What an insulin-to-carb ratio is, and who sets yours
An insulin-to-carb ratio is how much insulin covers a set amount of carbohydrate, and it is worked out with your team.
An insulin-to-carb ratio is a personal number that tells you how much insulin covers the carbohydrate in a meal. It’s a starting point for the dose you take with food, and it’s set with your diabetes team, not something you’re expected to guess.
- Your ratio matches your bolus (the fast insulin you take at meals) to the carbohydrate in what you’re about to eat.
- Too little insulin for the carbs leaves glucose high afterwards; too much can send you low, so the aim is a match, not a maximum.
- It’s your diabetes team who works your ratio out with you, and it’s normal for it to be revisited.
- The number isn’t fixed for life: it can shift with time of day, illness, activity, weight and hormones.
- Coming back to carb counting after a long gap is a normal reason to have your ratio looked at again, not a sign of falling behind.
What the ratio actually decides
Every time you eat carbohydrate, your glucose will rise. Bolus insulin is the fast-acting dose you take to cover that rise. The insulin-to-carb ratio is the link between the two: it’s the amount of carbohydrate that one unit of your insulin is expected to handle.
So the ratio doesn’t decide your dose on its own. It works alongside the carbs you count in the meal. Bigger meal, more carbs, more insulin. Smaller meal, less. The ratio is the constant you apply to whatever you’re eating, which is why getting it roughly right matters more than counting any single meal perfectly.
It’s worth being clear about what it is not. The ratio only covers the food. It doesn’t fix a glucose level that’s already high before you eat, and it doesn’t account for a hypo you need to treat first. Those are separate calculations, and your team sets those up with you too.
Who works your ratio out, and why it’s not yours to guess
Your ratio is worked out with your diabetes team, usually from your weight, your total insulin over a day, and what your glucose actually does after meals. Structured carb-counting courses like DAFNE (a five-day course) and BERTIE (available online) are built around teaching you to use it and to spot when it needs changing.
This is the clinical boundary, and it’s a real one. The ratio is a dose decision. Working it out from a formula you found online, or copying someone else’s, is how people end up chasing highs and lows they can’t explain. If your meals keep landing wrong in the same way, that’s the signal to bring it to your team, not to nudge the number yourself.
Why the number changes over time
A ratio that fit last year may not fit now, and that’s expected. Many people need a different ratio at breakfast than at their evening meal, because the body handles insulin differently across the day. Illness usually makes you more resistant to insulin, so the same food can push glucose higher than normal. Activity often does the opposite, and this can leave you low during the exercise itself or in the hour or two straight after, as well as more sensitive for the rest of the day.
Weight change, hormones, periods, growth and stress all move it too. None of this means you did anything wrong. It means the ratio describes your body at a point in time, and your body doesn’t stay still. The useful thing isn’t a perfect number, it’s noticing the pattern early enough to raise it. Extra carbohydrate is something you can manage yourself; insulin adjustments are worked out with your diabetes team.
What changes what your ratio needs to be
This is why there’s no single right ratio, even for one person. The same breakfast can behave differently on a normal day, a day you’re unwell, or the morning after a long walk. When you notice the same meal landing the same wrong way repeatedly, that repeat is the thing worth taking to your team, more than any one-off reading.
What a mismatch looks like day to day
Examples, not instructions or doses.
You eat the same thing most mornings, count it the same way, and two hours later you’re sitting at 12 mmol/L nearly every day. A consistent post-meal high after one particular meal is a pattern worth logging, not a one-off to ignore.
You’re dropping to around 3.5 mmol/L before lunch most days after your usual breakfast. A repeated low at the same time can mean the meal dose is doing too much, and that’s a pattern your team can look at.
Your mornings sit steady but evenings run high after a similar-sized meal. Needing a different ratio at different times of day is common, not a mistake.
Meals that used to land well have started running high or low over a few weeks. Bodies change with weight, hormones and time, and a ratio can simply need revisiting.