How to Use the Gamma Distribution with GAMMA.DIST in Excel

Use GAMMA.DIST to model positive, skewed amounts like monthly rainfall — and fit it from a mean and standard deviation.

You will learn to

• Find the chance of a value below a limit
• Find the chance of an extreme high value
• Derive shape and scale from a mean and SD

How it works

The gamma distribution fits quantities that are always positive and skewed to the right: monthly rainfall, insurance losses, time to complete several tasks in a row. GAMMA.DIST takes x, a shape (alpha) and a scale (beta); its mean is alpha × beta.

If you only know the mean and standard deviation of your data, you can still fit it: shape = (mean ÷ SD)² and scale = SD² ÷ mean. With TRUE the function returns the chance of x or less; subtract from 1 for the chance of more.

=GAMMA.DIST(40,2.5,30,TRUE)