Use a tape measure and the help of a friend to take the following measurements. When you're done, you can easily find out your US size.
Take your measurements
Use the tape to measure around the base of your neck, where it meets your shoulders. Put a finger between your neck and the tape measure for a more loose fit collar.
Bend your elbow and put your hand on your hip. Ask your friend to measure from middle of back of your neck, around shoulder and elbow to wrist bone.
This measurement should be taken beneath your armpits, around the widest part of your chest and shoulder blades. Make sure to keep the tape measure horizontal, and don't pull to tight or hold your breath while measuring.
Put the tape around your natural waistline, which should be close to your bellybutton. Put one finger between the tape and your body before you measure.
This is measured between your groin and your lower ankle. You might find that is most easily measured on a pair of pants that already suit you.