Insulin Correction Dose Calculator
Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM — September 2026
Estimates how many units of rapid-acting insulin are needed to bring an elevated blood glucose back to target.
Don't know your ISF? Calculate it here (1800 Rule) →
Active insulin still working from a previous dose. Estimate IOB →
The arithmetic of a correction dose is simple. The mistake that sends people to hospital is not in the arithmetic.
Correction dosing means dividing how far you are above target by how much one unit lowers your glucose. That part is easy. What gets missed is insulin still active from the previous dose — and correcting on top of it is the single most common route to severe hypoglycemia in insulin users.
Rapid-acting insulin works for three to five hours. Two hours after a meal bolus, roughly half of it is still to come. A glucose reading at that point is not a stable number needing correction; it is a moving one already being acted on. Correcting it anyway means giving two doses for one excursion — a pattern known as stacking, and the reason the low so often arrives a few hours later, frequently overnight.
The discipline is to subtract insulin on board before dosing, and to wait a full action cycle before correcting again.
The calculator below applies the correction formula. Beneath it you will find when correcting is appropriate, why IOB must be subtracted, how it works in mmol/L, and the common mistakes worth recognising.
The Correction Dose Formula
Then subtract IOB: Net Correction = Gross Correction − IOB
Example: BG = 230, Target = 100, ISF = 50, IOB = 1.5 units
Gross = (230 − 100) ÷ 50 = 2.6 units · Net = 2.6 − 1.5 = 1.1 units (round to 1.0)
When Should You Take a Correction Dose?
A correction dose — sometimes called a "correction bolus" — is extra rapid-acting insulin taken to bring an above-target blood glucose back down. It is separate from the insulin you take to cover food. Most people correct when their reading is meaningfully above their personal target and little or no insulin from a previous dose is still working.
Timing is everything. Correcting every time you see a high number is the most common path to insulin stacking, where overlapping doses drive blood glucose too low a few hours later. A practical rule many clinicians teach is to wait at least 3–4 hours between corrections unless told otherwise, because rapid-acting insulin keeps working for that long.
Why You Must Subtract Insulin on Board (IOB)
Insulin on board is the part of an earlier dose that is still lowering your blood glucose. This calculator subtracts it because forgetting active insulin is the single most frequent cause of correction-related hypoglycemia.
Worked example: your gross correction comes to 2.5 units, but you bolused for a meal 90 minutes ago and roughly 1.5 units are still active. Your net correction is only 2.5 − 1.5 = 1.0 unit. Taking the full 2.5 units would stack on top of the 1.5 already working — effectively a 1.5-unit overdose. Insulin pumps and many CGM-linked apps track IOB automatically; with pens and syringes you have to account for it yourself, which is exactly what the IOB field above does.
Not sure how much insulin is still active? Estimate it with the Insulin on Board Calculator, then come back and enter it here.
Correcting in mmol/L
Outside the United States, blood glucose and sensitivity factor are usually expressed in mmol/L. Toggle the unit switch above and every field — current glucose, target, and ISF — converts automatically, so you never have to do the ÷18 conversion by hand. The math is identical: (Current BG − Target BG) ÷ ISF, just in mmol/L. For example, BG 12.5, target 6.0, ISF 3.0 mmol/L per unit gives (12.5 − 6.0) ÷ 3.0 ≈ 2.2 units before subtracting IOB.
The 100 Rule: finding ISF in mmol/L
UK, Irish, Australian and most European clinics teach the 100 Rule rather than the 1800 Rule. You divide 100 by your total daily dose, and the answer tells you how many mmol/L one unit of rapid-acting insulin lowers your glucose.
ISF (mmol/L) = 100 ÷ TDD
Someone on 50 units a day gets 100 ÷ 50 = 2.0 mmol/L per unit. The same person calculated in US units gets 1800 ÷ 50 = 36 mg/dL per unit. Those are the same answer: 36 ÷ 18 = 2.0. The 100 Rule and the 1800 Rule are one rule expressed in two units, because 1800 ÷ 18 = 100. Knowing that saves you converting anything — pick the rule that matches your meter and the arithmetic stays simple.
Correction factor chart by total daily dose
Read across from your TDD to see the starting sensitivity factor in either unit. These are starting estimates from the rule, and real ISF is confirmed by testing corrections against actual readings.
| Total daily dose | ISF — 1800 Rule (mg/dL per unit) | ISF — 100 Rule (mmol/L per unit) |
|---|---|---|
| 20 units | 90 | 5.0 |
| 30 units | 60 | 3.3 |
| 40 units | 45 | 2.5 |
| 50 units | 36 | 2.0 |
| 60 units | 30 | 1.7 |
| 80 units | 22.5 | 1.25 |
| 100 units | 18 | 1.0 |
The pattern matters more than any single row: a higher total daily dose produces a smaller sensitivity factor. Someone on 100 units a day drops roughly 1 mmol/L per unit, so the same 4 mmol/L excursion needs about four units. Someone on 20 units a day drops 5 mmol/L per unit and needs less than one. Copying a correction dose from another person is unsafe for exactly this reason.
Regular (human) insulin uses 1500 instead of 1800, and about 83 instead of 100, because it lowers glucose less per unit than a rapid-acting analog. Work out either version with the insulin sensitivity factor calculator.
Common Correction-Dose Mistakes
- Ignoring IOB — the biggest cause of post-correction lows. Always subtract active insulin first.
- Correcting a post-meal spike too early — a high reading an hour after eating is often just food peaking, not a true correction target.
- Using one ISF all day — many people are more insulin-resistant in the morning, so a single sensitivity factor can over- or under-correct at different times.
- Correcting right before exercise — activity lowers glucose on its own and can turn a normal correction into hypoglycemia.
Want the deeper walkthrough of sensitivity factor and corrections? Read Insulin Sensitivity Factor Explained. Not sure what your ISF is? Work it out first with the Insulin Sensitivity Factor Calculator, then come back and enter it above.
Frequently Asked Questions
How do I calculate an insulin correction dose?
Subtract your target blood glucose from your current reading, divide by your insulin sensitivity factor (ISF), then subtract any insulin on board. The formula is (Current BG − Target) ÷ ISF − IOB. Example: (230 − 100) ÷ 50 = 2.6 units gross; minus 1.5 units of IOB leaves about 1.0 unit to take. Always confirm your ISF and target with your diabetes care team.
How do you calculate insulin sensitivity factor in mmol/L?
Divide 100 by your total daily dose. This is the 100 Rule, and the result is how many mmol/L one unit of rapid-acting insulin lowers your glucose. A total daily dose of 40 units gives 100 ÷ 40 = 2.5 mmol/L per unit. It produces the same answer as the 1800 Rule used in mg/dL, because 1800 ÷ 18 = 100. Regular human insulin uses 83 instead of 100.
What is insulin on board (IOB) and why subtract it?
IOB is rapid-acting insulin from an earlier dose that is still lowering your glucose. Subtracting it prevents "stacking" — adding a fresh correction on top of insulin that is already working — which is a leading cause of hypoglycemia a few hours later.
How often can I safely take a correction dose?
Most clinicians advise waiting at least 3–4 hours between corrections, because rapid-acting insulin keeps working for that long. Correcting more often risks stacking and lows. Follow the specific timing your care team gives you.
Is a correction dose the same as a meal bolus?
No. A meal bolus covers the carbohydrates you eat (carbs ÷ insulin-to-carb ratio). A correction dose separately lowers an above-target glucose. At mealtimes the two are often added together, but each is calculated independently.
Source
- Walsh J, Roberts R. Pumping Insulin. 5th ed. 2012.
- American Diabetes Association. Standards of Care in Diabetes — 2026. Section 9. Link
Written by Ryan Mitchell · Last updated: September 2026