How do you calculate home office square footage?

Measure the length and width of the space you use for work in feet and multiply them. A 10 ft × 12 ft office is 120 sq ft. Only count the area used regularly and exclusively for business — if you use part of a room, measure just that portion, not the whole room.

How does the home-office deduction use square footage?

Your office square footage sets the size of the deduction. The simplified method pays a flat $5 per square foot (up to 300 sq ft). The regular method divides office sq ft by total home sq ft to find a business-use percentage, which is then applied to your actual home expenses. This is general information, not tax advice.

What is the difference between the simplified and regular method?

The simplified method deducts $5 per square foot of office space, capped at 300 sq ft ($1,500 maximum), with no expense records needed. The regular method deducts a share of actual home expenses based on the business-use percentage. The regular method takes more paperwork but can yield a larger deduction.

What is the business-use percentage for a home office?

It's your office square footage divided by your total home square footage. A 120 sq ft office in a 1,500 sq ft home is 8% (120 ÷ 1,500). Under the regular method, you can deduct that percentage of eligible home expenses such as rent, utilities, and insurance.

Office Square Footage Calculator

Formula & guide

How it's calculated

An office is measured as area — length times width in feet. If your office isn't a clean rectangle, split it into rectangles, calculate each, and add them together.

1Measure the length of the dedicated office space in feet.
2Measure the width in feet, wall to wall.
3Multiply length × width to get the area in square feet.
4For a home-office deduction, apply the simplified rate ($5 per sq ft, capped at 300 sq ft) or divide office sq ft by total home sq ft for the regular method's business-use percentage.

Office length	10 ft
Office width	12 ft
Office area (10 × 12)	120 sq ft
Simplified deduction (120 × $5)	$600

Accuracy & assumptions

Measure only the space used regularly and exclusively for business — not the whole room if part of it is personal.
The regular method uses office sq ft ÷ total home sq ft as the business-use percentage applied to home expenses.
The simplified method caps the deductible area at 300 sq ft, so the most you can claim is $1,500 per year.
Commercial offices may quote usable square footage (your space) or rentable square footage (usable plus a share of common areas).

Key tips & rules

Simplified method: $5 per sq ft of office space, up to 300 sq ft — no expense receipts required.
Regular method: track actual home expenses and multiply by the business-use percentage (office sq ft ÷ home sq ft).
The $5/sq ft rate and 300 sq ft cap set a $1,500 ceiling on the simplified deduction.
The business-use percentage also decides how much of rent, utilities, and insurance the regular method lets you deduct.

How to measure office square footage

Office square footage is the floor area of the space you work in, measured in square feet. The calculation is the same one you'd use for any room: measure the length and the width in feet, then multiply them. A 10-foot by 12-foot office is 120 square feet. If your office is L-shaped or has a bump-out, divide it into rectangles, calculate each area, and add the results together for the total.

For a home office, measure only the part of the room you use regularly and exclusively for work. If you work at a desk in the corner of a spare bedroom, the deductible space is the desk area you use for business, not the entire bedroom. Being precise here matters, because this square footage feeds directly into whichever tax method you choose.

Using square footage for the home-office deduction

If you qualify for the home-office deduction, the IRS gives you two ways to calculate it, and both start with square footage. The simplified method multiplies your office area by a flat $5 per square foot, capped at 300 square feet — so a 120 sq ft office yields a $600 deduction, and the most anyone can claim this way is $1,500. It's fast because you don't have to track individual expenses.

The regular method instead uses your office as a share of your home. Divide the office square footage by your total home square footage to get a business-use percentage, then apply that percentage to actual home expenses like rent, mortgage interest, utilities, and insurance. Knowing your whole house area is essential for this method. The regular method takes more record-keeping but can produce a larger deduction for a big office or high home expenses. This is general information, not tax advice — confirm your situation with a tax professional or IRS guidance.

Office square footage for commercial leases

Commercial office space is leased by the square foot, so the same measurement drives your rent. Watch for two terms: usable square footage is the space you actually occupy, while rentable square footage adds a proportional share of shared spaces like lobbies, hallways, and restrooms. Because landlords typically quote and bill on rentable area, two offices with identical usable space can carry different rents. Ask which figure a quote is based on before you compare it to another building.

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