There is 'stuttering severity' scale which measures the percentage of syllables stuttered if you want an objective measure of outward stuttering. However it doesn't take into account a lot of other factors that can influence how someone's stutter impacts on them. For example, there may be people that score as 'mild' with outward stuttering but have an interiorised stutter where they spend so much time and energy avoiding stuttering, switching words, avoiding speaking situations and feeling negative about their speech that actually overall you could say that the stutter is having a severe impact on them. On the other hand there's people who may score as 'severe' with outward stuttering but they continue speaking, don't avoid words or any speaking situations and may not be as bothered. It really depends on the individual, as there's so many different factors that influence stuttering.