About the Area Calculator
An area calculator finds the two-dimensional space a shape covers. Area determines how much paint, flooring, turf, or fabric a job needs, and it is the basis for property measurement and material estimating of every kind.
The formula
Rectangle: A = l · w Triangle: A = ½ · b · hFor a rectangle, area is length times width. For a triangle it is half the base times the perpendicular height — not the slant length of a side.
How to use this calculator
- 1Enter your Length / Base. The field starts at
10, which you can overwrite. - 2Enter your Width / Height. The field starts at
5, which you can overwrite. - 3Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Length / Base | 10 |
| Width / Height | 5 |
Result
Rectangle Area: 50.00
Triangle Area: 25.00
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
The triangle formula is half the rectangle formula because any triangle is exactly half of the rectangle that encloses it on the same base and height. The height must be measured perpendicular to the base, which is the detail people get wrong: using the length of a sloping side instead of the true vertical height overstates the area.
Area scales with the square of linear size, so a room 20 percent larger in each direction needs 44 percent more flooring, not 20 percent. This is also why unit conversions must be squared: one square metre is 10.76 square feet, not 3.28.
Things worth knowing
- Keep every dimension in the same unit before multiplying.
- Break irregular spaces into rectangles and triangles, compute each, and sum them.
- Add 5 to 10 percent for waste on flooring and tiling, and more for diagonal or patterned layouts.
- One square metre is 10.764 square feet; one acre is 43,560 square feet.
- Subtract areas you are not covering — cabinets, fixtures, openings — before ordering material.
Frequently asked questions
How do I calculate the area of a rectangle?+
Multiply length by width in the same unit. A room 15 by 12 feet is 180 square feet.
How do I find the area of a triangle?+
Half the base times the perpendicular height. The height must be measured at a right angle to the base, not along a sloping side.
How much extra material should I order?+
Typically 5 to 10 percent above the calculated area for cuts and waste, rising to 15 percent for diagonal layouts or patterns needing alignment.
How do I handle an irregularly shaped room?+
Divide it into rectangles and triangles, calculate each area separately, and add them. Subtract any sections you are not covering.