Enter any three known angles (in degrees °), click calculate, and the remaining angle will be shown automatically.
This tool is based on a basic theorem in plane geometry: the sum of the four interior angles of any planar quadrilateral is always 360 degrees. This holds for rectangles, general convex quadrilaterals and simple concave quadrilaterals.
The formula is: ∠A + ∠B + ∠C + ∠D = 360°. If you know three angles, subtract their sum from 360° to obtain the remaining angle, which is very convenient for checking hand calculations and drawing.
The calculator works for many common shapes such as rectangles, squares, parallelograms, trapezoids and irregular quadrilaterals, and can be used in geometry homework, CAD drawing, layout design and engineering sketches.
When entering values, make sure the sum of the three known angles is less than 360°. If the sum is 360° or more, a planar quadrilateral cannot be formed. If the sum is very close to 360°, the fourth angle will be very small and measurement errors should be considered.
The four interior angles always sum to 360 degrees.
All four angles are 90 degrees.
You can split a quadrilateral into two triangles, each with 180° as interior angle sum.
360 − 80 − 100 − 110 = 70 degrees.
Enter three known angles, make sure their sum is less than 360°, then click calculate.
Yes. All inputs are in degrees, and radians are not supported.
The fourth angle becomes very small, so pay attention to measurement errors and geometric validity.
It can be combined with quadrilateral area, diagonal length, and trigonometric calculators.
Students, teachers, designers, engineers and anyone who needs quick angle checks.
They calculate quickly, reduce manual errors and support repeated trial calculations, improving efficiency.
Helps you find the fourth interior angle from three known angles. Useful for homework, drawing and engineering.
Scan the QR code to use this tool on mobile.