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How to measure an angle using a protractor (primary and geometry math help)

Updated on August 22, 2011

An angle can be measured using a protractor. Most protractors have two scales, an outside scale that runs from 0⁰ to 180⁰ going clockwise and an inside scale that runs from 0⁰ to 180⁰ going anticlockwise. It is vital that you use the correct scale when measuring an angle.

In order to measure an angle all you need to do is place the centre of the protractor onto the corner of the angle that you are measuring. The centre of the protractor is usually marked with a tiny cross at the bottom of the protractor (the middle of the sunrise). All you need to do now is make sure that the bottom line on the protractor (the 0⁰ to 180⁰ line) is positioned on one of the lines of the angle.

The angle be calculated now, by either going clockwise or anticlockwise, starting from 0⁰ until you reach the other line. Be careful that you use the correct scale on the protractor. If the angle you are measuring is on the left hand side of the bottom line then it is likely you will be using the inside scale (going anticlockwise). Or if the angle is on the right hand side of the line then you should be using the outside scale (going clockwise).

You can also check that the angle is correct by estimating the size of the angle before you use your protractor. An angle less that 90⁰ is acute and an angle more than 90⁰ is obtuse. For example, if you have measured an acute angle and got an angle of 130⁰, then you have probably used the wrong side of your protractor. Similarly, if you have an obtuse angle and measure it as 12⁰ then you have definitely used the wrong scale on the protractor.

Another tip when measuring angles is to make sure that the angle is directly in front of you. You can do this by turning your page around before measuring the angle. Also make sure that you have a decent protractor. Transparent protractors are best as you need to be able to see the centre of the angle that you are measuring. Also make sure that your protractor is not too small and too big, as this will make it harder to measure the angle. In most exam situations, you will be allowed to be within 1 degree of the true value.


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