How it works
For each x, find the two neighbouring rows and interpolate linearly between them:
y = yᵢ + (x − xᵢ) × (yᵢ₊₁ − yᵢ) / (xᵢ₊₁ − xᵢ)
Linear table lookup vs. a polynomial fit
Piecewise linear interpolation only uses the two nearest points, so it never overshoots. A Lagrange polynomial can be more accurate on smooth curves with a few points, but it oscillates with more than 6–8 points.
Tips for accuracy
- Closer rows give better results, especially where the data curves.
- The table tells you nothing about values beyond its ends, so be careful when extrapolating.
- For a 2D table (rows and columns), use bilinear interpolation.
FAQ
What format does the data need?
One x, y pair per line, separated by a comma, space or tab. You can paste two columns from Excel. A header row is ignored.
Do the x values need to be sorted?
No. The points are sorted for you. Each x must be unique.
What if x is outside the table?
It is reported as out of range. Tick “Extrapolate” to extend the first or last segment instead.