Most T1D resources explain bolusing for pumps. MDI users get a footnote. This guide covers the full calculation from the ground up: your insulin-to-carb ratio, sensitivity factor, insulin on board, CGM trend adjustment, and how fat and protein change everything.
A correct meal bolus on MDI requires four numbers. Get any one of them wrong and your glucose will reflect it. Get all four right and mealtime becomes predictable, not a guessing game.
The four variables are: your insulin-to-carb ratio (ICR), your insulin sensitivity factor (ISF), your insulin on board (IOB), and your CGM trend arrow. We will cover all four.
The ICR tells you how many grams of carbohydrate one unit of insulin covers. An ICR of 1:10 means one unit covers 10 grams. An ICR of 1:15 means one unit covers 15 grams. The higher the second number, the less insulin you need for the same meal.
The formula is simple: total carbs divided by your ICR equals your meal bolus.
If you eat 45 grams of carbs and your ICR is 1:10, your meal bolus is 4.5 units. If your ICR is 1:15, it is 3 units. Same meal. Very different doses. This is why knowing your ICR precisely matters.
If you do not know your ICR, start here. Take 500 and divide it by your total daily insulin dose — that is your basal and bolus combined over a typical day.
You take 25 units of long-acting insulin and average 15 units of rapid-acting per day. Total daily dose is 40 units. 500 divided by 40 equals 12.5. Your starting ICR is approximately 1:12, meaning one unit covers 12 grams of carbs.
This is a starting point, not a verdict. Your actual ICR needs to be verified through real meal testing. But it gives you a safe first number to work from.
This is one of the most important things most MDI guides skip. Cortisol peaks in the morning and makes you more resistant to insulin. The same meal at 7am needs significantly more insulin than the same meal at noon. Most people need a different ICR for at least three time periods: morning, midday, and evening.
If you are using a flat single ICR all day, that is the most likely reason your post-breakfast glucose is unpredictable.
The ISF tells you how many points your glucose drops per unit of rapid-acting insulin. If your ISF is 40, one unit drops your glucose by 40 mg/dL. This number is used to calculate correction doses when your glucose is above target.
Take 1800 and divide it by your total daily insulin dose.
Total daily dose of 40 units. 1800 divided by 40 equals 45. Your ISF is approximately 45 mg/dL per unit. If your glucose is 165 and your target is 110, the gap is 55 mg/dL. Divide 55 by your ISF of 45 to get 1.2 units of correction insulin.
IOB is the amount of insulin from a previous bolus that is still active in your body. Standard rapid-acting insulin (NovoRapid, Humalog) has a duration of action of roughly 4 hours, peaking at around 75 to 90 minutes.
If you bolused 3 units two hours ago and your insulin is still active, you cannot simply add a full correction dose on top without risking a serious low. The active insulin from your last bolus is already working.
IOB is deducted from your correction dose, not your meal dose. If the correction calculation says you need 1.5 units but you have 1 unit of IOB, your actual correction is 0.5 units. If your IOB is already enough to cover the correction, you give zero additional correction insulin.
Your current glucose number tells you where you are. Your trend arrow tells you where you are going. A flat arrow at 140 mg/dL is very different from a double up arrow at 140 mg/dL. In the second case, your glucose is rising fast and your dose needs to reflect that.
| Arrow | Rate of change | Dose adjustment |
|---|---|---|
| Double up | More than 2 mg/dL per minute | Add 10 to 20% to correction |
| Single up | 1 to 2 mg/dL per minute | Add 5 to 10% |
| Flat | Less than 1 mg/dL per minute | No adjustment |
| Single down | 1 to 2 mg/dL per minute falling | Reduce by 10 to 15% |
| Double down | More than 2 mg/dL per minute falling | Hold bolus, treat low first |
You are about to eat a meal with 55 grams of carbs. Your glucose is 160 mg/dL with a flat arrow. Your target is 110. Your ICR is 1:10, your ISF is 40, and you have 0.5 units of IOB from a snack two hours ago.
Meal bolus: 55 divided by 10 equals 5.5 units.
Correction: 160 minus 110 equals 50. 50 divided by 40 equals 1.25 units. Minus 0.5 IOB equals 0.75 units correction.
Total: 5.5 plus 0.75 equals 6.25 units, rounded to 6.5 units depending on your pen increments.
On MDI, you give this as a single injection 10 to 15 minutes before the meal for standard rapid-acting insulin.
High-fat and high-protein meals slow digestion and cause a secondary glucose rise 3 to 4 hours after eating. If your meal contains more than 20 grams of protein or more than 15 grams of fat, a standard pre-meal bolus will not be enough.
On MDI, the strategy is a split dose: give your standard carb bolus before the meal, then give an additional smaller dose 60 to 90 minutes later to cover the delayed fat and protein absorption. The Warsaw method converts fat and protein calories to a carb equivalent and doses using your ICR.
This is exactly what the Diabetic Wolf bolus calculator handles in the advanced mode, unlocked with your email.
Never bolus if your glucose is below 70 mg/dL. Treat the low first. Never bolus if your glucose is below 90 mg/dL and your trend arrow is falling. Your IOB is a ceiling on correction doses, not an addition to them. If your correction is negative after subtracting IOB, give zero correction. When in doubt, give less.
The Diabetic Wolf bolus calculator applies everything in this guide automatically. ICR by time of day, CGM trend adjustment, IOB correction, fat and protein split dosing, and exercise modifier.
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