One of the core skills of living with type 1 diabetes is calculating your mealtime insulin dose. It all comes down to one number: your insulin-to-carb ratio (ICR). In practice, though, it's a little more nuanced than it first appears.
What Is the ICR?
The ICR tells you how many grams of carbohydrates one unit of bolus insulin covers. If your ICR is 10, one unit covers 10 g of carbs. Eating a bowl of oatmeal with 50 g of carbs? You inject 5 units.
The formula:
Dose = Carbs (g) ÷ ICR
Simple enough. The complexity comes from the fact that ICR is individual and changes over time.
Why Your ICR Changes Throughout the Day
Most people with type 1 diabetes notice they need more insulin for the same carbs at breakfast than at dinner. This is driven by the "dawn phenomenon" — the morning surge of cortisol and growth hormone that reduces insulin sensitivity.
A typical pattern:
- Breakfast: ICR 8–10 (higher insulin need)
- Lunch: ICR 10–12
- Dinner: ICR 12–15 (better sensitivity)
This is exactly why Dia-Log stores separate ICR values for each period (breakfast, lunch, dinner) — the app accounts for the fact that your body at 8 am and 7 pm responds very differently.
How to Find Your ICR
If you're just starting out, or you're not sure your current values are right, there are a few approaches.
The Rule of 500
The classic starting-point method:
ICR = 500 ÷ total daily insulin dose (basal + bolus)
For example, if your total daily dose is 40 units: ICR ≈ 500 ÷ 40 = 12.5.
This is a starting point, not a final answer. The Rule of 500 gives you an approximation — you'll refine the number through observation.
The Observation Method
- Pick a food with a known carb count (e.g., 60 g plain oatmeal ≈ 40 g carbs).
- Before eating, confirm your blood sugar is in range (72–126 mg/dL) and there's no active insulin on board.
- Inject a test dose and check your blood sugar 2–3 hours later.
- If you're still in range — your ICR is correct. Too high? Lower your ICR. Too low? Raise it.
Repeat a few times to build a pattern. A single reading proves nothing.
How Dia-Log Helps You Dial In Your ICR
The app logs every meal: carbs, dose, and time. After 7–10 entries for a given meal period, you can see the average carb-to-dose ratio — that's your real-world ICR in action.
No mental math, no spreadsheets. Dia-Log surfaces your patterns automatically.
Other Factors That Affect Your Dose
ICR is the foundation, but it's not the only variable:
- Glycemic index of the food. Fast-acting carbs (white bread, juice) spike blood sugar quickly — sometimes you need to inject a bit earlier. Slow carbs (legumes, al dente pasta) — a bit later.
- Exercise. After a workout, insulin sensitivity stays elevated for 12–48 hours — your dose may be lower than usual.
- Stress and illness. Both increase insulin requirements.
- Pre-meal glucose. If your blood sugar is high before eating, you need an additional correction dose on top. If it's low, you may need to reduce your meal dose.
The Bottom Line
Calculating a mealtime insulin dose is a skill that sharpens over time. The Rule of 500 gives you a starting estimate, observation gives you accuracy, and tools like Dia-Log accelerate the process. Don't look for a single "correct answer" set in stone — your ICR will shift with your weight, activity level, season, and stage of life.
This content is for educational purposes only. All insulin dosing decisions should be made together with your doctor.