This area calculator computes 2D area for a rectangle, triangle, circle, or trapezoid. Choose the shape, enter side inputs, and read the area in squared length units matching your inputs.
Students, DIYers, and planners use it for rooms, plots, and simple geometry checks. Keep both lengths in the same unit so the squared result stays meaningful.
How each shape is computed
Rectangle area is a * b. Triangle area is 0.5 * a * b. Circle area is π * a * a with a as radius. Trapezoid area uses 0.5 * (a + b) * b in this implementation.
Worked example
Shape rectangle, side A = 10, side B = 5. Area = 50.
| Input | Value |
|---|---|
| Shape | Rectangle |
| Side A | 10 |
| Side B | 5 |
| Area | 50 |
How to use the fields
- Shape selects which formula runs.
- Side A is base, first length, or radius for circles.
- Side B is height, second length, or second base depending on shape.
Unit tips
If you measure in feet, area is square feet. If you need square meters, convert lengths first or convert the area after. Mixing feet and inches without conversion is a common flooring estimate error.
Practical uses
Estimate paint coverage, turf, or tile needs after you know area, then add waste percent outside this tool. For irregular rooms, split the floor into rectangles and sum areas from repeated runs.
Common mistakes
- Using diameter in the circle field that expects radius
- Mixing units between a and b
- Treating trapezoid inputs as something other than the plugin’s a, b convention
- Forgetting that triangle needs base and height, not two slanted sides alone
Classroom check
Compare a rectangle 10 by 5 against a triangle with the same base and height: the triangle should be half, or 25. A circle with radius 10 should be about 314.16. Those quick checks confirm you selected the right shape mode.
Compound shapes
Real rooms rarely match one rectangle. Split L shaped floors into two rectangles, compute each area, then add. For a circular patio with a rectangular walkway, compute each piece separately and sum. This calculator handles one shape per run, so keep a notepad total beside the browser.
When only a diameter is known for a circle, divide by two before entering radius. Entering diameter as if it were radius inflates area by a factor of four.
Material estimating extras
- Add waste percent after you know clean area.
- Confirm whether product coverage uses the same units you measured.
- Remeasure twice on long walls; small length errors square into larger area errors.
- For paint, area is wall area, not floor area, unless you are coating a floor.
Accurate area is the foundation; coverage rates and waste finish the shopping list.
Trapezoid and circle reminders
For trapezoids in this tool, remember the live formula uses 0.5 * (a + b) * b. Keep labels straight so a and b match the lengths you measured. When in doubt, split the trapezoid into a rectangle and a triangle and sum those areas with separate runs.
For circles, radius is half the diameter. A round rug labeled 10 feet across has radius 5, so area is pi * 5 * 5, not pi * 10 * 10. That single mistake is common in DIY shopping.
Landscaping quotes sometimes mix square feet and square yards. Divide square feet by 9 for square yards after you compute area in feet.
Walls and ceilings
Floor area does not equal wall paint area. Measure wall length times height for each wall, subtract large openings if you want a tighter estimate, then sum. Ceilings are usually length times width like a rectangle run on this calculator.
Write each surface area down before you buy materials so a single forgotten wall does not leave you short mid project.
For outdoor paving, add edge cuts and breakage waste after the clean area is known. The calculator gives the geometric area; suppliers still expect a waste allowance on top.
Recheck long diagonal measurements when a room is out of square so rectangle assumptions do not hide missing square footage.
Limitations
Results are flat shape areas only. They do not compute surface area of cylinders or spheres, and they do not convert units. Use them for clean 2D geometry first passes.
For related unit and percent checks while you plan materials, try the unit converter or the percentage calculator.