In Excel: click the chart, open the ➕ Chart Elements button and tick Trendline — then right-click the line ▸ Format Trendline to pick the type and show the equation and R².
6 pairs
R² = 0.9962
Intercept = 0.1667
What this does
A trendline is a fitted line drawn through a data series: linear by default, with exponential, logarithmic, polynomial, power and moving-average alternatives in the Format Trendline pane. Two checkboxes at the bottom of that pane matter more than the line itself — "Display Equation on chart" and "Display R-squared value on chart". The equation lets you reproduce the fit in a formula, and R² tells you how much of the variation the line actually explains, which is the difference between a trend and a decoration. Excel also exposes the same maths as worksheet functions: SLOPE, INTERCEPT, RSQ, LINEST and TREND. 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. The difference between a quick fix and a sheet you can trust is the extra minute you spend validating “excel graph trendline”. Start on a copy or a tiny sample, keep the affected chart 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 visual that makes the number obvious at a glance, but the practical win is that someone else can open the file and understand what happened without asking you.
A worked example
Twelve months of revenue in B2:B13 against month numbers in A2:A13. Click the chart ▸ ➕ ▸ Trendline ▸ Linear. Right-click the line ▸ Format Trendline ▸ tick Display Equation and Display R-squared. If the chart shows y = 3,120x + 41,500 and R² = 0.87, revenue is rising about €3,120 a month and the straight line explains 87% of the variation. The same numbers without a chart: =SLOPE(B2:B13,A2:A13) returns 3120, =RSQ(B2:B13,A2:A13) returns 0.87. A trendline turns a scatter of points into a stated rate of change, and the equation plus R² is what makes that statement defensible instead of a line someone drew through the noise. One habit worth forming early: name the cells that hold your inputs, so the formula reads in plain language instead of a string of cell addresses. A reviewer — or you in three months — can then follow the logic without decoding what B7 and D2 were supposed to mean, which is most of what makes a sheet maintainable.
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 graph trendline” 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
- Fitting a high-order polynomial because it hugs the points — an order-6 fit through 12 points has almost no predictive value and a flattering R².
- Adding a trendline to a bar chart of categories, where the x-axis has no numeric meaning.
- Quoting the slope without R², which hides how well the line actually fits.
- Forecasting far beyond the data and presenting the extension as a measurement rather than an assumption.
- Leaving unequal time gaps in the x values while treating them as evenly spaced — use a scatter chart with real x values.
Frequently asked questions
How do I add a trendline in Excel?
Click the chart, press the ➕ Chart Elements button and tick Trendline. On Mac use Chart Design ▸ Add Chart Element ▸ Trendline.
How do I show the trendline equation?
Right-click the trendline, choose Format Trendline, and tick "Display Equation on chart" and "Display R-squared value on chart" at the bottom of the pane.
What does R-squared mean?
The share of the variation in your data that the fitted line explains, from 0 to 1. R² = 0.87 means 87%. A low R² means the line is drawn but the relationship is weak.
Can I get the trend without a chart?
Yes — =SLOPE(known_ys, known_xs) and =INTERCEPT(known_ys, known_xs) give the same line, =RSQ() gives R², and =TREND() returns fitted values for new x inputs.