Excel Interpolate Data

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. Keep the inputs visible and clearly labelled and the whole thing stays auditable — anyone who opens the file later, including you, can see at a glance exactly what feeds the result and change one assumption without hunting through the formula. For “excel interpolate data”, the reliable version is a short checking loop, not just the first command that appears to work. Run it on a deliberately small range first, watch how the affected cells change, and only then apply the same setup to the full sheet. 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 is what makes a workflow that saves repeating the same clicks every week useful in real work: repeatable, auditable, and not dependent on memory or luck.

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. If there is any chance you will reuse this, drop it into a small template tab right now: a labelled input area on the left and the formula beside it, checked once against the tool above. Next time the same question comes up, the answer is a single paste away instead of a rebuild from memory.

In Google Sheets

Google Sheets handles this almost identically to Excel. The formula syntax above is the same, and the menu lives under a slightly different label rather than a ribbon tab. Use the platform toggle at the top of the page to switch every keyboard shortcut between Windows and Mac, and expect at most cosmetic differences in naming. The aim was to get you unstuck fast and leave you a little more capable than a copy-paste would. The answer is at the top, the tool proves it, and the detail above shows why it holds — so the next time a colleague asks, you can answer without reaching for search. Treat “excel interpolate data” as a small building block rather than a chore. Once the inputs sit in their own cells and the formula reads from them, the same setup answers a dozen related questions with a tweak, and Excel keeps every dependent figure current as the data changes. The tool above is there so you can rehearse and verify before committing anything to a real workbook; the steps and worked example are there so the logic sticks. Get it right once and it stops costing you time — it starts saving it, every time the question comes back around.

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