Overview — What This Simulator Models
In diabetes education, insulin settings are often introduced using weight-based calculation chains. This simulator lets you observe how those calculations work by adjusting a hypothetical weight and selecting a theoretical physiological profile, then watching the math cascade through three interconnected outputs:
A Basal Rate
Background insulin that covers your body's baseline metabolic needs regardless of meals. Expressed as units per hour.
B ICR
Insulin-to-Carb Ratio — the theoretical grams of carbohydrate that one unit of insulin is expected to cover at mealtime.
C ISF
Insulin Sensitivity Factor — the theoretical blood glucose drop expected from one correction unit of insulin.
D Waterfall Logic
All five steps cascade from a single starting input. The result of Step 2 (TDD) feeds every downstream calculation — ICR, ISF, and Basal all derive from it.
The Math — How Each Step Works
Weight Conversion
Standard insulin dosing formulas are built around kilograms. The first step converts the selected theoretical weight from pounds to kg.
Example: 150 lbs ÷ 2.2 = 68.2 kg
Total Daily Dose (TDD)
The central calculation that everything else flows from. A physiological factor is applied to the weight in kg to produce a Theoretical Total Daily Dose.
Example: 68.2 kg × 0.55 = 37.5 units/day
The Factor reflects observed physiological benchmarks: 0.45 for high sensitivity (honeymoon phase, high activity), 0.55 as a common textbook baseline, 0.65+ for puberty, illness, or higher insulin resistance.
Basal Rate (50% Split)
Standard insulin theory assumes approximately half the total daily insulin covers background metabolic needs (Basal) and half covers food (Bolus).
Hourly Rate = Daily Basal ÷ 24
Example: 37.5 × 0.50 = 18.75 units/day → 18.75 ÷ 24 = 0.78 units/hr
ICR — Rule of 500
The Rule of 500 (or 450 for a more aggressive estimate) divides a constant by TDD to produce a carb ratio. A higher TDD produces a lower ratio number — meaning each unit covers fewer grams.
Example: 500 ÷ 37.5 = 1:13.3 (1 unit covers ~13g carbs)
ISF — Rule of 1800
The Rule of 1800 (adjustable from 1500 for resistant to 2200 for sensitive) divides a constant by TDD to estimate how much blood glucose drops per unit.
Example: 1800 ÷ 37.5 = 48 mg/dL drop per unit
Why These Numbers Are Estimates — Not Prescriptions
While these formulas are standard in diabetes textbooks, human biology is highly variable. Here is why the theoretical numbers produced by this tool may not reflect real-world needs:
📐 The Law of Large Numbers
The Rule of 500 and Rule of 1800 are derived from population averages. They generally apply to the middle range of the population. Outliers — those who are extremely sensitive or extremely resistant — may find these rules too aggressive or too conservative in either direction.
⏰ Variable Resistance Across the Day
The model assumes insulin resistance is flat across 24 hours. In reality, many people experience "Foot on the Floor" morning resistance (higher insulin need) or greater afternoon sensitivity. This tool estimates a flat hourly basal rate, whereas real-world settings often vary hour by hour.
🏃 Lifestyle Variables
The simulator processes a hypothetical weight but cannot adjust for low-carb diets (which alter insulin needs), athletic training, medications like steroids that impact resistance, or illness. All of these can shift your real-world TDD significantly from the theoretical estimate.
💉 Absorption Variables
The math assumes 1 unit of insulin acts as 1 unit of insulin. It does not account for scar tissue (lipohypertrophy), site failures, or bad infusion sets — all of which can cause a full dose to behave like a partial dose in real life.
The Ghost Low Phenomenon
This is one of the most important concepts in understanding why the 50/50 split often doesn't work straight out of the math — and why basal testing is essential before trusting any ratio.
When you give a meal bolus, that insulin now does double duty — it covers the carbs and silently corrects the basal drift you didn't know you had. Your blood sugar lands in range, so it looks like your carb ratio is working.
But when you eat a low-carb meal, the bolus is small — there's not enough extra insulin to cover both the meal and the hidden basal gap. Blood sugar runs high. You conclude your carb ratio is wrong, and you strengthen it.
Now the stronger ratio overshoots on normal meals. You chase lows and highs wondering what's happening.
Basal Testing
A proper basal test involves fasting for several hours and observing whether blood sugar holds steady, rises, or falls without any meal bolus active. If it drifts, the basal rate needs adjustment — not the carb ratio. This is standard diabetes education from the Juicebox Podcast and beyond.
How to Use This Tool Responsibly
Use this simulator to understand the math behind insulin settings — not to set them. Here's the intended workflow:
- Learn how the variables connect. Adjusting the weight slider and profile factor lets you see how TDD drives every downstream number. That's the insight.
- Use it for conversations, not decisions. Bring the theoretical numbers to your endocrinologist as a starting point for discussion, not as a self-determined prescription.
- Layer in your real data. If the simulator suggests a basal of 0.78 units/hr but your real-world fasting data shows hypoglycemia, your physiology doesn't match the model. Listen to your body over the simulator.
- Test methodically. Basal testing, ICR testing, and ISF testing are separate processes, done in a specific order, with real glucose data. The math here is only ever a starting line.
All Five Formulas at a Glance
| Step | Output | Formula |
|---|---|---|
| 1 | Weight in kg | lbs ÷ 2.2 |
| 2 | Total Daily Dose (TDD) | kg × Factor (0.45 – 1.2+) |
| 3 | Hourly Basal Rate | (TDD × 0.50) ÷ 24 |
| 4 | ICR | 500 (or 450) ÷ TDD |
| 5 | ISF | 1800 (1500–2200) ÷ TDD |