How to linearly interpolate between two points
✓ Verified in LibreOffice 25.8.7.3 ✓ Verified in Google Sheets (2026-08-30)Estimate a value between two known data points — lookup tables, calibration curves, in-between dates.
The formula
| App | Formula | Notes |
|---|---|---|
| Excel (desktop) | =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. We then ran the same formulas in Google Sheets, executed 2026-08-30: a formula-only workbook goes into Google Drive, which converts it to a Sheet and recalculates every formula with Google’s own engine, and comes back out as .xlsx carrying the values Google computed. It returned 15 for the worked example, the same value LibreOffice produced. Both engines’ numbers on this page are executed results. The Excel formula follows Microsoft’s official documented syntax — we do not run desktop Excel.
Functions used
FORECAST.LINEAR — see full Excel, Google Sheets & LibreOffice compatibility for each.