Insulin Correction Dose Calculator

The Correction Dose Formula

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.

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.

Frequently Asked Questions