How to calculate logarithms in VBA

I will show you how to calculate logarithms in VBA, both base 10 and natural. Logarithms are useful for working with exponential and power functions, such as growth rates, decay rates, and compound interest.

What is a logarithm?

A logarithm is the inverse of an exponential function. It tells you what power you need to raise a base number to get another number. For example, the logarithm of 100 to the base 10 is 2, because 10^2 = 100. The logarithm of 8 to the base 2 is 3, because 2^3 = 8.

The base of a logarithm can be any positive number, but two common bases are 10 and e. The base 10 logarithm is also called the common logarithm, and it is often used in scientific notation and engineering applications. The base e logarithm is also called the natural logarithm, and it is often used in calculus and statistics. The constant e is approximately 2.718282.

How to calculate logarithms in VBA?

VBA does not have a built-in function for calculating base 10 logarithms, but it does have a function for calculating natural logarithms: Log(number). This function returns a Double value that represents the natural logarithm of the number argument, which must be a positive numeric expression.

To calculate base 10 logarithms in VBA, we can use a simple formula that relates the natural logarithm and the base 10 logarithm:

Log10(number) = Log(number) / Log(10)

We can define a custom function in VBA that implements this formula:

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Function FindLog10(number)
FindLog10 = Log(number) / Log(10)
End Function

Alternatively, we can use the WorksheetFunction object to access the Excel LOG function, which calculates the base 10 logarithm of a number:

Function FindLog10(number)
FindLog10 = Application.WorksheetFunction.Log(number)
End Function

Both functions will return the same result. For example, if we have the value 1000 in cell A1, we can use either function in cell B1 to get the base 10 logarithm of 1000, which is 3:

Range(“B1”).Value = FindLog10(Range(“A1”))

How to use logarithms in VBA?

Logarithms have many applications in mathematics and science. Here are some examples of how to use logarithms in VBA:

  • To convert a number from one scale to another, such as decibels, pH, Richter scale, etc., we can use the formula:

NewScale = Constant * Log(OldScale)

For example, to convert a sound intensity level from watts per square meter to decibels, we can use the formula:

Decibels = 10 * Log(WattsPerSquareMeter)

  • To solve for an unknown exponent in an equation of the form y = b^x, we can use the formula:

x = Log(y) / Log(b)

For example, to find the value of x that satisfies the equation 2^x = 32, we can use the formula:

x = Log(32) / Log(2)

  • To calculate the time needed for a quantity to grow or decay by a certain percentage, we can use the formula:

Time = Log(Final / Initial) / Log(1 + Rate)

For example, to find how long it takes for an investment of $1000 to grow to $1500 at an annual interest rate of 5%, compounded monthly, we can use the formula:

Time = Log(1500 / 1000) / Log(1 + 0.05 / 12)