Apportionment is an accounting term used to describe an allocation of a portion of a business's gross earnings. It is possible to calculate the apportionment rate of this allocation with some basic information about the business's earnings and total book value. Apportionment is generally expressed as a percentage and is often referred to as the apportionment rate.

Determine the number of days in the business's quarter. For example, assume there were 90 days in the quarter.

Calculate the gross income earned in the quarter. Gross income for a business is the sum of interest income and sales gain or loss for the quarter. For example, assume a business had interest income of $10,000 and gross sales of $100,000. $10,000 + $100,000 = $110,000.

Calculate the book value of the business. The book value is the net asset value of the business. To calculate book value, subtract the business' intangible assets and liabilities from the total physical assets of the business. Intangible assets include patents and goodwill. For example assume the book value of the business is $2,500,000.

Divide the gross income figure from Step 2 by the number of days in the quarter from Step 1. Continuing the same example, $110,000 / 90 = 1,222.22.

Multiply the figure from Step 4 by the number of days in a year. This number is typically 365 unless it is a leap year, when you'd use 366. Continuing the same example, 1,222.22 x 365 = 446,111.11

Divide the figure from Step 5 by the book value figure from Step 3. Continuing the same example, 446,111.11 / 2,500,000 = 17.84 percent. This figure represents the apportionment rate.