Home / Area and perimeter

Area and perimeter of a right triangle

The area is half of leg a times leg b. The perimeter is a + b + c. For a 3-4-5 triangle the area is 6 and the perimeter is 12.

Area

The legs meet at 90°, so one can be the base and the other the height. Any triangle’s area is half of base times height. You do not use the hypotenuse in the area.

area = (a × b) / 2

If the legs are in centimeters, the area is in square centimeters. The app shows the unit you selected, or no unit if you left the unit unset.

Perimeter

Add the two legs and the hypotenuse. You need all three sides first. If one side is missing, find the hypotenuse or find the missing leg before you add.

perimeter = a + b + c

Steps

1. Get all three sides, with c the longest.
2. Multiply the legs and divide by 2 for the area.
3. Add a, b, and c for the perimeter.
4. Keep area units squared and perimeter units the same as the sides.

Common mistakes

• Using (a × c) / 2. The height relative to base a is b, not the hypotenuse.
• Forgetting to divide by 2, which gives the area of the rectangle around the triangle instead.
• Adding only the legs and leaving out c.

How the app does it

After a successful Basic or Advanced solve, the measurements include area and perimeter along with the three sides and the three angles. The area formula used is (a × b) / 2. Perimeter is a + b + c. Decimal places affect how the numbers are shown, not the stored calculation.

Get Pythagorean Theorem Calculator on Google Play

Related guides

Questions