What Is an Angle?
An angle is the amount of turn between two arms that meet at a point, called the vertex. The length of the arms makes no difference: a small drawing and a large drawing of the same corner have the same angle. Children in Singapore start in Primary 3 by comparing angles with a right angle, the square corner of a book or a door, and in Primary 4 they learn to measure angles in degrees with a protractor. In Primary 5 the work moves to angles that are calculated rather than measured.
How to Use a Protractor
- Centre on the vertex. The small mark in the middle of the protractor’s straight edge goes exactly on the corner of the angle. On this page, let go close to the corner and the protractor snaps into place.
- Base line along an arm. Turn the protractor (drag the orange handle, or use the turn buttons) until the base line lies along one of the arms. It snaps when it is within a few degrees.
- Find the 0 on that arm. One of the two rows of numbers has its 0 on the arm. That is the row to read. Here the outer row is navy and the inner row is blue.
- Count round to the other arm. Follow the same row to where the second arm crosses it and read the number. If the arm is too short to reach the numbers, imagine it carried on in a straight line.
- Check that it looks right. An angle that is clearly smaller than a square corner cannot be 130°. If the reading and the look disagree, the other row was used.
Activities by Level
| Level | What your child practises |
|---|---|
| P3 | Right angles; angles smaller than or bigger than a right angle, drawn in any direction |
| P4 | Measuring angles up to 180° with a protractor, with one arm level and opening either way; acute, right, obtuse and straight angles |
| P5 | Measuring angles turned to any direction; reflex angles; unknown angles on a straight line, at a point, vertically opposite, in a right angle and in a triangle |
The levels follow the usual order in Singapore primary schools. Schools differ a little in pace, so move up or down freely.
Rules for Unknown Angles
- Angles on a straight line add up to 180°. If one angle is 35°, the angle next to it on the line is 145°.
- Angles at a point add up to 360°. All the angles round a point make one full turn.
- Vertically opposite angles are equal. When two straight lines cross, the angles facing each other are the same size.
- Angles in a right angle add up to 90°. A line drawn inside a right angle splits it into two angles that total 90°.
- Angles in a triangle add up to 180°. Take the two known angles away from 180° to find the third.
Diagrams in exam questions are often marked “not drawn to scale”, so unknown angles must be calculated, not measured. The page draws the diagrams to scale, which lets your child check that an answer looks sensible, but the marking always expects the exact calculated value.
Tips for Parents and Teachers
- Estimate first. Before measuring, ask whether the angle is smaller or bigger than a right angle. That one question catches almost every wrong-scale reading.
- Use a real protractor too. After a few questions on screen, draw an angle on paper and measure it with a real protractor, saying each step aloud.
- Find angles around the house. The hands of a clock, an open door and a slice of pizza all make angles. Ask which are acute, right or obtuse.
- Write the rule with the working. For unknown angles, teachers look for the reason, such as “angles on a straight line”, next to the calculation.
- Keep sessions short. Five questions a day builds the habit better than a long session once a week. Our area and perimeter page and the worksheet generator cover the rest of P3–P5 geometry.
Nothing to Install, Nothing Stored on Our Server
The practice runs entirely in your browser on any phone, tablet or computer. There is no account and no download. The last level and activity you chose are remembered on this device, and nothing about your child’s answers is sent to us.
Frequently Asked Questions
Put the centre mark of the protractor exactly on the corner (vertex) of the angle. Turn the protractor until its base line lies along one arm. Find the 0 that sits on that arm, then follow the same row of numbers round to the other arm and read the number there.
So it can measure an angle that opens to the left or to the right. The outer row starts at 0 on the left and the inner row starts at 0 on the right. Always use the row whose 0 lies on an arm of the angle. Reading the other row gives 180 minus the right answer, which is the most common protractor mistake.
P3 compares angles with a right angle. P4 measures angles with a protractor and names acute, right, obtuse and straight angles. P5 adds reflex angles, turned angles that need the protractor rotated, and finding unknown angles on a straight line, at a point, vertically opposite and in a triangle.
Answers within 1 degree of the true angle are marked correct, the same allowance most teachers give when marking a measured angle. Unknown angles that are worked out with a calculation must be exact.
Yes. There is no account, no download and no limit on rounds. It works in any browser on a phone, tablet or computer, and nothing about your child’s answers is sent to us.