Angles: Rules and Types Explained for KS3 Maths

An angle measures the amount of turn between two straight lines that share an endpoint called a vertex. Angles are measured in degrees (°), and a full turn equals 360°. Knowing the main angle types and the rules that connect them is fundamental to all geometry at KS3 and beyond.

Types of angles

Angle type | Size | Example
Acute | Between 0° and 90° | 35°, 72°
Right angle | Exactly 90° | Corner of a square
Obtuse | Between 90° and 180° | 110°, 153°
Straight (flat) | Exactly 180° | Angles on a straight line
Reflex | Between 180° and 360° | 200°, 310°
Full turn | Exactly 360° | Complete rotation

A right angle is usually marked with a small square in diagrams. Always identify the angle type before calculating — it tells you immediately whether an answer in the wrong range is unreasonable.

Core angle rules at KS3

Rule 1 — Angles on a straight line add up to 180°

When two or more angles sit on one side of a straight line and share the same vertex, their sum is 180°. This is sometimes called the supplementary angles rule.

Formula: a + b = 180° (angles on a straight line)

Worked example 1

A straight line has two angles sharing a vertex. One angle is 63°. Find the other.

63° + b = 180°
b = 180° − 63° = 117°

Answer: 117° (obtuse — makes sense)

Rule 2 — Angles around a point add up to 360°

All the angles at a single point, taken together, make a complete turn.

Formula: a + b + c + … = 360°

Worked example 2

Three angles meet at a point: 90°, 125°, and x. Find x.

90° + 125° + x = 360°
215° + x = 360°
x = 145°

Answer: x = 145° (obtuse — makes sense)

Rule 3 — Vertically opposite angles are equal

When two straight lines cross, they form two pairs of angles directly across from each other (at the vertex). Each pair of opposite angles is equal.

Formula: a = c and b = d (vertically opposite pairs)

Worked example 3

Two straight lines cross. One of the four angles is 48°. Find the other three.

Vertically opposite angle = 48°

The adjacent angle (on a straight line with 48°): 180° − 48° = 132°

The remaining angle (vertically opposite to 132°): 132°

The four angles are: 48°, 132°, 48°, 132°

Check: 48 + 132 + 48 + 132 = 360° ✓

Rule 4 — Angles in a right angle add up to 90°

If a right angle is divided into two or more parts, they sum to 90°. These pairs are called complementary angles.

Worked example 4

A right angle is split into two angles, one of which is 34°. Find the other.

34° + b = 90°
b = 56°

Answer: 56°

Angles formed by parallel lines

When a straight line (a transversal) crosses two parallel lines, three further rules apply. These are covered in more detail at upper KS3 and GCSE, but it is worth knowing the names.

Angle pair | Position | Rule
Corresponding angles | Same side, same position (F-shape) | Equal
Alternate angles | Opposite sides of transversal (Z-shape) | Equal
Co-interior (same-side interior) angles | Same side of transversal (C-shape) | Add up to 180°

Using angle rules together

Many KS3 problems require two or more rules in combination.

Worked example 5

Two straight lines cross. One angle is (3x + 10)°. Its vertically opposite angle is written as (5x − 20)°. Find x and both angle values.

Vertically opposite angles are equal:
3x + 10 = 5x − 20
10 + 20 = 5x − 3x
30 = 2x
x = 15

Angle = 3(15) + 10 = 45 + 10 = 55°
Check: 5(15) − 20 = 75 − 20 = 55° ✓

Answer: x = 15, each angle = 55°

Worked example 6

Angles a, b, and c are on a straight line. a = 2b and c = b + 15°. Find all three angles.

a + b + c = 180°
Substitute: 2b + b + (b + 15) = 180
4b + 15 = 180
4b = 165
b = 41.25°

a = 2 × 41.25 = 82.5°
c = 41.25 + 15 = 56.25°

Check: 82.5 + 41.25 + 56.25 = 180° ✓

Answer: a = 82.5°, b = 41.25°, c = 56.25°

How to set out angle-calculation working

KS3 mark schemes award a method mark for showing which rule you used. Write the rule name as part of your working:

"Angles on a straight line: 63° + b = 180°, so b = 117°."

This habit earns full marks even if you make an arithmetic slip.

Common mistakes to avoid

Mistake 1 — Using 360° instead of 180° for angles on a straight line.
Remember: 360° is a full turn (angles around a point), not a half turn.

Mistake 2 — Assuming all angles at a crossing are equal.
Only vertically opposite angles are equal. Adjacent angles at a crossing are supplementary (add to 180°).

Mistake 3 — Forgetting to name the rule used.
Examiners award marks for stating the rule. "Angles on a straight line add up to 180°" — write it every time.

Mistake 4 — Mixing up complementary and supplementary.
Complementary angles add to 90°. Supplementary angles add to 180°.

How angles fit the KS3 national curriculum

The Department for Education's KS3 mathematics programme of study requires pupils to "apply the properties of angles at a point, angles at a point on a straight line, vertically opposite angles." These rules are the building blocks for all further angle work, including polygons, parallel lines, and trigonometry at GCSE, as set out in the statutory national curriculum for Key Stage 3 and 4.

Frequently asked questions

What is the difference between acute and obtuse angles?

An acute angle is strictly less than 90°. An obtuse angle is strictly greater than 90° and strictly less than 180°. If an angle is exactly 90° it is called a right angle. Knowing these ranges lets you immediately check whether a calculated angle is reasonable — for example, an answer of 95° to a question asking for an acute angle signals an error.

Why do angles on a straight line add up to 180°?

A straight line represents a half turn (180° out of a full 360° rotation). Any angles placed on one side of a straight line, sharing the same vertex, together fill exactly that half turn. This is a geometric fact that holds for all straight lines, everywhere in Euclidean geometry — the type studied at KS3.

How do I spot vertically opposite angles in a diagram?

Look for two straight lines crossing to form an X. The angles diagonally across from each other — sharing only the vertex, not a side — are vertically opposite and always equal. They are sometimes highlighted with matching arc marks in exam diagrams.

What is a reflex angle and when does it appear at KS3?

A reflex angle is greater than 180° and less than 360°. It appears in KS3 questions when an angle is described as the "major" part of a rotation, or when a shape has an interior angle that bends inward (a concave polygon). To find a reflex angle, subtract the known angle from 360°.

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