PI formula in Excel is used to return the mathematical constant π (pi), which is the ratio of the circumference of a circle to its diameter.

**The function takes no arguments,** so the syntax of PI() is simply: =PI().

PI is a pre-defined function in Excel, meaning it is always available to be used in your formulas. This function is useful when making calculations involving circles or other circular objects. For example, you can use PI() in your formulas to calculate the area or circumference of a circle or even the volume of a sphere.

The value returned by PI() is an approximation, with 15 decimal places. It is accurate to within 10^-15, which is equivalent to 0.000000000000001.

The function will not update itself when the value of pi changes. If you need a more precise value for pi, you can use the DECIMAL() function. This function allows you to specify the number of decimal places to be included in the result.

In addition to being used in calculations involving circles, you can also use PI() in other mathematical calculations.

For example, you can use it to calculate the angle of a triangle or the slope of a line.