Fingers are puffier the first thing in the morning. Fingers are also smaller in colder temperatures. Take your measurment mid-morning/afternoon and at room temperature to get the truest fit.
If you don't know your size, you could visit any local store that sells rings. They have ring sizers to be able to size your finger, or you could have an existing ring measured.
You can also purchase our ring sizer kit to get an accurate measurement: