Irregular Shape Area Calculator
This irregular shape area calculator works out the square footage of a room made of multiple shapes, such as an L-shaped room, with any cutouts subtracted, and of an odd-shaped yard or bed, two ways. Break the space into rectangles, triangles, trapezoids, circles and ovals, adding or subtracting each one, or use the offset method: enter the length of the run and evenly spaced widths. You get the total in square feet, square yards, square meters and acres, plus the average width in offset mode.
Method 1: break the space into shapes
Split the space into regular shapes, work out each area, and add them. That's how the FAO field measurement manual handles irregular fields: divide into triangles, rectangles and other regular areas, then sum them. When every piece is a rectangle, circle or triangle, the main calculator does the adding too.
- Sketch the space and mark every corner.
- Draw dividing lines so the pieces meet with no overlaps and no gaps.
- Enter each piece as its own section, with its shape and measurements.
- Set holes to Subtract: a shed pad, a pond, a kitchen island.
| Shape | Area formula |
|---|---|
| Rectangle | length × width |
| Triangle | base × height ÷ 2 |
| Trapezoid | (parallel side A + parallel side B) ÷ 2 × distance between them |
| Circle | 3.14159 × (diameter ÷ 2)² |
| Oval | 3.14159 × (length ÷ 2) × (width ÷ 2) |
Check it two ways. The yard above is also 40 × 30 = 1,200 minus the 12 × 15 garage (180), which is 1,020 again. If the two routes disagree, a piece is missing or counted twice.
For ovals, Clemson Cooperative Extension uses length × width × 0.8. On a 60 by 30 foot oval that gives 1,440 square feet against 1,414 from the exact formula the calculator uses, about 2 percent high.
Method 2: the offset method for curved and ragged edges
Average the width across the space and multiply by its length. Run a straight baseline along the space, measure straight across at evenly spaced points, and enter the run and the widths in offset mode.
Area = average width × length of the run
Measuring at the middle of each 10 foot section gives each width an equal share of the run. More measurements give a closer answer: Clemson's guide notes that the closer the offset lines, the more accurate the estimate.
Method 3: irregular polygons from corner coordinates
If you have corner coordinates from a plat, survey or GPS, the shoelace formula gives the exact area of any polygon whose sides don't cross. The calculator doesn't run it, so here's the method, as stated in MIT's geometry lecture notes.
- List the corners in order around the edge, clockwise or counterclockwise, never jumping across.
- For each corner, multiply its x by the next corner's y, and subtract its y times the next corner's x.
- Add the results, ignore any minus sign, and halve it.
Shoelace working for the lot above
| Corner to next corner | x × next y minus y × next x | Result |
|---|---|---|
| (0, 0) to (60, 0) | 0 × 0 minus 0 × 60 | 0 |
| (60, 0) to (75, 40) | 60 × 40 minus 0 × 75 | 2,400 |
| (75, 40) to (0, 50) | 75 × 50 minus 40 × 0 | 3,750 |
| (0, 50) to (0, 0) | 0 × 0 minus 50 × 0 | 0 |
| Total | 6,150 |
Half of 6,150 is 3,075 square feet. As a check, the same lot splits into two triangles of 1,200 and 1,875 square feet.
Rooms, lots, and what changes between them
A room is measured wall to wall for materials, a lot from its plat in acres, and a house to the outside of its exterior walls.
Split at bump-outs and alcoves. For simple L-shapes and closets the room calculator takes feet and inches.
Work from the plat, not the fence line. One acre is 43,560 square feet; the calculator shows acres, or use square footage to acres.
Official size follows the ANSI Z765 standard, measured outside the walls. What counts as square footage covers the exclusions.
Common mistakes
Two rectangles that share a strip count it twice. Draw the dividing lines first.
A triangle's height is perpendicular to the base. The triangle calculator also works from three sides.
On the lot above, average length × average width gives about 3,145 square feet instead of 3,075.
7 inches is 0.58 feet, not 0.7. Divide inches by 12.
For round sections on their own, the circle calculator also does half and quarter circles. Every other shape is covered in how to calculate square footage.
Frequently asked questions
How do you calculate the area of an irregular shape?
Split it into rectangles, triangles, trapezoids and part circles, work out each area, add them, and subtract any holes. For a curved edge, average evenly spaced widths and multiply by the length.
What is the formula for an irregular shape?
There's no single formula. You add up regular shapes, average widths for curved edges, or use the shoelace formula when you have corner coordinates.
How do I find the square footage of an odd shaped room?
Divide it into rectangles at each bump-out and add them. A 10 by 12 room with an 8 by 6 alcove is 120 + 48 = 168 square feet.
How do you calculate the area of an irregular polygon?
List the corners in order, multiply each x by the next y and subtract each y times the next x, add the results and halve the total. A lot with corners at (0, 0), (60, 0), (75, 40) and (0, 50) feet is 3,075 square feet.
How do I measure an irregular lot for acreage?
Get the square footage from the plat's coordinates or by splitting the lot into triangles, then divide by 43,560. A 3,075 square foot lot is about 0.07 acres.
Can I just use length times width on an odd shaped space?
Only for a true rectangle. On the L-shaped yard above, 40 × 30 gives 1,200 square feet, but only 1,020 is yard.