SqftKit
SqftKit · Shapes

Area calculator

Rectangle, triangle, circle, trapezoid

An area calculator is for the shapes a room list cannot describe: a round patio, a triangular gable, a trapezoidal side yard. Pick the shape, enter the perpendicular dimensions, and read the area in the unit you measured plus square feet, square meters, and acres.

Inputs

Result

Pick a shape and enter its dimensions.

Disclaimer: This is an estimate for planning. Measure the real space, read the product label, and confirm anything structural, legal, or financial with a qualified person. Nothing you type is sent to a server.

Odd outdoor shapes often become mulch or concrete. Volume, not area, is what those jobs need: the ClearQuip mulch calculator and concrete calculator.

How to use

  1. Choose the shape that matches the outline, not the object’s name. A bay window that is a half-circle uses the circle formula on the half, which you can do by halving the result.
  2. Use the perpendicular height for a triangle or trapezoid. A slanted side is longer than the height and will overstate the area.
  3. Measure in feet or meters and set the unit menu to match. Do not mix units in one calculation.
  4. Press Calculate. Reset clears the numbers.

How it's calculated

Rectangle = length × width
Triangle = ½ × base × height
Circle = π × radius²
Trapezoid = (base A + base B) ÷ 2 × height

These are the standard plane-geometry formulas. Pi is the math constant used by the browser, not a rounded 3.14 typed by hand, though your tape measure is coarser than that precision. When the dimensions are in meters, the area is in square meters and the page also divides by 0.3048² to show square feet.

Several rectangular rooms are faster on the square footage calculator, which keeps a running total. A lot stated in acres is faster on the acre converter. This page is the one to open when the outline is not a list of rooms.

Worked example

A circular patio with a 10 foot radius has area π × 10² = 314.16 square feet, about 0.0072 acres. A triangle with a 20 foot base and a 12 foot height is ½ × 20 × 12 = 120 square feet. A trapezoid with parallel sides of 30 and 18 feet and a height of 10 feet is (30 + 18) ÷ 2 × 10 = 240 square feet.

Assumptions and sources

Height means the perpendicular distance between bases, which is how the triangle and trapezoid formulas are defined in plane geometry. The circle uses the radius, not the diameter; if you measured across the patio, divide by 2 before you type. The page does not subtract islands, trees, or buildings. The foot-to-meter factor is the international foot, 0.3048 m.

FAQ

What if I measured the diameter instead of the radius?

Divide the diameter by 2 and enter that as the radius. A 20 foot diameter is a 10 foot radius.

Can I add a circle to a rectangle?

Calculate each shape, then add the square feet yourself. The result panel shows square feet for every shape so the units match.

Why does a triangle use one-half?

A triangle is half of a parallelogram with the same base and height. The ½ is part of the formula, not a waste factor.

Is this a survey?

No. It is arithmetic on the distances you type. Property lines, curves, and rights-of-way need a surveyor.

Related tools