Calculate insulin doses including basal, bolus, correction factor, and carb ratio using the 1800 rule, 500 rule, and weight-based TDD estimation.
Insulin dosing is one of the most complex aspects of diabetes management, requiring calculation of multiple components: total daily dose (TDD), basal-to-bolus split, insulin sensitivity factor (ISF), insulin-to-carb ratio (ICR), and correction doses. Errors in any component can lead to dangerous hypoglycemia or inadequate glucose control, making a systematic calculation approach essential for both patients and clinicians.
This insulin dosage calculator estimates TDD using weight-based formulas or accepts known TDD values, divides the daily dose into basal (long-acting) and bolus (mealtime) components using the standard 50/50 split, calculates ISF using the 1800 rule (for rapid-acting insulin), derives the ICR using the 500 rule, and computes both carb-based meal boluses and correction doses in real-time. It covers Type 1, Type 2, and gestational diabetes with appropriate weight-based dosing factors.
For Type 1 diabetes, typical TDD ranges from 0.4 to 0.8 units/kg/day, with 0.5–0.6 being average for adults not in the honeymoon phase. For Type 2 diabetes, initial TDD is often 0.5–0.7 units/kg/day but may escalate to 1.0–2.0 units/kg/day due to insulin resistance. The calculator provides both estimated and customizable factors, allowing users to override auto-calculated ISF and ICR values with their physician-prescribed ratios while maintaining the framework for understanding how each component is determined.
Insulin dosing involves multiple interdependent calculations — TDD, basal/bolus split, ISF, ICR, carb counting, and correction — that are easily confused or miscalculated. This calculator performs all these computations simultaneously, showing how each component is derived and providing reference tables for ISF and ICR across a range of TDD values. It serves both as a clinical initiation tool and a patient education resource.
TDD (weight-based): weight (kg) × factor (0.5–1.0 U/kg). Basal = 50% of TDD. Bolus = 50% of TDD ÷ 3 meals. ISF (1800 rule): 1800 ÷ TDD = mg/dL drop per 1 unit of rapid insulin. ICR (500 rule): 500 ÷ TDD = grams of carbs covered by 1 unit. Correction dose = (current BG - target BG) ÷ ISF. Meal bolus = carbs (g) ÷ ICR.
Result: TDD: 56 units. Basal: 28 units. Meal bolus: 6.7 units + correction: 3.1 units = 9.8 units total.
An 80 kg Type 2 patient: TDD = 80 × 0.7 = 56 units. Basal = 28 units. ISF = 1800 ÷ 56 = 32 mg/dL per unit. ICR = 500 ÷ 56 = 1:9. Meal bolus for 60g carbs: 60 ÷ 9 = 6.7 units. Correction: (220 - 120) ÷ 32 = 3.1 units. Total mealtime: 9.8 units.
Basal insulin provides a steady background insulin level that suppresses hepatic glucose production between meals and overnight. Long-acting insulins like glargine (Lantus, Basaglar), detemir (Levemir), and degludec (Tresiba) are used for this purpose. The basal rate should keep fasting blood glucose within target without causing nocturnal hypoglycemia. Basal insulin titration protocols typically increase the dose by 1–2 units every 3 days until fasting BG is at target (typically 80–130 mg/dL).
Bolus (mealtime) insulin covers the glucose rise from food intake. Rapid-acting analogs — lispro (Humalog), aspart (NovoLog), and glulisine (Apidra) — are given 0–15 minutes before meals. Accurate carb counting is essential for bolus calculation. One "carb choice" = 15g carbohydrates. Common meals range from 30g (light meal) to 90g+ (large meal). The insulin-to-carb ratio determines how many units are needed per carb gram.
Beyond the basic rules, several factors modify insulin needs: (1) exercise reduces insulin sensitivity for 24–48 hours; (2) illness and infection dramatically increase insulin resistance; (3) the dawn phenomenon (early morning glucose rise) may require split basal doses or pump rate adjustments; (4) high-fat meals delay glucose absorption, requiring extended or split bolus dosing; and (5) alcohol inhibits gluconeogenesis, increasing hypoglycemia risk hours after drinking. Insulin pump therapy and automated insulin delivery systems (hybrid closed-loop) use programmable basal rates and bolus calculators to account for these variables.
The 1800 rule estimates the insulin sensitivity factor (ISF) for rapid-acting insulin. Divide 1800 by the total daily dose (TDD): ISF = 1800 ÷ TDD. The result tells you how many mg/dL your blood glucose will drop per 1 unit of rapid-acting insulin. For regular insulin, the 1500 rule is used instead.
The 500 rule estimates the insulin-to-carb ratio (ICR): divide 500 by TDD. The result tells you how many grams of carbohydrates are covered by 1 unit of rapid-acting insulin. For example, TDD of 50 units → ICR = 1:10, meaning 1 unit covers 10 grams of carbs.
The standard starting split is 50% basal / 50% bolus. Some patients may need a 40/60 or 60/40 split based on their meal patterns and glucose profiles. The bolus portion is typically divided equally among three meals unless meal sizes vary significantly.
A correction dose (also called a corrective or supplemental dose) is additional rapid-acting insulin given to lower an elevated blood glucose back to target. It's calculated as: (current BG - target BG) ÷ ISF. It's added to the meal bolus at mealtime or given alone between meals.
Most guidelines recommend starting with basal insulin alone at 0.1–0.2 U/kg/day (or 10 units/day) and titrating up by 2 units every 3 days until fasting BG reaches target. Mealtime insulin is added later if postprandial targets are not met despite optimized basal insulin.
Insulin resistance fluctuates with illness, stress, activity level, hormonal changes, medications (like steroids), and weight changes. Regular dose adjustment based on blood glucose monitoring is fundamental to diabetes management. Many patients adjust doses daily based on carb intake and pre-meal BG levels.