Supposing you have recorded the experiments data as left screenshot shown, and to add best fit line or curve and figure out its equation (formula) for a series of experiment data in Excel 2013, you can do as follows:

1. Select the experiment data in Excel. In our case, please select the Range A1:B19, and click the Insert Scatter (X, Y) or Bubble Chart > Scatter on the Insert tab. See screen shot:

2. Select the scatter chart, and then click the Add Chart Element > Trendline > More Trendline Options on the Design tab.

3. In the opening Format Trendline pane, check the Polynomial option, and adjust the order number in the Trendline Options section, and then check the Display Equation on Chart option. See below screen shot:

Then you will get the best fit line or curve as well as its equation in the scatter chart as above screen shot shown.

In most cases, you may get multiple sets of experiment data. You can show these sets of data in a scatter chart simultaneously, and then use an amazing chart tool – Add Trend Lines to Multiple Series provided by Kutools for Excel – to add the best fit line/curve and formula in Excel.

1. Select the sets of experiment data, and click Insert > Scatter > Scatter to create a scatter chart.

2. Now the scatter chart is created. Keep the scatter chart, and click Kutools > Charts > Chart Tools > Add Trend Lines to Multiple Series. See screenshot:

Now the trendline is added to the scatter chart. If the trendline does not match with the scatter plots, you can go ahead to adjust the trendline.

3. In the scatter chart, double click the trendline to enable the Format Trendline pane.

4. In the Format Trendline pane, tick the trendline types one by one to check which kind of trendlines is the best fit. In my case, the Polynomial trendline fits best. And tick the Display Equation on chart as well.