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.
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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.
Need it as an auditable file?
Ships inside the linked template — formula-driven, unlocked, audit-ready.
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. The difference between a quick fix and a sheet you can trust is the extra minute you spend validating “simple interpolation formula excel”. Start on a copy or a tiny sample, keep the affected formula visible, and compare the result with the tool above before you touch the real workbook. 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. The point is a calculation you can defend to a CFO or an auditor, but the practical win is that someone else can open the file and understand what happened without asking you.
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. Nothing on this page is behind a login or a download: the exact answer is at the top, and the detail below it is there for when you need it. That is the whole promise here — the answer first, and just enough context to make it stick. Here is the takeaway for “simple interpolation formula excel”: copy the answer if you are busy, but if you have a spare few minutes, rebuild the example in Excel yourself with the tool above open beside it. That single pass — type it, run it, watch the result move when you change an input — is what turns a formula you found into a technique you trust. Keep your inputs labelled and referenced, never hard-coded, and the same sheet stays correct and auditable as it grows. Done that way, you will not need to look this up again, and you will be the person others ask.
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.