Triangle Calculator

How to use

1. For the first part, type Base and Height. The sample is base 10 and height 5. Press Calculate Area. Area is base times height divided by 2, so the sample prints Area = 25.
2. Keep both measurements in the same unit. The page does not convert inches to feet.
3. For the second part, type Side a, Side b, and Side c under the heading Area from 3 sides (Heron). The sample 3, 4, 5 is a right triangle. Press Calculate, which runs Heron's formula. The sample prints Area = 6.
4. If the three lengths cannot close, Calculate prints Invalid triangle. Sides 1, 1, and 3 do that. A degenerate line such as 1, 1, and 2 is also rejected, because the check uses a less-than-or-equal test on each pair of sides against the third.

When it helps

Use base and height when a sketch already has an altitude, including an obtuse triangle if you measure the height outside the shape to the extended base. Use the three-side part when you only paced the boundaries and do not know the height.

Neither button reduces a fractional area. If base 1/2 and height 1/3 are the real measurements, multiply those fractions on the fraction calculator and then halve, or type decimals such as 0.5 and 0.333 into Base and Height. A square root or an angle check that this page does not offer can go on the scientific calculator.

Limits

Heron needs a valid triangle. Impossible side lengths do not produce a quiet zero or a made-up area. The second button says Invalid triangle and stops. Calculate Area does not apply that test: any base and height, including zero, simply follow base times height divided by 2.

The page does not solve angles, it does not find a missing side from SAS or ASA, and it does not tell you the height that matches three sides. Perimeter is not listed as its own result. The two buttons do not share numbers, so changing Side a does not change Base.