The easiest and most accurate way to measure a Euro cylinder is to use a cylinder gauge, which can be purchased here. However if you do not want to use to gauge, it is still possible to measure what size cylinder you need with a tape measure or ruler:

1. When you open your door, you will see the below:

The first thing to do is to locate the retaining screw (this is the screw that is keeping your current cylinder in place.)

2. If you are using a cylinder gauge, put the centre of the cylinder gauge (B) into the retaining screw hole on your door (there is no need to remove the retaining screw):

Adjust lever (A) and (C) on the gauge to the end of your door handle.

Do not measure what the existing cylinder is at it may be the wrong size and cause the cylinder to overhang, which makes it vulnerable to burglars.

3. On side A, read off the measurement (i.e. 45mm.) On side C, read off the measurement (i.e. 35mm.) Depending on which type of cylinder you want to use, it may only fit one way round, so make a note of which side is the external side (outside.) In this example, we will say that the external side is side A.

3A. If you are using a ruler or tape measure, measure from the centre of the retaining screw to the end of your handle on side A, and repeat with side C. Always round your measurements to the nearest 5mm, so if you get a measurement of 43mm round it up to 45mm, or if the measurement is 42mm round it down to 40mm.

4. You will now have 2 measurements: Side A = 45mm and Side C = 35mm. Adding these measurements together will give you an overall measurement of 80mm.

5. If you are planning on purchasing a key/key cylinder that can only be fitted one way round (for example the UAP 3* Kinetica cylinders) then the cylinder you need is a 80mm 35/45EX cylinder (the EX shows which side is external.) However if you are puchasing a cylinder that can be fitted either way round, you can purchase either 80mm 35/45 or a 80mm 45/35 cylinder (they are both exactly the same.)

5A. If you are planning on purchasing a thumb turn cylinder, it can only be fitted one way round with the thumb turn side being on the internal side (inside.) You would therefore need a cylinder which is a 80mm 35T/45 cylinder (the T shows which side the thumb turn goes on.)

Please note - you will see the cylinder options set out as Xmm X/X (X-10-X). For example, 80mm 35/45 (30-10-40). The X-10-X measurement is just another way that some people measure cylinders (they count the cam in the middle of the cylinder as 10mm and then deduct 5mm for each side.) However you do not need to take notice of this measurement when ordering.