In short
Measure each room wall to wall at floor level with a tape measure or a laser, write down the length and width, and multiply them. Keep garages, porches and basements on their own line, apart from the finished rooms.
What you need
- A 25 ft tape measure or a laser measure
- Paper or your phone for notes
- A rough sketch of each floor, with the rooms numbered
- A helper for long walls (not needed with a laser)
Step by step
- Sketch the floor and number the rooms.
- Measure the length of each room at floor level, from wall to wall.
- Measure the width at a right angle to the length.
- Write each size in feet and inches, for example 12′ 6″, next to the wall on your sketch.
- Measure closets, nooks and the bump of a bay window as their own parts.
- Multiply length by width for each part and add the parts together.
Rooms that are not rectangles
Split the room into rectangles and triangles and measure each part. For an L-shaped room, measure the full rectangle and the missing corner; the room is the rectangle minus the corner.
Floors, stairs and basements
Measure each floor on its own and add the finished rooms per floor. Under ANSI Z765 a floor with any part below ground is below-grade area, listed apart from the living area even when it is finished.
Where a ceiling slopes, note where it is 5 ft and 7 ft high: parts lower than 5 ft do not count, and the room counts only when at least half of it is 7 ft or higher.
How precise is precise enough
Round each measurement to the nearest inch and round only the final total to a whole square foot. A laser measure is quick and precise over room distances; a tape is fine when you keep it straight and level against the floor.
Figure 1. One floor of 25 × 17 ft. The rooms add up to 425 sq ft.
Sources
- ANSI Z765-2021, Square Footage: Method for Calculating, Home Innovation Research Labs · Checked against the source on September 24, 2026
About this page · Written by the Squaremetry team, based on ANSI Z765-2021, Square Footage: Method for Calculating. Last checked on September 26, 2026. Found a mistake? Send us a message.