In Excel: give FORECAST.LINEAR only the two bracketing points — =FORECAST.LINEAR(x,{y1;y2},{x1;x2}) — and it returns the straight-line value between them.
On this page7
Sort the table by the x column ascending; everything below depends on that order.
Locate the row below the target with =MATCH(E2,$A$2:$A$13,1), which returns the position of the largest x not greater than the target.
Use INDEX with that position and the one after it to build the two-point ranges for FORECAST.LINEAR.
Or compute it arithmetically: y1 + (x - x1) * (y2 - y1) / (x2 - x1), which makes each term visible on the sheet.
Guard the edges with IF so a target outside the table returns a message rather than a silent extrapolation.
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. Check the referenced cells, argument order and result type in your workbook. Start with the worked example before changing an input, then compare the changed result with the calculation you expected. An error value or a result of a different type is a reason to inspect the inputs and arguments.
In Google Sheets
Google Sheets support for this task has not been verified. Check the function, arguments and workflow in the target product before transferring the example. Matching formula notation does not establish identical behaviour. Keep this page bookmarked for the next time the same question comes up. Better still, repeat the steps once in your own workbook — doing it yourself is what turns a copied answer into something you remember. For “excel linear interpolation function”, keep the input cells and the result visible while you check the formula in Excel. Follow the worked example and change one input at a time. The steps explain the workbook calculation; this page does not run a live calculator for that task.
Common mistakes
- Passing the entire dataset to FORECAST.LINEAR and calling the result an interpolation.
- Leaving the x column unsorted, which makes
MATCHwith 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
Is FORECAST.LINEAR the same as TREND?
For a single new x they return the same value. TREND accepts an array of new x values and returns an array of results.
How do I find the two rows to interpolate between?
MATCH with match type 1 on a sorted x column gives the row below; the row after it is the upper bracket.
What about non-linear data?
Add a trendline, tick Display Equation on chart, and evaluate that equation — or interpolate between points close enough together for a line to be reasonable.
Can I interpolate a date?
Yes. Dates are numbers, so a date x column works directly, as long as the cells are real dates rather than text.