How to Easily Calculate Square Footage of a Building
To calculate the square footage of a simple rectangular building, multiply its length by width. For multi-story or irregular buildings, calculate each floor or section separately and add the results. This guide covers homes, offices, garages, and commercial buildings in cities from Oklahoma City to Kansas City.
Rectangular building: Length × Width. Multi-story: add each floor's area. Irregular: divide into sections, calculate each, sum them. A 60×40 ft one-story building = 2,400 sq ft. Two stories = 4,800 sq ft. Always confirm what to include (garage, basement, porch) based on the purpose of your calculation.
Building Square Footage Formula
Example: One-Story Building
A building in Oklahoma City measuring 60 feet long × 40 feet wide:
If the building has two identical floors:
Step-by-Step Measurement Method
- Obtain a floor plan, property survey, or measuring tape.
- Measure the length and width of each rectangular section.
- Convert all dimensions to feet.
- Multiply length × width for each section.
- Measure additional floors separately.
- Add all areas together.
- Subtract any areas excluded by your measurement standard (e.g., unfinished garage for living space calculations).
Measuring an Irregular Building
Many buildings have a main rectangle plus an attached garage, rear extension, or side addition. Measure each section separately.
Example: Building With Three Sections
| Section | Length | Width | Area |
|---|---|---|---|
| Main building | 50 ft | 30 ft | 1,500 sq ft |
| Rear extension | 20 ft | 15 ft | 300 sq ft |
| Attached office | 12 ft | 10 ft | 120 sq ft |
| Total | — | — | 1,920 sq ft |
For triangular sections, use: Base × Height / 2, then add to the rectangular total.
Interior vs. Exterior Square Footage
The phrase "building square footage" can mean different things. Clarify the purpose before measuring.
| Measurement Type | What It Includes | Common Use |
|---|---|---|
| Building footprint | Ground-level area covered | Site planning, permits |
| Interior floor area | Usable space inside exterior walls | Flooring, HVAC, real estate |
| Gross building area | Total area within exterior walls + common areas | Commercial leasing |
| Exterior wall area | Wall surface (perimeter × height) | Siding, paint, insulation |
Multi-Story Building Example
A two-story building in Kansas City, Missouri:
- First floor: 80 ft × 45 ft
- Second floor: 80 ft × 45 ft
- Attached garage: 24 ft × 24 ft
Second floor: 80 × 45 = 3,600 sq ft
Garage: 24 × 24 = 576 sq ft
Including garage (total building area): 3,600 + 3,600 + 576 = 7,776 sq ft
Excluding garage (heated living space): 3,600 + 3,600 = 7,200 sq ft
Should You Include the Garage, Basement, or Porch?
The answer depends entirely on the purpose of the calculation. Measure these areas separately so you can include or exclude them as needed:
- Attached and detached garages
- Finished and unfinished basements
- Attics and lofts
- Screened porches and patios
- Balconies and decks
- Mechanical rooms and storage areas
For real-estate listings, appraisals, permits, or tax documents — use the measurement rules required by the relevant authority. Standards vary by jurisdiction and property type.
Calculate Building & Room Areas Instantly
Use our free construction calculators for concrete, slabs, cubic yards, and more — designed for US homeowners and contractors.
Common Building Measurement Mistakes
- Adding length and width instead of multiplying them.
- Counting the same extension twice.
- Including a garage when the project requires living area only.
- Forgetting a second or third floor.
- Mixing interior and exterior measurements.
- Using feet-and-inches values incorrectly without converting.
- Measuring only the largest rectangle and ignoring smaller sections.
Keep all measurements in a written table — this makes the calculation easy to verify and update if you find an error.
Frequently Asked Questions
Related Articles
- How to Calculate Square Feet of a Room — for measuring individual interior spaces
- How to Calculate Roof Square Footage — pitch factors, roofing squares, and waste
- Home Addition Cost Calculator — estimate renovation cost by square footage
- Concrete Calculator — estimate concrete for foundations and slabs
Disclaimer: This guide provides general measurement information and is not a substitute for an official survey, appraisal, architectural plan, or local building-code requirement.