Free estimate — verify against local code before building
Square Footage Calculator
Measure several spaces, subtract excluded areas, and audit every shape in the total.
What this calculator includes
Build a total from up to twelve independent rectangles, circles, triangles, or trapezoids. Add rooms and sections, deduct openings or excluded zones, and enter contractor-style feet-and-inches measurements. Each shape stays visible in the calculation trail, followed by exact square-yard, square-meter, and acre conversions. The tool does not assume that shapes overlap or share boundaries, so divide irregular plans into non-overlapping pieces before entering them.
How to use this square footage calculator
- 01
Split the plan into simple shapes
Divide an irregular floor, wall, lot, or roof plan into non-overlapping rectangles, circles, triangles, or trapezoids. Add each section separately instead of guessing one average length and width.
- 02
Enter feet and inches
Measurements accept entries such as 12, 12' 6", or 12' 6-1/2". For triangles and trapezoids, height means the perpendicular distance to the base, not a sloped side.
- 03
Deduct excluded zones
Choose Subtract for openings, shafts, islands, or other zones already contained inside an added area. Do not subtract a space unless its footprint is part of an added shape.
- 04
Check the audit trail
Confirm each shape's formula and contribution, then compare added area, deductions, and net square footage before using the conversion results.
Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula review completed July 13, 2026. What this review covers
- NIST Handbook 44 — Appendix C, General Tables of Units (opens in a new tab)
U.S. customary area relationships, including square feet, square yards, and acres.
- NIST Special Publication 811 — Guide for the Use of SI (opens in a new tab)
The exact foot-to-meter relationship behind the square-meter conversion.
Frequently asked questions
How do I calculate the square footage of an irregular room?
Split it into non-overlapping simple shapes, calculate each area, and add the pieces. Use deductions only for zones that are already inside an added shape.
How many square feet are in a square yard?
One square yard equals exactly 9 square feet because one yard is 3 feet and area scales in both directions: 3 × 3 = 9.
How many square feet are in an acre?
One acre is 43,560 square feet. Divide square feet by 43,560 to convert to acres.
What height should I use for a triangle or trapezoid?
Use the perpendicular distance from the base to the opposite point or parallel base. A sloped side is not the height unless it is perpendicular to the base.