How to Measure the Square Footage of a House

This is the method appraisers use, from ANSI Z765-2021, written for an owner or agent with a tape measure. It takes a helper, a notebook and some patience with the corners. If you have the floor plans, skip to working from a plan.

What you need

• A long tape (100 ft is easiest outside) or a laser distance measurer, plus a 25 ft tape for short walls.
• A helper to hold the end of the tape.
• Graph paper or a notebook, a pencil and a clipboard.
• A calculator, or a blank sketch sheet in FloorFigures that adds it up as you go.

The steps

1. 1

   Sketch each level

   A rough outline is enough: one sketch per level, with every corner drawn. Note any level that has ground against part of its walls.
2. 2

   Measure the outside walls

   Measure every exterior wall at floor level, to the outside face of the siding, brick or stucco. Write each length on its wall as you go, in feet and inches.
3. 3

   Capture offsets and check closure

   Measure every jog and bump-out. Then check that the wall lengths on opposite sides of the house add up to the same total. If they don't, a measurement is wrong.
4. 4

   Measure the garage, porches and decks separately

   Measure an attached garage from the garage-side face of the wall it shares with the house. Porches and decks get their own lines. None of them are finished area. See does a garage count.
5. 5

   Adjust upper and lower levels

   Subtract openings to the floor below but keep stairwells, apply the ceiling-height rules under sloped ceilings, and keep any level that is partly below grade separate.
6. 6

   Calculate and round

   Split each level into rectangles, multiply length by width, add them up, and round each total to the nearest whole square foot.
7. 7

   Write it down

   Above-grade and below-grade finished area on separate lines, how and when you measured, and any declaration the method requires.

Measuring the outside, wall by wall

ANSI Z765 measures each level at floor level to the exterior finished surface of the outside walls, so the tape goes to the face of the siding, brick or stucco. Keep the tape level and tight, and measure to the inch or the tenth of a foot. Walk the house in one direction and write each length on its wall in the sketch.

• Bay windows count only if they have a floor at that level. A floor-to-ceiling window counts; a box bay that sits above the floor doesn't.
• Chimneys outside the wall line don't count.
• The garage wall: measure the house to the garage side of the shared wall, so the wall belongs to the house.

Handling offsets

Every jog in the wall is a separate measurement, and offsets are where most errors hide. The fix is a closure check. Walls that face the same way on opposite sides of the house have to add up to the same total. In the example below, the front pieces (18'-0" + 12'-4" + 22'-2") and the back pieces (20'-0" + 14'-0" + 18'-6") both add up to 52'-6". If they disagree, measure again before you calculate anything. Then split the outline into rectangles that only touch, as in L-shaped and irregular rooms.

Worked example: a ranch with a bump-out and a rear wing

A one-story house on a slab: a 52'-6" × 28'-0" main block (measured to the garage side of the shared wall), a 12'-4" × 3'-6" bump-out at the front, and a 14'-0" × 10'-9" bedroom wing at the back. To convert inches, divide by 12: 6 in = 0.5 ft, 4 in = 0.333 ft, 9 in = 0.75 ft.

The garage and porch are measured and written down, but they aren't in the 1,664. If you'd added up the room sizes on a plan instead, you'd get a lower number, because room sizes leave out walls, closets and halls.

What to leave out

Upper floors and basements

You can't run a tape up the outside of a second floor. Where the upper walls sit directly over the lower ones, the outline is the same. Where they don't (an overhang, or a smaller second floor), measure inside and add the exterior wall thickness, which you can measure at a window or door opening. Then subtract any opening to the floor below, keep the stairwell, and check sloped ceilings against the 7 ft and 5 ft lines.

A basement, or any level with ground against part of its walls, is measured the same way but reported on its own line as below-grade area, never added to the above-grade total.

Rounding and writing it down

Measure to the nearest inch or tenth of a foot, carry the decimals through, and round each total to the whole square foot at the end (in metric: 0.01 m and 0.1 m²). Then write a statement, with above-grade and below-grade finished area separate, because ANSI Z765 doesn't allow one without the other:

Finished above-grade area: 1,664 sq ft. Finished below-grade area: none. Garage: 528 sq ft (not included). Measured on site on [date] by [name], to the exterior finished surface of the outside walls, following ANSI Z765-2021.

ANSI Z765 also requires a declaration when the figures come from (1) exterior measurements only, with the interior not inspected, (2) plans, or (3) estimated dimensions. Leaving a required declaration out voids any claim that the figure follows the standard.

Working from a floor plan

If you have the plans, you can measure from them instead, as long as you say so. The plans declaration reads:

"Finished square footage calculations for this house were made based on plan dimensions only and may vary from the finished square footage of the house as built."

FloorFigures does this for free in your browser, and the plan never leaves your device. Open the PDF or an image of the plan. On a PDF it sets the scale from the printed dimensions; on an image, click both ends of a known length. Press Shift+A to trace the building outline, which leaves out the garage and porch, mark each level above or below grade, and read the totals, rounded to the whole square foot, with a statement you can copy. Measuring with a tape instead? Start a blank sketch sheet and type each wall length as you draw. The Compare tab then checks your figure against the listed or county size. The full method is in square footage from a floor plan, and if you already have the areas, the ANSI Z765 GLA calculator sorts them.