As a person with chronic illness, I get asked this question a lot. During a difficult period, I found I was wishing for a way to more reliably distinguish between, say, a 5 on Tuesday and a 7 on Thursday.
The two pain scales (below) are still subjective, mostly, but they're consistent, and now I don't have to guess.
Physical Self-Awareness Pain Scale
I know a few medical folks who like this scale, and so for them, here's an acronym: P-SAPS. You know how medical people love a good acronym.
The P-SAPS (cough) came from years of chronic pain, punctuated by a very few events at level 9, and one brief procedure that went weird and became a 10, for a few minutes there. (That was a big surprise, and very enlightening!)
Mental and Emotional Self-Awareness Pain Scale
Sticking with the acronym pattern, this one gets to be the ME-SAPS, I suppose.
This one is also very personal. During a different sort of difficult period, I was wishing I had a similar scale for mental and emotional pain. The ME-SAPS is based on the physical scale, and adapted from my own experience (and that of a friend or two).
This is not an official scale, but a personal one. Nobody will know what the numbers mean to you unless you tell them. So, if you read one of these scales and think "yes, this is what it's like!", share it with your provider. Let it be a conversation starter. (See Disclaimer)
License: I own the copyright for these pain scales, and have made them available for free. I hear they're really useful, so I've given them the sort of license that keeps them free no matter what. There's a creative commons license specified at the bottom of each document.
First, understand that since these are personal, whether a contribution is accepted or not will depend on whether it improves how this describes my experience. If your experience is very different, I encourage you to make a copy (or fork it), and adapt it so it's just right for you. Just be sure to follow the license at the bottom of each one. (Or take the inspiration, and write your own from scratch!)
If you find it basically accurate for you, but you have some improved wording to suggest, please open an issue here (click "New Issue" in the top right), or, if you're the kind of nerd who makes pull requests, you can make a pull request.
I am not a medical professional. These are based on my own experience, and are not vetted by any medical establishment whatsoever. Think of them as poetry. Words to help you communicate. Ask your doctor for their opinion, etc. Take care of yourself. You deserve it.