Modular (flip-face) helmets are mostly round oval, due to the hinge mechanism, though newer technologies are now allowing them to fit intermediate shapes better. A helmet that is the wrong shape will cause pressure points on your forehead (too round) or the sides of your head (too oval).
So, how tight should a new motorcycle helmet be?
The helmet should fit snugly in the "baseball cap area" highlighted in the photo above. RevZilla photo. A new helmet should be snug around any part of your head that would be covered by a baseball cap. This area should be about as firm as a tight knitted winter hat — slightly compressed, but not painful or squeezing.
How do you size a motorcycle helmet?
Use it to find which size of helmet suits you: Wrap the measuring tape around the fullest part of the head - this will be just above the ears and about a half-inch above the eyebrows for most - and take the measurement at the forehead. To get the most accurate measure, have a friend help with this step.