Excel Linear Interpolation Function

What this does

Interpolation estimates a value at an input that lies between two measurements, on the assumption that the relationship is straight between them. Passing FORECAST.LINEAR the whole table instead of the bracketing pair does something quite different: it fits a least-squares line through every point and returns a value from that fit, which is a regression estimate rather than an interpolation. Most people learn this as a sequence of clicks and forget it by next week; learning it as a pattern instead is what lets you apply it to the next, slightly different version of the problem without starting from scratch. That is the difference this page is trying to make. Treat “excel linear interpolation function” as a small repeatable workflow rather than a one-off click you hope to remember next time. Use a small test block before the live file, so any surprise in the affected formula shows up while it is still harmless. When a formula is involved, keep the inputs labelled beside it, reference cells instead of typing values, and apply number formatting only after the result checks out. That turns a calculation you can defend to a CFO or an auditor into a method you can reuse, explain, and defend when the workbook leaves your screen.

A worked example

Viscosity is 4.2 at 20 °C and 3.1 at 30 °C. At 24 °C the interpolated value is 4.2 + (24-20) × (3.1-4.2) / (30-20) = 3.76, and =FORECAST.LINEAR(24,{4.2;3.1},{20;30}) returns the same 3.76. Handing the function all twelve rows of the table would return 3.68 — a different number answering a different question. Rate tables, calibration curves and price breaks are all published at intervals, and reading the nearest row instead of interpolating is a rounding error nobody documents. A practical tip before you scale it up: build it once on a small block of test data, confirm the number against the tool on this page, and only then point it at your real sheet. That one habit catches almost every mistake while it is still cheap to fix, long before a wrong figure reaches a report or a colleague.

In Google Sheets

Everything above works in Google Sheets too. Excel and Sheets share the formula syntax used here; only the surrounding menus are arranged differently. That portability is deliberate — learn it once and it follows you between the two tools and across Windows and Mac. Keep this page bookmarked for the next time the same question comes up. Better still, rebuild the example once in your own sheet — doing it yourself, with the tool above to check against, is what turns a copied formula into a technique you own. The short version of “excel linear interpolation function”: the answer is at the top of this page, the tool proves it on your own numbers, and the sections above explain why it holds so the next variation does not stump you. Excel rewards people who reference cells instead of typing values and who keep inputs separate from formulas, because that is what makes a result you can audit months later. Build it once, deliberately, with the live tool as a check, and you convert a one-off lookup into a reusable skill — which is the whole point of learning the why and not just the what.

Common mistakes

• Passing the entire dataset to FORECAST.LINEAR and calling the result an interpolation.
• Leaving the x column unsorted, which makes MATCH with match type 1 return the wrong bracketing row without any error.
• Extrapolating past the ends of the table, where a straight-line assumption has nothing supporting it.
• Interpolating a curved relationship over a wide gap — fit a curve, or interpolate between closer points.

Frequently asked questions