Insulin Sensitivity Factor Calculator

Introduction to the insulin sensitivity factor

The insulin sensitivity factor (ISF), sometimes called the correction factor, estimates how far one unit of rapid-acting insulin is expected to lower your blood glucose. It is the number that turns a high reading into a specific correction dose instead of a guess: if your ISF is 40 mg/dL, a single unit should pull your glucose down by roughly 40 points, so a reading 80 points above target points toward a 2-unit correction.

Sensitivity varies enormously between people. Children, adults newly diagnosed with type 1 diabetes, and very lean or active people are often highly sensitive and may drop 75 mg/dL or more per unit. People with more insulin resistance, common in type 2 diabetes or during a course of steroids, see a much smaller drop, sometimes under 20 mg/dL per unit. Your ISF is personal, and that is precisely why a fixed correction dose rarely serves everyone.

ISF is one of three settings that drive modern bolus math. The basal rate or long-acting dose holds glucose steady between meals, the carbohydrate-to-insulin ratio covers food, and the ISF closes the remaining gap between where glucose is now and where you want it. Pumps, smart pens, and bolus-calculator apps all store an ISF somewhere in their settings, which is why it is worth understanding the number rather than treating it as a black box.

Where the 1500, 1700, and 1800 rules come from

Before continuous glucose monitors and sophisticated pumps were widely available, clinicians sought simple formulas to approximate ISF. The 1500 rule emerged first for use with regular human insulin. It suggested dividing 1500 by the total daily dose to estimate how many milligrams per deciliter a single unit would lower glucose.

As rapid-acting analog insulins became the standard for bolus dosing, researchers observed they were slightly more potent per unit over the window that matters for a correction. Updated heuristics, the 1700 and later the 1800 rule, adjusted the constant upward to reflect this. Dividing 1800 by the total daily dose typically provides a reasonable starting ISF for modern rapid-acting insulins like lispro, aspart, or glulisine.

These rules are not carved in stone. They provide a baseline that must be tailored to the individual through experience and data. Some endocrinologists prefer the 1700 rule for highly active individuals or for those who find the 1800 rule overestimates sensitivity. The calculator lets you explore all three constants and even input a custom value if your care team recommends a different number.

How to use the ISF calculator on this page

Enter the total insulin you use across a typical day, counting basal plus every mealtime and correction bolus, then choose the rule that matches your insulin type or type your own value under "Custom constant." Pull the total daily dose from a pump download, a connected pen, or a written log rather than from memory, because an undercounted basal dose is the single most common way to end up with a misleading ISF.

The two glucose fields are optional. If you fill in a current reading and a target, the calculator also shows the correction dose that the estimated ISF implies for that gap, which is useful for sanity-checking a factor against real numbers you recognise. Leave them blank if you only want the factor itself. Every result is reported in both mg/dL and mmol/L, since the two units describe the same drop once you divide by 18.

The ISF formula: constant divided by total daily dose

The whole calculation is one division. The chosen constant is divided by the total daily dose, and the answer is the expected glucose drop per unit:

Formula: ISF = C / TDD

where $C$ is the chosen constant (1500, 1700, 1800, or custom) and $\mathrm{TDD}$ represents the total daily dose. Because the constant carries mg/dL units, dividing it by units of insulin leaves mg/dL per unit, which is exactly what a correction factor is.

To convert the answer into mmol/L, divide by 18, the approximate factor between milligrams per decilitre and millimoles per litre for glucose:

Formula: ISF_mmol = ISF_mg / 18

Once you have a factor, the correction dose for a high reading is the distance above target divided by the ISF:

Formula: units = (G_now − G_target) / ISF

And the glucose you would expect once that dose, plus any insulin still active from an earlier bolus, has finished working is:

Formula: G_end = G_now − (dose + IOB) × ISF

Worked example: 50 units a day on the 1800 rule

Say you average 50 units a day and use the 1800 rule. Dividing 1800 by 50 gives an ISF of 36 mg/dL, or 2.0 mmol/L, per unit. With a target of 110 mg/dL and a current reading of 200, you are 90 points high, and 90 divided by 36 works out to about 2.5 units of correction. Switch to the 1500 rule and the ISF becomes 30, turning that same 90-point gap into a 3-unit correction.

That half-unit swing from one constant to the next is exactly why the rule you pick should match your insulin and be checked against real results, dosing to the nearest whole or half unit your pen or pump allows. Now add a complication: suppose 1 unit from lunch is still active. At an ISF of 36 that unit is already worth another 36 mg/dL of drop, so a full 2.5-unit correction on top of it would aim for roughly 110 − 36 = 74 mg/dL instead of the 110 you wanted. Subtracting the active insulin first leaves about 1.5 units, which is the reasoning every insulin-on-board feature automates.

Fine-tuning your factor over time

No single ISF fits every hour of every day. Dawn hormones tend to make mornings less sensitive, while the hours after aerobic exercise can leave you more sensitive and prone to late lows. Illness and infection usually raise insulin resistance, which is why some people keep a lower "sick-day" correction factor. Pumps and smart pens let you store different factors by time block to capture these swings, and they track insulin-on-board so a fresh correction accounts for insulin still working from the last dose.

Treat the calculated number as a hypothesis and let your own data sharpen it. If a 2-unit correction reliably drops you 120 points when you expected 80, your true ISF is higher than the formula suggested, and continuous glucose monitor trends make that pattern easy to spot. Your carbohydrate-to-insulin ratio works alongside ISF for full bolus math, covering the meal with the carb ratio and adding a correction from the ISF, and both settings drift with weight change, new medications, pregnancy, or menopause. Recalculate whenever your average daily dose shifts noticeably or corrections stop landing where you expect them to.

Common pitfalls that distort ISF

A handful of everyday mistakes quietly throw off a correction:

• Stacking corrections: dosing again before the previous bolus has peaked layers insulin on top of insulin and overshoots into a low. Let insulin-on-board tracking or a simple timer guard against it.
• An inaccurate total daily dose: undercounting basal or a few boluses skews the constant division and every ISF it produces. Pull the real number from pump downloads or written logs.
• Miscounted carbohydrates: if a high reading came from underestimated carbs rather than a true excess, correcting on ISF alone can push you too low.
• Site absorption: injecting into scar tissue or a poorly perfused spot slows absorption, making a correction look weaker than your ISF really is.
• Reading a rising or falling arrow as a flat number: a correction sized for a steady 250 mg/dL behaves very differently when glucose is already dropping fast.

Working safely with your care team

This calculator is educational and is not a substitute for your clinician's judgment. Share the number you get with your endocrinologist or diabetes educator, who can confirm the constant suits your insulin and adjust it against your labs, lifestyle, and other conditions; most start people on the 1800 or 1500 rule and refine from a few weeks of glucose data. Whatever ISF you settle on, confirm no active insulin remains before adding a correction, and for severe highs with ketones, vomiting, or laboured breathing, follow your sick-day plan or seek emergency care rather than leaning on a correction dose alone.