Understanding your result
The primary result is the side selected in Find. Perimeter uses the same length unit as your inputs; area uses its square. The two additional angles are the acute angles; the third angle is the known right angle of 90°.
To put both sides into matching units first, use the Length Converter before applying the right-triangle formula.
The formula
The legs a and b meet at the right angle, while c is the hypotenuse opposite it. Triangle area is ab/2 and perimeter is a + b + c. The angle opposite the entered known leg is arctan(a/b), expressed in degrees.
A triangle with legs 3 and 4
Choose Hypotenuse and enter 3 and 4. The missing side is √(9 + 16) = 5. Its area is 6 square units and perimeter is 12 length units. The angle opposite the leg of length 3 is about 36.869898°.
How to use this calculator
- Confirm that the triangle has a right angle.
- Choose the missing side and enter both known lengths in one unit.
- Calculate the missing length and use square units for the area.
Identify the hypotenuse first
The hypotenuse is always opposite the right angle and is the longest side. In missing-leg mode, the second input must therefore exceed the known leg. Equal values would produce a zero-length leg, not a proper triangle.
A sketch helps avoid swapping a leg with the hypotenuse. The formula does not establish that an arbitrary triangle is right-angled; that fact must come from the problem or a reliable measurement.
Use the result for a diagonal or a height
A rectangle’s diagonal forms a right triangle with its width and height. Enter those two perpendicular dimensions as legs to obtain the diagonal. The same idea can apply to a vertical rise and horizontal run.
Keep all lengths in the same unit before calculating. For example, combine metres with metres rather than metres with centimetres. Measurement uncertainty remains even when the displayed calculation has several decimal places.
Assumptions & limitations
What this calculation assumes
- The triangle is flat and has one angle of exactly 90°.
- Both entered sides are positive and share one length unit.
What to keep in mind
- Non-right triangles require additional information and a different formula.
- The calculator does not account for measurement tolerances or curved surfaces.
Common questions
Can the hypotenuse be shorter than a leg?
No. Its square is the sum of two positive squared legs, so it must exceed either leg.
Why is the area in square units?
Multiplying two lengths produces an area. Inputs in centimetres give square centimetres for area.
Can I enter inches and feet together?
Convert them to one unit first. This form treats both numbers as the same unit.