Calculating an insulin dose based on your blood sugar uses a simple formula: subtract your target blood sugar from your current reading, then divide by your correction factor. Your correction factor tells you how many mg/dL one unit of insulin will lower your blood sugar, and it’s personalized to you based on how much insulin you use in a day. The math is straightforward once you know the pieces involved.
The Correction Dose Formula
The core calculation looks like this:
Correction dose = (Current blood sugar − Target blood sugar) ÷ Correction factor
Your target blood sugar is the number you and your care team have agreed on, often somewhere around 100 to 120 mg/dL. Your correction factor (also called an insulin sensitivity factor, or ISF) tells you how much one unit of rapid-acting insulin will drop your blood sugar. So if your current reading is 250 mg/dL, your target is 120 mg/dL, and your correction factor is 50, the math works out to: (250 − 120) ÷ 50 = 2.6 units. You’d round that to the nearest half or whole unit depending on your pen or syringe.
How to Find Your Correction Factor
If you don’t already have a correction factor from your care team, the standard starting estimate uses what’s called the 1800 Rule:
Correction factor = 1800 ÷ Total Daily Dose (TDD)
Your TDD is the total number of insulin units you take in a typical day, including both long-acting (basal) and rapid-acting (mealtime) insulin. If you take 36 units total per day, your correction factor would be 1800 ÷ 36 = 50. That means one unit of rapid-acting insulin should lower your blood sugar by roughly 50 mg/dL.
The 1800 Rule applies to rapid-acting insulin, which is what most people use for corrections today. An older version called the 1500 Rule was designed for regular insulin, which acts more slowly. If you use regular insulin, you’d divide 1500 by your TDD instead. Most modern insulin regimens use rapid-acting analogs, so the 1800 Rule is the one you’ll typically need.
A Step-by-Step Example
Say your total daily insulin dose is 45 units across all injections. Your target blood sugar is 110 mg/dL. You check your blood sugar before dinner and it reads 240 mg/dL.
First, find your correction factor: 1800 ÷ 45 = 40. One unit will drop you about 40 mg/dL. Next, find the gap: 240 − 110 = 130 mg/dL above target. Finally, divide: 130 ÷ 40 = 3.25 units. You’d give about 3 units as a correction dose.
If you’re also eating a meal, this correction dose gets added on top of your mealtime dose (covered below) for one combined injection.
Adding Carb Coverage at Meals
At mealtimes, your total bolus is usually two parts combined: a correction dose for high blood sugar plus a carb dose to cover what you’re about to eat. The carb dose uses a separate ratio called the insulin-to-carb ratio (ICR), which tells you how many grams of carbohydrate one unit of insulin covers.
The standard starting estimate for ICR uses the 500 Rule:
Insulin-to-carb ratio = 500 ÷ Total Daily Dose
With a TDD of 45, that’s 500 ÷ 45 = roughly 11. So one unit of insulin covers about 11 grams of carbohydrate. If your meal contains 55 grams of carbs, you’d need 55 ÷ 11 = 5 units for the food.
Now combine that with the correction from the earlier example: 5 units for carbs + 3 units for the high reading = 8 units total before dinner.
Why These Numbers Are Starting Points
The 1800 and 500 Rules give you a reasonable estimate, but your actual sensitivity to insulin is unique. Some people need more insulin per unit of carbohydrate; others need less. Your care team will adjust these ratios over time based on how your blood sugar actually responds. Factors like physical activity, stress, illness, and hormonal changes can shift your sensitivity day to day. A correction factor of 50 might work perfectly on a normal Tuesday but overcorrect on a day you went for a long run.
These formulas also assume a relatively stable routine. If your total daily dose changes significantly, both your correction factor and carb ratio should be recalculated.
Avoiding Insulin Stacking
One of the most common mistakes is giving a second correction dose too soon after the first. Rapid-acting insulin starts working within 10 to 30 minutes, peaks around one to two hours, and stays active in your body for about four hours total. If you check your blood sugar an hour after a correction and it’s still high, injecting more insulin means there’s now overlap between the two doses. This is called insulin stacking, and it’s a leading cause of unexpected low blood sugar.
The general guideline is to wait three to four hours before giving another correction. Your blood sugar will very likely still be coming down during that window, even if progress feels slow. Insulin pumps handle this automatically with a feature called “insulin on board” (IOB), which tracks how much active insulin is still working and subtracts it from the next recommended dose. If you’re on injections, you need to track this timing yourself.
Recognizing Low Blood Sugar
Overcorrecting pushes your blood sugar too low, which can be dangerous. Blood sugar below 70 mg/dL is considered low. Below 54 mg/dL is classified as severely low and needs immediate treatment. Symptoms include shakiness, sweating, confusion, fast heartbeat, and irritability. If you’re new to calculating correction doses, erring slightly conservative is safer than aggressive. A blood sugar of 160 mg/dL after a correction isn’t perfect, but it’s far less risky than dropping to 55.
Keeping fast-acting glucose (tablets, juice, or regular soda) nearby whenever you take a correction dose is a practical safety net, especially as you’re learning how your body responds to your calculated doses.
How Insulin Pumps Simplify the Math
If you use an insulin pump, the built-in bolus calculator does all of this automatically. You enter your current blood sugar and the carbs you plan to eat, and the pump applies your pre-programmed correction factor, carb ratio, and target blood sugar to recommend a dose. It also subtracts any insulin still active from a previous bolus, which eliminates the stacking risk. The formulas behind the scenes are identical to what’s described above. The pump just runs them faster and accounts for IOB in a way that’s hard to do reliably with pen and paper.