Square Footage Calculator

Start by measuring the longest wall (length) and the wall perpendicular to it (width) using a tape measure. For rectangular rooms, multiply length × width — a 12 ft × 15 ft room is 180 sq ft.

For L-shaped or irregular rooms, break the space into simple shapes (rectangles, triangles), calculate each separately, and sum them. Always measure to the inside of the walls, not the outside.

Our square footage calculator supports four shapes and lets you add unlimited rooms for a combined total.

• Flooring installation typically costs $3–$12 per square foot depending on material (vinyl at $3–$5, hardwood at $8–$12, tile at $5–$10).
• Interior painting runs $1.50–$3.50 per square foot.
• New construction averages $150–$350 per square foot in the US depending on location and finish level.
• Commercial office space ranges from $15–$55 per square foot per year in rent.

Plug your cost per square foot into the calculator to get an instant total estimate.

To calculate the square footage of a rectangular room, multiply length by width: A = L × W. Measure both dimensions in feet, then multiply — a room 14 ft long and 11 ft wide equals 14 × 11 = 154 square feet.

For other basic shapes:

• For a triangle use A = ½ × base × height
• For a circle use A = π × r² (π ≈ 3.14159 per NIST)
• For a trapezoid use A = ½ × (a + b) × h, where a and b are the parallel sides

Break irregular or L-shaped rooms into these basic shapes, compute each, and add the results. HyperPhysics (Georgia State University) and Khan Academy both teach area as the product of two length dimensions, giving square units.

Area is a derived quantity equal to length squared, so its SI unit is the square meter (m²), defined by BIPM from the metre — the distance light travels in 1/299,792,458 of a second (NIST).

Square footage (ft²) is the US customary equivalent, based on the international foot fixed at exactly 0.3048 m. Because 1 ft = 0.3048 m, one square foot equals 0.3048² = 0.09290304 m², which rounds to 0.0929 m². Conversely, 1 m² = 10.7639 ft².

Larger land units build on these:

• 1 acre = 43,560 ft²
• 1 hectare = 10,000 m²

Always keep every room in the same unit before summing to avoid conversion errors.

Converting area units means multiplying by the correct factor derived from the linear conversion squared.

• To go from square feet to square meters, multiply by 0.0929 (since 0.3048² = 0.0929); to reverse it, multiply square meters by 10.7639. For example, 750 ft² × 0.0929 = 69.68 m².
• For land, divide square feet by 43,560 to get acres, so a 21,780 ft² lot is exactly 0.5 acre.
• To reach hectares, divide square meters by 10,000.

These factors come from exact definitions maintained by NIST and BIPM, not approximations, so results stay precise as long as you carry enough decimal places in the linear factor before squaring.

For a circular room, patio, or pool, the area formula is A = π × r², where r is the radius (half the diameter). A circle 20 ft across has r = 10 ft, so A = 3.14159 × 10² = 314.16 ft².

For a trapezoid — common with angled walls or tapered lots — use A = ½ × (a + b) × h, where a and b are the two parallel sides and h is the perpendicular distance between them. Parallel sides of 12 ft and 8 ft with a height of 10 ft give ½ × (12 + 8) × 10 = 100 ft².

These standard geometry relations are documented by Encyclopaedia Britannica and Khan Academy. Never use the slanted side as the height.

Irregular rooms are handled by decomposition: split the floor plan into non-overlapping rectangles and triangles, compute each area with its own formula, then add them.

For an L-shaped room, draw a line to divide it into two rectangles — for example a 20 ft × 12 ft section plus a 10 ft × 8 ft section gives 240 + 80 = 320 ft². For diagonal corners, add a right triangle using ½ × base × height.

This additive method reflects the geometric principle that area is conserved under partition, taught by Khan Academy and HyperPhysics. Sketch the layout, label every measurement, and double-check that partitioned pieces neither overlap nor leave gaps before summing.

Square footage drives decisions across many fields.

• In real estate, gross living area determines listing price and appraisal, following the ANSI Z765 standard used by US appraisers.
• In flooring, tile, and paint projects, area sets how much material to buy, so contractors typically add 5–10% for cuts and waste.
• HVAC sizing uses conditioned square footage as a primary input for heating and cooling load estimates.
• Landscaping, roofing, and concrete work all price by area, while commercial leases quote rent per square foot per year.
• Solar installers estimate panel counts from usable roof area.

Accurate area figures, expressed in ft² or m², keep budgets, permits, and material orders aligned with the actual space.

Not all measured space counts the same way.

Under the ANSI Z765 standard referenced by the National Association of Realtors, gross living area (GLA) includes only finished, heated space above grade:

• bedrooms
• living areas
• kitchens
• bathrooms
• closets
• hallways

Garages, unfinished basements, attics, and covered porches are measured and reported separately, not folded into GLA. Below-grade finished basements are typically listed as separate finished square footage rather than added to above-grade GLA.

This distinction matters because appraisers and buyers compare homes on GLA, so including excluded areas inflates the figure and can trigger appraisal disputes. Always note which spaces are finished and heated when you record measurements.

• The most frequent error is mixing units — measuring some rooms in feet and others in meters, then summing without converting. Convert everything to one unit first (1 m² = 10.7639 ft²).
• Another mistake is using a slanted wall or diagonal as the height in triangle and trapezoid formulas; the height must be the perpendicular distance.
• People also forget to square the conversion factor, wrongly multiplying square feet by 0.3048 instead of 0.0929.
• Measuring to the outside of walls instead of the inside overstates livable area.
• Finally, treating diameter as radius in A = π × r² quadruples the error.

Sketch the room, label measurements, and re-check each formula before trusting a total.

Precision requirements depend on purpose. For appraisals and real estate listings, ANSI Z765 guidance calls for measurements accurate to within a few inches, since area errors compound across a whole home.

For material ordering, small errors are usually absorbed by the standard 5–10% waste allowance, but underestimating a large floor can force a costly second order in a mismatched dye lot.

Use a laser measure or steel tape rather than a cloth tape, which stretches, and record dimensions to the nearest inch (0.01 ft is 1/8 inch). Because area scales with the square of length, a 2% error in each linear measurement produces roughly a 4% error in area — so tighten linear accuracy first.