Do you measure square footage from the inside or outside walls?

For a home's total, measure from the outside: ANSI Z765, the US standard for measuring houses, uses the exterior finished surface of the outside walls. Room dimensions on a plan are usually interior, wall face to wall face, so the rooms add up to less than the total.

Why don't the room sizes add up to the house's square footage?

Room sizes are measured inside the walls and usually leave out halls, closets, stairs and the walls themselves. A house's total is measured around the outside, so it includes all of that. On our sample plan, the eight rooms with printed sizes add up to 1,401 sq ft, while the house measures 1,728 sq ft around the outside.

How do I find the square footage of an L-shaped house?

Split the L into two rectangles that don't overlap, multiply length by width for each, and add the two areas. For example, a 40' × 30' block plus a 20' × 12' wing is 1,200 + 240 = 1,440 sq ft.

Can I calculate square footage from a floor plan without a scale?

Yes, if any dimension is known. Measure a printed dimension, a standard door (usually 3 ft wide) or an exterior wall you've measured on site, and use it to set the scale. FloorFigures lets you click both ends of any known length to scale the whole plan.

Guide

How to Calculate Square Footage From a Floor Plan

Calculate square footage from a floor plan or blueprint in four steps: check the scale, measure the outline, split odd shapes, and subtract what doesn't count.

Updated October 12, 2026 · FloorFigures

Short answer

Check the plan's scale against a printed dimension, measure the outside walls, split the outline into rectangles, multiply length × width for each and add them up. Then subtract the garage, porches and anything open to the floor below. A 48' × 36' house is 48 × 36 = 1,728 sq ft.

A floor plan already holds everything you need: the outline of the house, the dimensions and a scale. The work is in reading it correctly and knowing what counts. Here is the method appraisers and estimators use, step by step, and the mistakes that throw the total off.

1
Check the scale
Find the scale note (for example 1/4" = 1'-0") or any printed dimension, and measure it on the plan. If 48' doesn't measure 12 inches at 1/4-inch scale, the plan was resized and the note is wrong; use the dimension instead. See how to scale a PDF drawing.
2
Measure the outside walls
For the home's total, measure to the exterior faces of the outside walls, one level at a time. For individual rooms, measure the inside faces.
3
Split the shape into rectangles
Divide the outline into rectangles that don't overlap. Use triangles for angled walls (half of base × height). Multiply and add.
4
Subtract what doesn't count
Leave out the garage, porches, decks, unfinished space and openings to the floor below, and report any below-grade level separately.

Which square footage are you after?

"Square footage" means different things to different people, and most disagreements come from mixing them up. Decide which one you need before you measure.

Figure	Measured to	Includes	Used for
Gross living area (GLA)	Exterior wall faces	Finished, above-grade space only	Appraisals, listings
Total finished area	Exterior wall faces	GLA plus finished below-grade space	Listings (shown separately)
Gross building area	Exterior wall faces	Everything under roof, including the garage	Construction, insurance
Room area	Interior wall faces	One room's floor	Flooring, paint, room dimensions

In the US, the standard for measuring single-family homes is ANSI Z765-2021, and Fannie Mae requires it for most appraisals. Here's what it says, and how gross living area is calculated.

Splitting an irregular outline

Few houses are a single rectangle. Break the outline into rectangles that don't overlap, work out each one, and add them. An L-shaped house with a 40' × 30' main block and a 20' × 12' wing:

Split the outline into rectangles that only touch, then add them: 1,200 + 240 = 1,440 sq ft.

Piece	Length × width	Area
Main block	40 × 30	1,200 sq ft
Wing	20 × 12	240 sq ft
Total		1,440 sq ft

For an angled wall, treat the angled part as a triangle: half of base × height. For a bay window or a curved wall, a few short straight segments get you within a square foot or two. More in L-shaped and irregular rooms.

What to leave out

Garages, even finished or heated ones. See does a garage count.
Porches, decks, patios and balconies, covered or not.
Openings to the floor below, such as a two-story foyer. They count once, on the lower floor. See open to below.
Unfinished space: storage, mechanical rooms, unfinished attics.
Below-grade levels, which are reported separately from above-grade living area. See does a basement count.
Low ceilings: under ANSI Z765, finished area needs a 7-foot ceiling, with rules for sloped ceilings. See ceiling height rules.

Why the rooms don't add up to the total

Room sizes printed on a plan are measured between the inside faces of the walls, and they usually skip halls, closets and stairs. The house total is measured around the outside, so it also includes the walls. On our 48' × 36' sample plan, the eight rooms with printed sizes add up to 1,401 sq ft; the house is 1,728 sq ft. That's not an error, it's two different measurements.

Doing it in FloorFigures

FloorFigures follows the same four steps, automatically where it can. It's free and runs in your browser.

Open the plan: a PDF, an image, or a photo of a printed plan.
Scale: on PDFs it reads the printed dimensions and sets the scale itself. On images, click both ends of any known dimension.
Trace: press A to find every room, or Shift+A to trace the building outline. Click inside a single room to trace just that one.
Read the totals: gross living area, below-grade and garage areas, each room, and a takeoff for flooring, paint and drywall. Export a PDF report or a spreadsheet.

Common mistakes

Mistake	What happens	Fix
Trusting the scale note on a resized PDF	Every measurement is off by the same factor	Check against a printed dimension
Adding up room sizes for the total	Total comes out low (walls, halls and closets missed)	Measure the outside walls
Counting a two-story space twice	Total too high	Count openings once, on the lower floor
Including the garage or porch	Total too high	Measure them separately
Overlapping rectangles	Some area counted twice	Split the outline so pieces only touch

Questions

How do I calculate square footage from a blueprint?: The same way as any floor plan: confirm the scale from a printed dimension, measure the outside walls, split the outline into rectangles, multiply length by width for each, and add them up. Then subtract the garage, porches and anything open to the floor below.
Do you measure square footage from the inside or outside walls?: For a home's total, measure from the outside: ANSI Z765, the US standard for measuring houses, uses the exterior finished surface of the outside walls. Room dimensions on a plan are usually interior, wall face to wall face, so the rooms add up to less than the total.
Why don't the room sizes add up to the house's square footage?: Room sizes are measured inside the walls and usually leave out halls, closets, stairs and the walls themselves. A house's total is measured around the outside, so it includes all of that. On our sample plan, the eight rooms with printed sizes add up to 1,401 sq ft, while the house measures 1,728 sq ft around the outside.
How do I find the square footage of an L-shaped house?: Split the L into two rectangles that don't overlap, multiply length by width for each, and add the two areas. For example, a 40' × 30' block plus a 20' × 12' wing is 1,200 + 240 = 1,440 sq ft.
Can I calculate square footage from a floor plan without a scale?: Yes, if any dimension is known. Measure a printed dimension, a standard door (usually 3 ft wide) or an exterior wall you've measured on site, and use it to set the scale. FloorFigures lets you click both ends of any known length to scale the whole plan.