Insulin Calculator: Accurate Bolus and Basal Dosing

How to Use Insulin Calculator: Accurate Bolus and Basal Dosing

1. Step 1: Enter carbs (g)

   Type the grams of carbohydrate for your meal or snack. If you need help estimating, use food labels or a trusted carb‑counting app.

2. Step 2: Set glucose and units

   Enter your current blood glucose and choose mg/dL or mmol/L. Pick a realistic target (e.g., 100–120 mg/dL) with your clinician.

3. Step 3: Provide ICR and ISF (or estimate)

   If you know your insulin‑to‑carb ratio (ICR) and correction factor (ISF), enter them. Otherwise, enable “Estimate from TDD” to derive ratios.

4. Step 4: Account for IOB

   If you have recent rapid‑acting insulin on board (IOB), enter it so the calculator subtracts it from any correction dose.

5. Step 5: Calculate and review

   Tap Calculate. Review meal bolus, correction, total, rounding, and any safety notes before dosing.

Key Features

• Accurate dose calculation
• Integrated carb counting
• Customizable correction factor
• Basal context (reference only)
• Mobile‑first, clear results

Understanding Results

Formula

Total bolus = meal bolus + correction. Meal bolus is carbohydrate grams ÷ insulin‑to‑carb ratio (ICR). Correction is the difference between your current and target glucose divided by your insulin sensitivity factor (ISF). In symbols: Meal = carbs ÷ ICR; Correction = max(0, (current − target) ÷ ISF − IOB). We round to your selected step for practical dosing. If current glucose is below target, no correction is suggested and the tool highlights low‑glucose safety.

Reference Ranges & Interpretation

ICR and ISF are individualized. As a rough starting point used in diabetes education, the “500 rule” estimates ICR ≈ 500 ÷ total daily insulin (TDD), and the “1800 rule” estimates ISF ≈ 1800 ÷ TDD (mg/dL per unit). For mmol/L, ISF ≈ (1800 ÷ TDD) ÷ 18. These are educational heuristics—not prescriptions. Clinicians refine targets based on age, activity, hypoglycemia risk, and monitoring data. After calculation, compare the result with your history and current context (food type, activity, illness) and discuss adjustments with your care team.

Assumptions & Limitations

This tool assumes rapid‑acting insulin taken before meals, stable absorption, and an accurate carb count. It cannot account for gastroparesis, unusual insulin action, steroids, infections, or device algorithms. IOB entry is manual and approximate; pumps estimate IOB differently. Do not use the tool to change prescriptions or for emergency care. Severe highs or lows require clinician‑directed action.

Complete Guide: Insulin Calculator: Accurate Bolus and Basal Dosing

Use our insulin calculator to estimate mealtime bolus from your carb ratio and correction factor. Enter carbs, glucose, target, and IOB to get a dose.

This guide pairs the tool with plain‑English explanations so you can double‑check the math, understand what the inputs mean, and avoid common pitfalls like insulin stacking or unrealistic targets. It does not replace medical advice. Use the results to prepare questions for your clinician and to compare with your patterns from daily checks or a CGM alongside your A1C and blood sugar results.

What bolus insulin does

Bolus insulin is the rapid‑acting dose you take before eating (or to correct a high). Its job is to match the glucose rise from carbohydrates and to bring a higher reading down toward a chosen target. Behind the scenes, your body is also running a basal influence—either a long‑acting injection or a pump’s steady micro‑pulses—to cover the background glucose your liver releases. The calculator on this page focuses on the bolus part of that picture.

Because meals and days vary, there is no single “right” number. The same lunch might require different insulin during illness or after exercise. Think of the bolus as a set of dials you can adjust: one dial for carbs (ICR), one dial for high‑glucose correction (ISF), and one dial for timing and delivery. The tool helps you set the first two dials in a consistent way using your inputs.

How this insulin calculator works

The calculation has two parts: a meal bolus and a correction. The meal bolus is simply the carbohydrate grams divided by your insulin‑to‑carb ratio (ICR). If your ICR is 12 g/u, then 60 g of carbs calls for 5 units for the meal. The correction addresses the gap between your current reading and your target, using your insulin sensitivity factor (ISF). If your current is 180 mg/dL and your target is 110 mg/dL, the gap is 70. With an ISF of 50 mg/dL per unit, 70 ÷ 50 ≈ 1.4 units of correction. If you have insulin on board (IOB) from a recent dose, this tool subtracts that amount from the correction portion to reduce stacking. We never subtract IOB from the meal bolus because food still needs coverage.

To fit everyday decision‑making, the tool rounds to the step you choose—typically 0.5 units for pens or 0.1 for pumps. If your current glucose is already below target, no correction is suggested; the tool highlights that you might need fast‑acting carbs if you have symptoms, per your clinician’s advice. The math stays transparent so you can see how each part contributes to the total.

Picking units and a target

Use the unit toggle to work in mg/dL or mmol/L. Under the hood, the calculator converts mmol/L to mg/dL using the standard factor 18 so the correction math is consistent. Your target is personal. Many adults use a pre‑meal target around 100–120 mg/dL (5.6–6.7 mmol/L), but targets are individualized for safety. If you experience frequent lows, your clinician might recommend a higher target. A stable, realistic target beats chasing a single number that invites hypoglycemia.

You can also flip between units to get a feel for the same number expressed differently—helpful if your meter and clinician notes use different units. If you only need unit conversion without dosing, see our glucose converter and the blood sugar ranges tool for context.

Estimating ICR and ISF

Not everyone knows their ICR and ISF. Education programs often teach two rules of thumb as starting points: ICR ≈ 500 ÷ total daily insulin (TDD), and ISF ≈ 1800 ÷ TDD (mg/dL per unit). For example, if your TDD is 40 units, ICR ≈ 500 ÷ 40 = 12.5 g/u; ISF ≈ 1800 ÷ 40 = 45 mg/dL per unit (≈ 2.5 mmol/L per unit). Our calculator can estimate these automatically when you enable the TDD option. These are heuristics; clinicians refine them over time using logs, CGM traces, and experiences like post‑meal rises or activity‑related dips.

Some people use body weight to arrive at a tentative TDD before refining it with data, often in the range of 0.4–0.6 units/kg/day for type 1 diabetes in adults, with wide variation by context. If you need a quick weight reference, our ideal body weight calculator can provide a benchmark to compare with your real weight when discussing dosing.

Insulin on board and stacking

Insulin on board (IOB) refers to rapid‑acting insulin that is still active from a previous bolus. If you correct again before the earlier dose has worn off, the effects stack and can drive a low. Pumps and some apps estimate IOB based on an insulin‑action curve; our calculator takes a simpler approach: it subtracts the IOB you enter from the correction portion only. That encourages caution while still allowing you to cover carbs. When in doubt, many clinicians prefer waiting for the previous dose to peak before adding more unless you have clear guidance.

Timing matters, too. For many rapid‑acting insulins, dosing 10–20 minutes before eating can better match the glucose rise—unless you are close to or below target, in which case dosing with or after the first bites can be safer. A CGM trend arrow can inform timing, but the math in this tool stays the same: meal coverage + correction, then rounding.

Rounding and delivery methods

Most pens deliver 0.5‑unit or 1‑unit increments. Pumps and patch devices can dose in finer steps, like 0.1 units. The calculator lets you pick the rounding step so what you see is practical to deliver. If you round down, consider how the food’s speed (fast carbs vs. mixed meals) and your sensitivity trend might affect the result; similarly, rounding up can be appropriate for slower meals or if your glucose is trending higher. The key is consistency so you can learn how your body responds and then adjust with your clinician’s input.

Basal vs. bolus context

Basal insulin covers background needs; bolus covers food and corrections. Many adults’ total daily insulin ends up roughly balanced between basal and bolus, around 40–60% each, but the split changes with diet, activity, and goals. When you enter a TDD in the calculator, it shows a basal reference (40–50% of TDD) as context—not as a prescription. Some days you will see meal boluses dwarf basal; other times—during illness or when eating low‑carb—basal dominates. What matters is whether your patterns are comfortable and safe.

If you are exploring nutrition patterns, calculators like our carb calculator and calorie calculator can help estimate intake. Use them as context when reviewing your logs—never as dosing rules by themselves.

Special cases and limitations

Certain situations shift insulin needs: exercise (both during and many hours after), fever or infections, steroid medications, menstrual cycles, stress, sleep debt, and travel across time zones. Digestion issues like gastroparesis can delay glucose rises; high‑fat meals can cause prolonged elevations. Pumps with automated algorithms may recommend doses that do not match a simple ICR/ISF calculation because they account for predicted trends and prior micro‑boluses. Use this calculator as a clear snapshot, then layer device guidance and clinical advice on top.

Children, pregnancy, and older adults require tailored targets and safety margins. If you fall into these groups—or if you are newly diagnosed—work closely with your care team. This site provides tools and education; it does not manage emergencies or change prescriptions. For severe highs or lows, follow your clinician’s plan or local emergency instructions.

Examples and walkthroughs

Suppose you plan a meal with 60 g of carbohydrate. Your ICR is 12 g/u, your current is 180 mg/dL, your target is 110 mg/dL, and your ISF is 50 mg/dL per unit. Meal bolus = 60 ÷ 12 = 5. Correction = (180 − 110) ÷ 50 = 1.4. If you have 0.5 units of IOB, the correction after IOB is 0.9. Total = 5 + 0.9 = 5.9, which rounds to 6.0 if you select 0.5‑unit rounding. If your meter reads in mmol/L, the calculator transparently converts those numbers to mg/dL behind the scenes so the same logic applies.

If you do not know your ICR/ISF but your TDD averages 40 units, the tool estimates ICR ≈ 12.5 g/u and ISF ≈ 45 mg/dL per unit. With the same 60 g meal, the meal bolus is ≈ 4.8 units, and a 70 mg/dL gap would suggest ≈ 1.6 units of correction. After 0.5 units IOB, that leaves ≈ 1.1 units of correction, for a total ≈ 5.9 units—very similar to the previous example. Consistency across methods gives you confidence that the math lines up with your experience.

To see how these numbers relate to long‑term averages, compare occasional checks and CGM trends with your A1C result. And if you need quick conversions between mg/dL and mmol/L outside of dosing, try our glucose converter.

Frequently Asked Questions