42A: By-Company Apportionment Factor Details and Instructions 2025 (2025)
Idaho · Idaho Tax Commission · Used for tax years 2025
Idaho Form 42A provides a company-by-company breakdown of the Idaho apportionment amounts reported on Form 42, Part I. It reports each company’s Idaho property, sales, payroll, and apportionment factor and is included with Form 42.
Who files this form
Form 42A must list each company identified as “transacting business in Idaho” in column F of Form 41A. It provides a by-company breakdown of the numerators reported on Form 42, Part I, Apportionment Formula.
What you'll need
Have these forms and details available to complete Form 42A:
- Form 41A, column F, to identify companies transacting business in Idaho.
- Form 42, including the Total column amounts used to calculate the property, sales, and payroll percentages.
- Each company’s name and federal Employer Identification Number (EIN).
- Beginning- and end-of-year Idaho inventories and owned real and tangible personal property at original cost, plus rents paid for rented property.
- Idaho sales and gross receipts details, including sales shipped or delivered to Idaho purchasers, “throwback” sales, services, and other business receipts.
- Idaho wages and salaries.
How to file
Include Form 42A with Idaho Form 42.
Enter each company’s name and EIN, check the box for each company using three-factor apportionment, and complete each field for each company transacting business in Idaho. See the Form 42 instructions for factor calculation details.
Common mistakes
- Do not leave out companies identified as transacting business in Idaho in Form 41A, column F.
- Remember to check the box for each company using three-factor apportionment.
- The Idaho Combined Corporation Total column should not exceed the sum of all companies listed on Form 42A.
- The Idaho combined corporation total on Form 42A should match the total of the Idaho column on Form 42. Include companies using both three-factor and single-sales apportionment.
- Remember to include Form 42A with Form 42.
Questions people ask
How many decimal places should I use for the factor percentages?
Compute the property, sales, and payroll percentages on lines 9, 16, and 18 to four decimal places.
How is rented property reported?
Line 7 reports rented property capitalized at eight times the rents paid. Add lines 6 and 7 to get total property on line 8.
How do three-factor taxpayers calculate line 20?
Divide the total percentage on line 19 by three. If a factor does not apply to your business, divide by the number of factors used instead. The instructions give having no employees anywhere as an example that reduces the number of factors by one.
Where can I ask questions?
Call (208) 334-7660 in the Boise area or (800) 972-7660 toll free. The hearing-impaired TDD number is (800) 377-3529. You can also visit tax.idaho.gov/contact.