Square footage calculator

How to calculate square footage

Square footage is the number of 1 x 1 foot squares that fit on a surface. For a rectangle you multiply length by width. For anything else, you break the space into simple shapes, find each area, and add them up. Here is the math the calculator runs on its starting values: a living room 15 ft 6 in by 13 ft and an L-shaped kitchen.

1. 1

   Turn inches into feet

   Divide the inches by 12. 6 inches is 0.5 ft, so 15 ft 6 in is 15.5 ft. Never multiply "15.6" by anything: 15 ft 6 in is not 15.6 ft.

2. 2

   Multiply length by width

   Living room: 15.5 × 13 = 201.5 sq ft.

3. 3

   Handle the odd shapes

   The kitchen is a 14 x 12 ft rectangle with a 6 x 4 ft corner missing. 14 × 12 = 168, minus 6 × 4 = 24, leaves 144 sq ft.

4. 4

   Add the areas and convert

   201.5 + 144 = 345.5 sq ft. Divide by 9 for square yards (38.4 sq yd) or by 10.764 for square meters (32.1 m²).

5. 5

   Add extra if you are buying material

   With 10% for cuts: 345.5 × 1.10 = 380.05, so plan on 381 sq ft. At $3.00 per sq ft that is $1,143.

Area formulas for every room shape

Use the same unit for every side. If you measured in feet and inches, convert to decimal feet first.

A bay window, an angled wall or a walk-in closet is just another small shape. Measure it and add it as its own area. For a half circle, such as a rounded bay, the easiest route is a triangle or trapezoid that matches it closely, or enter the full circle and halve the result on your sheet. For a triangle, the height is measured square to the base, not along the sloped side.

Square feet in common room sizes

Tap a size to load it into the calculator.

Converting square feet to other units

Inches to decimal feet, for quick work on a phone: 1 in = 0.083 ft, 2 in = 0.167, 3 in = 0.25, 4 in = 0.333, 6 in = 0.5, 8 in = 0.667, 9 in = 0.75 and 10 in = 0.833.

How to measure a house the way appraisers do

Listing sites, tax records and builders often disagree on a home's size because they measure differently. Lenders settled the question for single-family homes: since 2022, Fannie Mae and Freddie Mac require appraisers to follow ANSI Z765-2021, the American National Standard for measuring houses. Its main rules:

• Measure the outside. Detached and attached houses are measured on the exterior walls, to the nearest inch or tenth of a foot. The total is reported to the nearest whole square foot.
• Only finished space counts. Garages, unfinished rooms, porches, decks and patios are left out, even when they are heated or attached.
• Below grade is separate. Any level that is partly or fully below ground, such as a walk-out basement, is reported as below-grade area, not as living area.
• Ceilings need 7 feet. Finished space must have a ceiling at least 7 ft high, except under beams and ducts. In a room with a sloped ceiling, at least half the floor must have 7 ft of height, and no part under 5 ft counts.
• Stairs count once. A staircase counts on the floor it goes down from. Openings to the floor below, like a two-story foyer, do not count on the upper floor.
• Condos are different. A condo in an apartment-style building is measured on the inside of its walls, not with ANSI.

For your own use, sketch each floor, measure the outside walls with a 100 ft tape or a laser, and split the footprint into rectangles. Enter each one here, subtract open-to-below areas, and add each finished floor. Your number should land within a few percent of a good appraisal.

What to do with the number

• Buy floor covering. Measure inside the room, add 10% for a straight lay and 15–20% for diagonal or herringbone, then divide by the box coverage. The flooring calculator and tile calculator do the boxes, trim, mortar and grout.
• Paint and drywall use wall area, not floor area. Multiply the room's perimeter by the wall height and take out doors and windows. The paint calculator does that for you.
• Compare home prices. Divide the price by the finished living area. A $400,000 house with 2,000 sq ft is $200 per sq ft. Only compare homes measured the same way, in the same area.
• Check a quote. Contractors price floors, roofs, decks and concrete by the square foot. Knowing your own number lets you spot a padded estimate.

Measuring tips and common mistakes

• Measure twice, at two heights. Old walls are rarely straight. Measure each wall at the floor and at chest height and use the larger number.
• Round up, not down. Round each side up to the next inch. Short by an inch on a 12 ft wall is a full extra row of tile you didn't buy.
• Don't mix inches and decimals. 10 ft 9 in is 10.75 ft, not 10.9. This is the most common error we see.
• Include closets you will cover. Flooring runs into closets. Finished closets also count toward a home's living area.
• Draw it first. A rough sketch with every wall labeled stops you from counting a hallway twice or skipping a nook.
• Lots need a survey. For a property line or a fence on the boundary, use the recorded survey or plat, not a tape across the yard.

Common questions

Sources and assumptions

Rules checked in October 2026. The material price and the home price are only examples; change them to your own.

• House measuring rules (exterior walls, nearest inch or tenth of a foot, 7 ft ceilings, the 50% and 5 ft sloped-ceiling test, stairs, open-to-below areas, below-grade area, condos measured inside): Fannie Mae, Standardizing Property Measuring Guidelines (ANSI Z765-2021 FAQ), September 2025.
• The standard itself and who publishes it: Home Innovation Research Labs, ANSI Z765 square footage standard FAQ.
• Unit conversions (1 ft = 0.3048 m exactly, so 1 m² = 10.7639 sq ft; 1 acre = 43,560 sq ft): NIST, US customary and metric conversions.
• Waste allowances (10% straight, 15–20% for angled patterns) are common flooring trade practice. Check the install guide for your product.
• The calculator rounds material up to the next whole square foot (or tenth of a square meter). Areas are shown to one decimal place; the inside math keeps full precision.