# Interpolation of a Curve in Excel

#### ferrethouse

##### New Member
Basic statistics help please. I have the following values. I need to know what the value of the first column is given the value of the second column being 55.3 (sorry for the poor formatting "55.3" should be in the second column)...

2000 74.4
55.3
1000 18.2
850 13.5
500 6.5
425 5.5
250 3.9
180 3.3
150 3.1
125 2.8
75 2.2
63 2.1
44 1.8
31 1.5
20 1.2
16 1.1
11 0.9
7.8 0.7
6.6 0.6
3.9 0.4
2 0.2
1.3 0.1

### Excel Facts

Move date out one month or year
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Take a look at the LINEST function. It allows you to interpolate data.
First, try plotting teh data as an X-Y Scatter and inserting a trend line. Play with linear and third-order fits; Back when I was in a lab we used third-order polynomials for curve fitting if it was meant to be linear -- it made allowance for variation in the readings and gave a better fit.
Once you know whether linear or third-order looks better, the LINEST function will let you specify so that you can calculate the interpolated value.

Denis

Little bit of addition to Denis's suggestion:
Assuming data in column A are x-values and data in column B are y-values, its looks like you want x-value for known y-value. Since trendline gives you y = f(x) and you really are interested in x=f, you might want to plot column B as x-values and column A as y-values on chart and then pass the trendline. Make sure "Display equation on chart" is selected. You can use this equation to find out x-values for your known y-values.
Also, while playing with trendlines, you might want to try to extend the curve forward and backward several units to see its behavior and change the order of polynomial (or the curve type) if the behavior doesnt seem right.

Last edited:
Worked like a charm. Thanks guys!

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