💉 Insulin Sensitivity Factor (ISF) Calculator

Enter your Total Daily Dose (TDD) of insulin to estimate an Insulin Sensitivity Factor (ISF, or correction factor) using the "1800 Rule" (rapid-acting insulin) or "1500 Rule" (regular insulin). This is only a starting-point estimate of how much one unit of insulin lowers blood sugar.

⚠️ Medical Disclaimer - Please Read

This tool is for educational and reference purposes only and cannot replace medical diagnosis or treatment decisions. This calculator gives only a rough starting-point estimate - your actual ISF can vary significantly based on individual blood sugar response.

⚠️ Dose Change Warning

Do not adjust your correction insulin dose based on this calculator's result alone. Miscalculating correction insulin can cause serious hypoglycemia. Always consult your doctor or diabetes care team before applying it.

Insulin Sensitivity Factor (ISF)

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Blood glucose drop (mg/dL) per 1 unit of insulin

⚠️ Dose Change Warning

GUIDE

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What Is the Insulin Sensitivity Factor (ISF)?

The ISF, or correction factor, indicates how many mg/dL one unit of insulin lowers blood sugar. It is used to calculate the correction dose needed when blood sugar is above target, independent of meals.

The 1800 Rule and 1500 Rule

1800 Rule: ISF(mg/dL) = 1800 ÷ TDD
A common starting-point formula for patients using rapid-acting insulin or who are insulin-sensitive.

1500 Rule: ISF(mg/dL) = 1500 ÷ TDD
Traditionally used for patients on regular (non-rapid-acting) insulin or with insulin resistance.

Example: TDD of 60 units
1800 Rule: 1800 ÷ 60 = 30 mg/dL per unit

How to Use the ISF

Correction dose = (current blood sugar - target blood sugar) ÷ ISF. For example, with an ISF of 30, a current blood sugar of 250 mg/dL, and a target of 100 mg/dL, the estimated correction dose is about 5 units. This is calculated separately from mealtime insulin.

Choosing Between the 1800 and 1500 Rules

Generally, the 1800 Rule is considered for rapid-acting insulins (e.g., lispro, aspart, glulisine), while the 1500 Rule is considered for regular insulin or insulin-resistant patients (e.g., obesity, type 2 diabetes). Which rule fits you must always be decided together with your medical team.

Signs the ISF May Need Adjustment

Discuss re-adjusting your ISF with your medical team if:
• Blood sugar does not reach target or drops too low after correction doses
• Hypoglycemia occurs repeatedly
• Body weight, activity level, or illness status changes significantly
• You start or stop other medications (e.g., steroids)

Precautions When Using Correction Insulin

Correction insulin can cause hypoglycemia ("insulin stacking") if active insulin still working in the body is not accounted for. Always follow your medical team's guidance before giving an additional correction dose within the duration of action of a previous dose.

Frequently asked questions

Can I give a correction dose directly based on this calculator?

No. This calculator only gives a starting-point estimate based on the 1800/1500 rules. Your actual ISF and correction dose must be determined through your doctor's or diabetes care team's prescription and ongoing blood sugar monitoring.

Should I use the 1800 Rule or the 1500 Rule?

Generally, rapid-acting insulin users consider the 1800 Rule, while regular insulin users or those with insulin resistance consider the 1500 Rule. This is only a starting point - always consult your medical team to decide which rule fits you.

Can I give extra correction doses frequently?

No. Repeatedly giving correction insulin while active insulin remains in your body can cause "insulin stacking" and serious hypoglycemia. Always follow your medical team's guidance on the timing and frequency of correction doses.