With the exception of a leap year, each month has exactly the same number of days each year. Ranging from 28 to 31 days each month, the Gregorian Calendar used in the United States measures the number of days and weeks in a year for a total of 365 days -- or 366 days in a leap year. With a simple mathematical calculation, you can calculate how many weeks are in each month.

Count the number of days in the month and divide that number by 7, which is the number of days in one week.

For example, if March has 31 days, there would be a total of 4.43 weeks in the month. (31 / 7 = 4.43).

Calculate the number of weeks in a month without a calculator by counting the number of full weeks by hand and dividing the remaining days by 7.

Take the number of full weeks and the answer for the remaining days and add them together.

For example, since there are 30 days in April, there are exactly four full weeks and two remaining days.

Divide the 2 by 7 to get 0.29.

Add the two numbers together and you can calculate the month of April has 4.29 weeks (4 + 0.29 = 4.29).