpy and scipy Which function to use, and the silent default that turns bad input into a flat line.