Triangle Square Feet Calculator
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Triangle Diagram
InteractiveHow to Calculate Square Feet of a Triangle
Every triangle — whether it forms a gable roof, an oddly shaped room corner, or a wedge of yard space — obeys two core area formulas. Pick the one that matches the measurements you already have.
Base × Height Method
The most common approach: multiply the base length by the perpendicular height, then divide by two. The perpendicular height is the straight-line distance from the base to the opposite vertex, measured at a 90-degree angle to the base.
Triangle Area Formula
FormulaExample: A triangle with a 20 ft base and 10 ft height = ½ × 20 × 10 = 100 sq ft
Heron's Formula (Three Sides Known)
When you know all three side lengths but not the height, Heron's formula handles it. First, find the semi-perimeter (half the sum of all three sides). Then plug into: Area = √[s(s−a)(s−b)(s−c)]. This approach works for any triangle, regardless of shape.
Right Triangle Shortcut
For right triangles, the two sides forming the 90-degree angle act as base and height. Multiply them and divide by two — no extra height measurement needed.
Practical Uses for Triangle Square Footage
Roofing and Gable Ends
A gable end is a triangle sitting above the flat wall line. Measuring the span (base) and the peak height gives you the area for siding, insulation, or paint estimation. Contractors routinely calculate square feet of triangular gable sections before quoting roofing materials.
Landscaping and Odd Lot Sections
Corner lots and pie-shaped parcels often contain triangular zones. Calculate the triangle area separately, then add it to any rectangular sections to get your total property square footage.
Architectural Features
Dormer windows, A-frame walls, and cathedral ceilings all involve triangular surfaces. Knowing the square feet of these sections prevents over-ordering drywall, tiles, or cladding.
Flooring Cuts and Waste Planning
Bay windows and angled hallways create triangular floor areas. Calculating these zones accurately helps you buy the right amount of flooring material and anticipate cutting waste.
Triangle Types and Their Square Feet Calculation
Equilateral Triangle
All three sides are equal. The height equals (√3 / 2) × side length. So the area formula simplifies to: Area = (√3 / 4) × side². For a 10 ft equilateral triangle, area = 43.30 sq ft.
Isosceles Triangle
Two sides are equal. Calculate the height using the Pythagorean theorem: height = √(equal_side² − (base/2)²). Then use the standard ½ × base × height formula.
Scalene Triangle
All sides differ. Heron's formula is the go-to method here since determining the height geometrically requires extra construction lines. Input all three sides and the calculator handles the rest.
Right Triangle
The two legs (non-hypotenuse sides) serve as base and height. Area = ½ × leg₁ × leg₂. The simplest triangle to calculate.
How to Measure a Triangle for Square Feet
Step 1: Identify the Base
Any side of the triangle can serve as the base. For roof triangles, the base is usually the bottom edge (the wall span). For lot measurements, pick the side you can measure most easily with a tape.
Step 2: Measure the Height
The height must be perpendicular to your chosen base. Drop a vertical line from the opposite corner straight down to the base line. For roofs, this is the ridge height above the wall plate.
Step 3: Apply the Formula
Multiply base × height, then divide by 2. Convert units if needed — our calculator handles mixed units automatically. If your base is in feet and height is in inches, enter each in its native unit and the tool converts everything to square feet.
Frequently Asked Questions
Multiply the base by the perpendicular height and divide by two. For example, a triangle with a 15 ft base and 10 ft height has an area of 75 square feet (15 × 10 ÷ 2).
Heron's formula calculates area from three side lengths. First find the semi-perimeter: s = (a + b + c) / 2. Then Area = √[s × (s−a) × (s−b) × (s−c)]. It works for every triangle shape.
Yes. Calculate the area in square inches first (½ × base_in × height_in), then divide by 144 to convert to square feet. Our calculator handles this conversion automatically.
Area = ½ × 20 × 15 = 150 square feet.
The height (also called altitude) is the perpendicular distance from the base to the opposite vertex. A side length is the edge of the triangle. They're only the same in a right triangle where one leg acts as the height.
Measure the base (the span of the gable wall) and the height (from the wall plate to the peak). Use ½ × base × height. For pitched roof surfaces, measure the actual rafter length instead for the sloped surface area.
Yes. The ½ × base × height formula works universally for equilateral, isosceles, scalene, and right triangles — as long as you use the correct perpendicular height for your chosen base.
If you can measure all three boundary lengths, use Heron's formula. If you can identify a base and drop a perpendicular from the opposite corner, use the ½ × base × height method. For very irregular lots, a surveyor's GPS measurements paired with Heron's formula gives the most accurate result.
Calculate the triangle's square footage, then add 10–15% for cutting waste. Triangular sections require angled cuts, which generate more offcuts than rectangular rooms.