Finger size can vary from day to day or even throughout the day due to a variety of factors including diet, hydration, exercise, allergies, temperature, etc. It can be difficult to get an accurate size even when using a ring sizer, so I recommend measuring your finger more than once, ideally at different times of day. Later in the day is generally best, as your finger is likely to be its largest after a day of working with your hands and being in the heat of the day.
In addition, be sure the sizer slides comfortably over your knuckle. If the ring you wish to order is over 1/4” (6mm) wide, I would recommend ordering a slightly larger ring to compensate for the width. Wider rings constrict more of the finger and so will fit tighter than a narrower ring of the same size. Go up 1/4 size if you are ordering a ring between 1/4” (6mm) and 5/16” (68mm) wide. Go up 1/2 size if you are ordering a ring that is 3/8” (9.5mm) wide or wider.