There is no universal formula
Why doses and ratios come from your own patterns, not a chart, and why they keep changing.
Your insulin doses, your ratios, the amount of carbs that fixes a low: none of it comes off a chart that works for everyone. It gets set for you, with your team, and it keeps changing because your body keeps changing. If yours look different from someone else’s, that is normal, not a mistake.
- Your insulin-to-carb ratio (how much insulin covers a set amount of carbohydrate) is personal, and someone else’s numbers are no guide to yours.
- The same reading can need a different response on a different day, because activity, illness, stress and sleep all move glucose.
- A dose that worked last year may not fit now: your needs shift over months and years, and that is expected.
- When a number does not do what you thought it would, that is information, not a failure.
- Anything that changes a dose (a ratio, a correction) is set with your diabetes team, never copied from another person.
Why there isn’t one set of numbers for everyone
People often expect diabetes to work like a recipe: eat this many carbs, take this much insulin, land on this number. It would be easier if it did. But the amount of insulin your body needs for a given amount of food, or to bring a high down, depends on things that are specific to you and that no chart can know in advance.
Your weight, how long you’ve lived with type 1, how much insulin your own body still makes (if any), how active you are, how you sleep, even the time of day: all of it feeds into the numbers your team sets. Two people the same age, eating the same meal, can need genuinely different doses and both be doing exactly the right thing. Your numbers are yours because they were fitted to you.
What this changes about how you read your own data
The practical upshot: stop comparing your doses, your readings or your time in range to anyone else’s. A friend’s ratio tells you nothing useful about yours, and a number that looks “high” on someone else’s screen might be exactly where their team wants them.
It also means a reading that surprised you is worth a second look, not a telling-off. If lunch sent you higher than expected, the question is what was different (a bigger meal, a lie-in, a stressful morning), not whether you got it wrong. Over a week or two, patterns show up that a single reading never could. That pattern is the thing you take to your team, and it’s what they use to adjust the numbers with you.
Why the right number keeps moving
Even once your team has set your numbers, they don’t stay fixed forever. Insulin needs drift with the seasons, with a growth in activity or a drop in it, with illness, with hormones, and with the slow changes that come from living with type 1 over years. This is not you slipping. It’s the normal reason people have review appointments in the first place.
So if something that worked reliably for months starts missing, that’s usually a signal to look at the pattern and talk to your team, not a sign you’ve done anything wrong. The numbers are meant to be revisited. Coming back to check them after a long time away from all this is a normal thing to do, not starting over.
What changes the numbers that fit you
Every one of these can shift how much insulin you need, and they shift in both directions. More activity often means less insulin and a real hypo risk (a low, glucose dropping too far), during the activity or straight after and sometimes hours later, including overnight. Illness and stress can push needs the other way. This is why a fixed formula can’t hold: the inputs keep moving, so the answer does too.
When the same situation needs a different answer
Examples, not instructions or doses.
One day your usual lunch lands you around 7 mmol/L after. Another day the exact same food leaves you higher. The difference is often what else was going on: less sleep, a stressful morning, less movement. Same meal, different day, different result.
You corrected a high the way you always do and it barely moved. That’s not a personal failure. Insulin can act differently depending on the day, and one reading rarely tells the whole story. It’s worth noting, then watching whether it repeats.
A friend with type 1 shows you their doses and they’re nothing like yours. That difference is expected: their numbers were fitted to their body, not yours. Neither set is 'right' or 'wrong'.