Constants in Excel VBA are values that don’t change during the execution of a program. They are useful for defining fixed values such as mathematical constants, file paths, or error codes.

To declare a constant in VBA, use the `Const` keyword followed by the name of the constant and its value. For example, to declare a constant named “pi” with a value of 3.14159, you would write:

Const pi As Double = 3.14159

The `As` keyword is used to specify the data type of the constant. In this case, we are using the `Double` data type to represent a floating-point number.

Once you have declared a constant, you can use it in your VBA code by referring to its name. For example, if you wanted to calculate the circumference of a circle with a radius of 5 using the constant “pi”, you would write:

Dim radius As Double

Dim circumference As Double

radius = 5

circumference = 2 * pi * radius

In this example, we first declare two variables `radius` and `circumference` as type `Double`. We then set the value of `radius` to 5 and calculate the value of `circumference` using the formula 2 * pi * radius. Because we declared `pi` as a constant, we can use its name directly in the calculation.

One thing to keep in mind when using constants is that their values cannot be changed during the execution of the program. Attempting to assign a new value to a constant will result in a compile-time error.

That’s a brief introduction to using constants in Excel VBA. Constants can be very useful for simplifying your code and making it more readable.