How to Calculate the Square Footage of a House

A home’s square footage affects its price, its property taxes, its insurance and how it compares with other homes for sale. Yet two people measuring the same house can come up with different numbers. The difference almost always comes down to what they counted and where they measured from. This guide explains the standard approach used in the US and how to apply it to your own home.

Gross living area, explained

The figure on most listings and appraisals is the gross living area (GLA): the finished, habitable space above ground level. In the US, many appraisers follow the ANSI Z765 measuring standard, and Fannie Mae has required it for most appraisals since 2022. Its main rules are:

• Measure to the outside of the exterior walls.
• Count only finished space: walls, floors and ceilings finished to a standard similar to the rest of the house.
• Report above-grade and below-grade (basement) areas separately.
• Leave out garages, unfinished areas, and porches, decks and balconies.
• Exclude areas with very low ceilings; the standard sets minimum ceiling heights.

Step-by-step: measuring a house from the outside

1. Sketch the footprint of each floor, showing every corner of the exterior walls.
2. Measure each exterior wall with a long tape or laser, and write the lengths on your sketch.
3. Split the footprint into rectangles, so that every part is covered exactly once.
4. Calculate each rectangle (length × width) and add them up for that floor.
5. Repeat for upper floors. Upper floors are often a different shape from the main floor, so measure them separately rather than assuming they match.
6. Subtract open-to-below areas such as a two-storey foyer or living room. That floor area is already counted downstairs.
7. Add the floors together for the above-grade total, and list any finished basement on its own line.

Worked example

You can reproduce this in the house square footage calculator: add each section, tick Subtract for the foyer, and the total appears automatically.

What to include and what to leave out

Usually included

• Bedrooms, living rooms, kitchens, bathrooms, hallways and closets
• Stairs, counted once on the floor they lead down from
• Finished attic rooms with enough headroom and permanent stairs
• Enclosed, heated sunrooms finished like the rest of the house

Usually excluded or listed separately

• Garages, even when attached or converted to storage
• Basements (reported separately as below-grade area, even if finished)
• Unheated porches, decks, patios and balconies
• Unfinished attics, crawl spaces and utility areas

Inside versus outside measurements

Measuring each room from the inside and adding them up leaves out the thickness of every wall. In a typical home that can make the interior total several percent smaller than the exterior figure. Interior measurements are the right choice for flooring, paint and furniture planning; exterior measurements are the right choice when comparing with listings and appraisals.

Using records and plans

If you don’t want to measure, you can find a figure in county assessor records, a previous appraisal, the builder’s plans or an old listing. Treat these as estimates: assessor records in particular may be decades old, may miss additions, or may have been calculated differently.

When to hire a professional

For a sale, a refinance, a dispute or an insurance claim, use a licensed appraiser or a professional measuring service. Their figure carries weight with lenders and buyers, and they apply the standard consistently. For planning, budgeting and checking a listing, your own careful measurement is usually close enough.