Insulin Dosage Calculator
Enter your meal carbohydrates, current blood glucose, target blood glucose, carb-to-insulin ratio and insulin sensitivity factor to calculate your total mealtime bolus dose. The calculator breaks the result into the carbohydrate coverage dose and the blood-glucose correction dose, and shows the full working. If you do not know your carb ratio or sensitivity factor, enable the estimator and enter your weight to get values derived from your total daily insulin dose.
Formula
Worked example
A person eating 60 g of carbs with a CIR of 15 g/unit needs 60/15 = 4 units for the meal. Their blood glucose is 150 mg/dL and target is 100 mg/dL with an ISF of 50 mg/dL/unit, so the correction is (150-100)/50 = 1 unit. Total mealtime dose = 4 + 1 = 5 units of rapid-acting insulin.
What this calculator computes
This tool calculates the mealtime bolus dose of rapid-acting insulin using two components: the carbohydrate coverage dose and the blood-glucose correction dose. The carbohydrate coverage dose accounts for the carbs in your meal; the correction dose accounts for any deviation from your target blood glucose. Adding the two gives the total units you need to inject or bolus before eating. The math follows the formulas taught by the American Diabetes Association and published in standard clinical practice guidelines.
How to use the calculator
Choose your blood-glucose unit (mg/dL or mmol/L), enter the grams of carbohydrate in your upcoming meal, your current blood glucose reading, and your target blood glucose. If you already know your carb-to-insulin ratio (CIR) and insulin sensitivity factor (ISF) from your care team, keep the toggle on and enter those values directly. If you do not yet have personalised values, switch the toggle off and enter your body weight: the calculator will estimate your total daily insulin dose (TDID) using 0.55 units/kg, then derive a starting CIR via the 500 rule and a starting ISF via the 1800 rule. These estimates are a starting point and must be validated with your clinician.
The 500 rule and 1800 rule explained
When a patient does not yet have a verified CIR, clinicians often use the "500 rule" as a first estimate: divide 500 by the total daily insulin dose (TDID) to get the grams of carbohydrate one unit covers. Similarly, the "1800 rule" divides 1800 by TDID to estimate the sensitivity factor in mg/dL/unit (for mmol/L users this is roughly 100 divided by TDID). These rules assume roughly half of the daily dose is basal and half is bolus, and are regularly used as starting points for Type 1 and Type 2 patients beginning carbohydrate counting. Actual values vary significantly and should be refined through structured self-monitoring.
Understanding the correction dose
The correction dose (also called the high blood sugar correction or supplemental dose) adjusts for blood glucose that is already above or below your target before a meal. A positive correction means your current glucose is above target, so extra insulin is added. A negative correction means your glucose is already below target, reducing the total dose. If the combined total calculates to zero or below, do not skip eating a carbohydrate-containing meal without speaking to your care team first, as this situation can quickly lead to hypoglycemia.
Limitations and safety
Insulin dosing is a medical decision. This calculator is an educational tool that applies published formulas correctly, but it cannot account for day-to-day variation caused by illness, exercise, stress, other medications, pump occlusions or changes in insulin sensitivity across the menstrual cycle. Always confirm a calculated dose with the guidance your prescribing clinician has given you. If a calculated dose seems unusually high or low compared with your typical doses, double-check your inputs and consult your care team before administering.
Common insulin-to-carb ratio and sensitivity factor ranges
| Parameter | Typical range | Derived by |
|---|---|---|
| Carb-to-insulin ratio (CIR) | 5-20 g/unit | 500 / TDID (500 rule) |
| Insulin sensitivity factor (ISF) | 20-100 mg/dL/unit | 1800 / TDID (1800 rule) |
| Total daily insulin dose (TDID) | 0.4-0.7 units/kg/day | Weight x 0.55 (starting estimate) |
| Basal insulin share | ~40-50% of TDID | Set by prescribing clinician |
| Bolus insulin share | ~50-60% of TDID | Split across meals |
These are population starting-point ranges. Your actual values must be determined through structured testing with your care team.
Frequently asked questions
What is a carb-to-insulin ratio (CIR)?
The carb-to-insulin ratio is the number of grams of carbohydrate that one unit of rapid-acting insulin can cover. For example, a CIR of 15 g/unit means you need 1 unit for every 15 g of carbs you eat. Your care team determines your personal CIR through structured meal testing, but a common starting estimate is derived from the 500 rule: 500 divided by your total daily insulin dose.
What is the insulin sensitivity factor (ISF)?
The insulin sensitivity factor (ISF), also called the correction factor, is how much one unit of rapid-acting insulin lowers your blood glucose. An ISF of 50 mg/dL/unit means one unit brings your glucose down by about 50 mg/dL (2.8 mmol/L). The 1800 rule gives a starting estimate: 1800 divided by your total daily insulin dose. Actual sensitivity varies with time of day and many other factors.
What is the difference between basal and bolus insulin?
Basal insulin (long-acting, such as glargine or detemir) provides a steady background level that covers glucose released by the liver between meals and overnight, typically 40-50% of the total daily dose. Bolus insulin (rapid-acting, such as lispro, aspart or glulisine) is taken at mealtimes to handle the carbs eaten, typically 50-60% of the total daily dose split across meals. This calculator helps with the bolus (mealtime) dose only.
How do I convert mg/dL to mmol/L?
Divide mg/dL by 18 (more precisely 18.0182) to get mmol/L. For example, 180 mg/dL is approximately 10 mmol/L. This calculator accepts both units and converts automatically when computing the correction dose, so you only need to switch the unit selector and enter your values as your meter displays them.
Why might my total dose be zero or negative?
A zero or negative total can result when your blood glucose is already well below your target, making the negative correction dose larger than the carb coverage dose. In practice, this means your blood glucose is low and a straight bolus for the meal may cause hypoglycemia. The correct action is to treat the low first (or reduce the meal dose as directed by your care team) rather than skipping carbs or injecting insulin.
Does this calculator work for Type 1 and Type 2 diabetes?
Yes. The carbohydrate counting method applies to any person using rapid-acting insulin at mealtimes, whether they have Type 1, Type 2, LADA or gestational diabetes, and whether they use multiple daily injections (MDI) or a continuous subcutaneous insulin infusion (pump). The formulas are the same; only your personal CIR and ISF values differ.
When should I take my mealtime insulin?
Timing depends on the type of rapid-acting insulin prescribed. Most modern analogs (lispro, aspart, glulisine) are designed to be taken 0-15 minutes before starting a meal when blood glucose is in range, and can be taken during or immediately after the meal if glucose is low or the meal size is uncertain. Regular (human) insulin is typically taken 30 minutes before eating. Always follow your prescribing clinician's timing instructions.