Please Note: This article is written for users of the following Microsoft Excel versions: 97, 2000, 2002, and 2003. If you are using a later version (Excel 2007 or later), this tip may not work for you. For a version of this tip written specifically for later versions of Excel, click here: Using the FORECAST Function.
The FORECAST function can be used whenever you have an existing set of data pairs (x-values and y-values) and you want to calculate an estimated y-value to a new x-value. Excel performs a linear regression based on the existing values and then inserts the x-values into the expression for the regression which gives you an y-value. The new x-value can have any value and is not restricted to be larger than the existing x-values.
The FORECAST function uses the following syntax:
=FORECAST(X, Yrange, Xrange)
In this usage, X is the X value for which you want FORCAST to return a Y value. The Yrange and Xrange parameters are sets of know Y and X values.
As and example, let's say that you are going on a diet, and you decide to keep track of your weight each day. Every day you enter the date into column A and the weight for that day in column B. After getting about 10 days or so of measurements, you can use these data pairs to forecast when you will hit your target weight. If your target weight is 160 lbs., you could use the following formula:
=FORECAST(160, A2:A11, B2:B11)
The result is the anticipated date when you will reach the target weight.
Excel calculates the "trend line" (using linear regression) of the points in A2:B11 (i.e., it assumes there is a linear relationship between the dates and the weights. (This trend line is the same as you would get from plotting the data pairs and adding a trend line to the chart.)
ExcelTips is your source for cost-effective Microsoft Excel training. This tip (2573) applies to Microsoft Excel 97, 2000, 2002, and 2003. You can find a version of this tip for the ribbon interface of Excel (Excel 2007 and later) here: Using the FORECAST Function.
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Please Note:
This article is written for users of the following Microsoft Excel versions: 97, 2000, 2002, and 2003. If you are using a later version (Excel 2007 or later), this tip may not work for you. For a version of this tip written specifically for later versions of Excel, click here: 
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