How to Measure the Square Footage of a House, Step by Step

The square footage on a listing affects the asking price, property taxes and even insurance quotes, yet two people measuring the same house can arrive at different numbers. The differences come from where you measure and which spaces you count. This guide follows the approach used by most US appraisers, based on the ANSI Z765 standard for single-family homes.

What counts as finished square footage

Finished area is space that is heated and cooled, has finished walls, floors and ceilings, and is directly accessible from other finished space. Under ANSI Z765:

• Measure to the exterior face of the exterior walls, not the inside of the rooms.
• Report above-grade finished area separately from finished space below grade (basements).
• Garages, unfinished areas, open porches and decks are not included in finished area.
• Finished space generally needs a ceiling height of at least 7 feet, with allowances for beams and ducts, and rules for sloped ceilings in which areas under 5 feet are excluded.
• Openings to the floor below, such as a two-story foyer, are not counted on the upper floor; stairs are counted on the floor from which they descend.

Since 2022, Fannie Mae has required appraisers to use this standard for most loans it buys, which is why it has become the reference point for residential appraisals. Local real estate practice can still differ, so ask your agent or appraiser which method a listing used.

Tools you need

• A 100-foot tape measure or a laser distance measurer
• Graph paper or a notes app for a sketch
• A helper for long walls

Step-by-step

1. Sketch the footprint. Walk around the outside and draw the shape of each floor, noting every corner.
2. Measure each exterior wall and write the length on your sketch. Round to the nearest inch, then convert to decimal feet (6 inches = 0.5 ft).
3. Break the footprint into rectangles. An L-shaped house becomes two rectangles; a house with a bump-out becomes three.
4. Calculate each rectangle (length × width) and add them for that floor.
5. Subtract excluded areas inside the footprint, such as an attached garage or an open porch under the main roof.
6. Repeat for each floor, measuring the upper floor's exterior where possible, and subtract open-to-below areas.
7. Total the above-grade floors. List finished basement area separately.

Worked example

A one-story ranch house has a main rectangle of 48 ft × 28 ft and a bedroom wing of 14 ft × 12 ft. A 22 × 24 ft garage is attached but separate from the main rectangle.

Section	Calculation	Area
Main block	48 × 28	1,344 sq ft
Bedroom wing	14 × 12	168 sq ft
Garage	22 × 24 (not counted)	528 sq ft
Finished above-grade area		1,512 sq ft

You can enter the sections in the square footage calculator as separate rectangles; mark any area you want to exclude as "subtract" if it lies inside your measured outline.

Measuring from inside instead

If you cannot reach the exterior, measure every room wall to wall and add 5–10% for interior and exterior wall thickness, closets and hallways you may have missed. This gives a rough figure for planning, not a number to use in a listing. For flooring and paint, interior room measurements are actually what you want; see how to measure irregular rooms.

Why your number may differ from public records

• Tax records sometimes include garages or porches, or have not been updated after additions.
• Older listings may have used interior measurements.
• Finished basements are sometimes added to the total in marketing materials.

If the difference is significant, an appraiser's measurement is the most reliable. National statistics on home sizes are published by the U.S. Census Bureau in its Characteristics of New Housing report.

Next steps

Once you have the total, you can compare value using price per square foot, and convert to metric with our guide to square feet and square meters.