Many factors affect the size of fingers, from temperature to stress, from activity using the hands to pregnancy. The size can vary drastically from one day to the next. Before being fitted for a ring, ideally you would take a measurement when your finger was larger so the ring would always be comfortable, but that is usually not the case. Over the course of time, rings can become dangerously tight. The goal is to remove the ring before that time comes.
You Cannot Turn the Ring on Your Finger
You should be able to turn the ring in a circle around your finger. This means that the ring is not stuck to your skin underneath it. This also allows the skin underneath the ring to breathe. If you cannot turn the ring, ice your finger with ice pack for 10 minutes, then allow your finger to dry. If you can comfortably move the ring after icing, the fit is OK. If you still can't move the ring, it is too tight.
You Cannot Move the Ring Upward
Being able to move the ring upwards on your finger indicates a good fit. Ideally, you should be able to take it off from time to time to allow the finger to breathe, but that's not always possible, especially after years of wearing the ring. If the ring won't budge, try icing the finger as above, then apply some olive oil or body wash to the skin around the ring. If this doesn't work, then seek the help of your doctor.
Finger is Tingly or Numb
Your finger with the ring should feel just like your other fingers. If it feels tingly or numb, it's too tight. If the tingly feeling is accompanied by a bluish or white tinge of color above the ring, you might need to consult a doctor.
Skin is Ballooning Over the Ring
The ring should ideally sit on top of the skin, not in it. If the ring is so tight that it appears to severely indent the skin underneath it, it is too tight. A small amount of indentation is expected, especially in the summer, but the ring should be able to be nudged. If skin appears to be growing over the ring, consult a doctor immediately.