90 Degree Angle – Measurement, Definition With Examples
reviewed by Janice S. Armas
Updated on July 29, 2026
A 90-degree angle, also called a right angle, is formed when two lines meet at a perfect square corner — that is, when they are perpendicular to each other. It measures exactly 90 degrees and is marked with a small square (∟) rather than a curved arc. You see right angles wherever walls meet floors, at street intersections, and in the corners of books and screens.
The right angle is a cornerstone of geometry, appearing in squares, rectangles, and right triangles, so it’s worth making sure kids can both recognize and construct one — something a geometry tutor can reinforce with hands-on practice.
What Is a 90-Degree Angle?
A 90-degree angle is an angle that measures exactly 90 degrees, formed where two perpendicular lines or rays meet. It looks like the corner of a square and is usually labeled with a small square symbol instead of an arc. It is also known as a right angle, and there is no special ‘formula’ for it — it is defined simply by its measure of 90°.
Right angles are everywhere in the built world because they create stable, square corners: the meeting of two walls, the corner of a page, and the intersection of two streets are all right angles.
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How to Measure a 90-Degree Angle
To measure a 90-degree angle, line up one ray of the angle with the baseline of a protractor and place the protractor’s center on the vertex. Read where the second ray crosses the scale — a right angle lands exactly on the 90° mark at the top center of the protractor.
Because a right angle is the midpoint of a protractor’s 180° range, it’s one of the easiest measures to check: if the second ray points straight up from a horizontal baseline, you have 90°.
90-Degree Angles in Geometry
Right angles are common throughout geometry. They form the four corners of squares and rectangles, appear in the right-angled triangle, and sit at the meeting of the horizontal and vertical axes on a coordinate grid. The right angle is also central to the Pythagorean theorem. Splitting a right angle in half gives two 45 degree angles.
How Does a 90-Degree Angle Compare to Other Angles?
A 90-degree angle is the dividing line between acute and obtuse angles. Angles are grouped by how their measure compares to a right angle and a straight angle:
| Angle type | Measure |
| Acute angle | Less than 90° |
| Right angle | Exactly 90° |
| Obtuse angle | Between 90° and 180° |
| Straight angle | Exactly 180° |
| Reflex angle | Between 180° and 360° |
How to Draw a 90-Degree Angle
You can draw a 90-degree angle with a protractor or, for a more exact construction, with a compass. Here are both methods.
With a Protractor
- Draw a straight line.
- Place the protractor’s base on the line, find 90° on the scale, and mark it.
- Remove the protractor and draw a line from the vertex to your mark.
If you need a refresher on reading the scale, see this guide on how to use a protractor.
With a Compass (Perpendicular Bisector)
- Draw a horizontal line and mark a point P in the middle where the angle will sit.
- Place the compass point on P and draw two small arcs on the line, one on each side — label them A and B.
- Open the compass wider than the distance from P to A. From A, draw an arc above the line; without changing the width, draw another from B so the two arcs cross at point Q.
- Use a ruler to draw line PQ. It is perpendicular to the base, forming a 90° angle.
Solved Examples
Example 1. A square has four corners, each a right angle. What is the sum of all four corner angles? Since each is 90°, the total is 90 × 4 = 360°.
Example 2. Angle A and Angle B sit next to each other and together form a right angle. If Angle A is 30°, what is Angle B? Subtract from 90°: 90 − 30 = 60°.
Example 3. A triangle has one 90° angle and two 45° angles. What kind of triangle is it? Because it contains a 90° angle, it is a right-angled triangle (specifically a 45-45-90 triangle).
90-Degree Angle Worksheets
Put this into practice with related Brighterly worksheets:
- Types of angles worksheets
- Measuring angles with a protractor worksheet
- Measuring angles worksheets
- Pairs of angles worksheets
90-Degree Angle: Practice Math Problems
- Draw a 90° angle on paper using only a protractor.
- List three objects in your home that have a right angle.
- If an angle is twice the size of a right angle, what is its measure? (Answer: 180°.)
Frequently Asked Questions About the 90-Degree Angle
What Is a 90-Degree Angle?
A 90-degree angle is an angle that measures exactly 90 degrees, formed when two lines or rays meet perpendicularly to create a square corner. It is the angle you see at the corner of a book, a window, or a piece of paper. On diagrams it is usually marked with a small square rather than a curved arc.
What Is Another Name for a 90-Degree Angle?
A 90-degree angle is most commonly called a right angle. The two terms mean exactly the same thing. The name ‘right angle’ reflects its role as the standard, ‘upright’ angle against which other angles are compared — acute angles are smaller than a right angle, and obtuse angles are larger.
How Is a 90-Degree Angle Marked in Diagrams?
A 90-degree angle is marked with a small square drawn in the corner where the two lines meet, rather than the curved arc used for other angles. This square symbol (∟) is a quick visual signal that the angle is exactly 90° and that the two lines are perpendicular to each other.
Is There a Formula for a 90-Degree Angle?
No, there is no formula for a 90-degree angle — it is defined simply by its measure of 90 degrees. What does have a formula is the relationship it creates: in a right triangle, the 90° angle is what makes the Pythagorean theorem (a² + b² = c²) work, linking the lengths of the triangle’s sides.
What Shapes Have 90-Degree Angles?
Squares and rectangles each have four 90-degree angles, one at every corner. Right-angled triangles have exactly one. Other shapes can include right angles too, such as certain L-shaped polygons. The coordinate grid also forms a 90° angle where its horizontal and vertical axes cross.
How Do You Draw a 90-Degree Angle Without a Protractor?
You can draw a 90-degree angle without a protractor by using a compass to construct a perpendicular bisector. Draw a line, mark two equal points on either side of a center point, then draw intersecting arcs above the line from each. A line from the center up to where the arcs cross meets the original line at exactly 90°.
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