Free Tool
Insulin-to-Carb
Ratio Calculator
Estimate your insulin-to-carb ratio (ICR) and correction factor for type 1 diabetes, based on your daily insulin use. A useful starting point that takes under 2 minutes.
⚕️ Medical disclaimer: This calculator provides an estimate based on widely-used T1D formulas (the 500 Rule, and the 100 Rule or 1800 Rule). Results are a starting point for discussion with your diabetes care team. Never adjust insulin doses without clinical guidance. Individuals vary significantly.
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Basics
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Insulin
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Targets
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Results
About you
A few basics to personalise your estimate. Everything you enter stays in your browser. Your inputs and results are never sent to us or stored.
kg
Used to cross-reference estimates.
yrs
Helps contextualise your results.
💉 MDI
Multiple daily injections
⚙️ Pump
Insulin pump / CSII
🔄 Hybrid Loop
Closed-loop system
Other
Not listed above
Your insulin use
We use the 500 Rule (ICR) and the 100 Rule for mmol/L or 1800 Rule for mg/dL (correction factor), the most widely used starting-point formulas for T1D.
units/day
Add all basal + all bolus doses taken on a typical day. Use a 7-day average if possible.
units/day
Your long-acting insulin total per day (or pump basal rate total). Optional, helps refine the estimate.
per day
Including meals and corrections. Usually 3–5 for most people.
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Your glucose targets
Used to calibrate your correction factor and interpret your results.
mmol/L
Your desired "in range" mid-point. Typical target: 5–7 mmol/L (90–126 mg/dL).
🧘 Low
Mostly sedentary
🚶 Moderate
Some exercise
🏃 Active
Regular exercise
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Common Questions
The 500 Rule states that 500 ÷ TDD (Total Daily Dose of insulin) gives you your insulin-to-carb ratio, i.e. how many grams of carbohydrate one unit of insulin covers. For example, if your TDD is 50 units: 500 ÷ 50 = 10, meaning 1 unit covers 10g of carbs. This is a starting estimate, not a prescription.
The correction factor (also called insulin sensitivity factor, or ISF) estimates how far one unit of insulin lowers your blood glucose. For mmol/L, 100 ÷ TDD. For mg/dL, 1800 ÷ TDD. Example: TDD 50 gives 2.0 mmol/L (36 mg/dL) per unit.
These formulas are population-level starting estimates. Your actual ratio is influenced by your diet composition, activity levels, stress, sleep, illness, hormones (especially relevant for women), and your specific insulin type. Your clinic's ratio is based on real-world observation of your response, which is always more accurate. Use this result as a reference point, not a replacement for clinical guidance.
Use a 7-day average where possible. If your TDD varies significantly (e.g. between training days and rest days), calculate both and note the range. On higher-activity days, 1 unit often covers more carbs, so the ratio number is larger (e.g. 1:12 instead of 1:10) and the correction factor is larger. Always agree changes with your care team.
Children's insulin requirements change rapidly with growth and development. These formulas can be used as a rough reference for paediatric T1D, but any ratio adjustments in children must be made with a paediatric diabetes team. Do not adjust children's insulin based solely on this calculator.
T1Fit stores your ICR and correction factor in your profile. When you log a meal, it calculates a Suggested IOB based on the carbs entered and your ICR. Over time, T1Fit Pro can help you see whether your actual glucose responses match your estimated ratios, surfacing when they may need reviewing.