How to linearly interpolate between two points
✓ Verified in LibreOffice 25.8.7.3Estimate a value between two known data points — lookup tables, calibration curves, in-between dates.
The formula
| App | Formula | Notes |
|---|---|---|
| Excel | =FORECAST.LINEAR(D2,B2:B3,A2:A3) | D2 = the x to estimate; A2:A3 known x values; B2:B3 known y values. |
| Google Sheets | =FORECAST.LINEAR(D2,B2:B3,A2:A3) | Identical; plain FORECAST also still works. |
| LibreOffice Calc | =FORECAST.LINEAR(D2,B2:B3,A2:A3) | Identical. |
How it works
With exactly two known points, FORECAST.LINEAR draws the straight line through them and reads off the y for your x: halfway between x=2 (y=10) and x=3 (y=20) is 15. The manual form is =B2+(D2-A2)*(B3-B2)/(A3-A2) — same answer, useful when you need to see the algebra. For a bigger table, first bracket the x with the closest-match lookup pattern, then interpolate between those two rows.
Verified, not just documented
We ran =FORECAST.LINEAR(D2,B2:B3,A2:A3) in LibreOffice 25.8.7.3 (headless, with forced recalculation) and it returned 15 — exactly the expected result. Every formula here is confirmed by actually executing it.