How To Calculate Correction Factor For Insulin | Safer Bolus

If you’re trying to learn how to calculate correction factor for insulin, you’re after predictable corrections when your glucose runs high.

A correction factor (also called an insulin sensitivity factor) estimates how much 1 unit of rapid- or short-acting insulin will drop blood glucose. It’s the number behind the “correction” line in many bolus calculators.

This is general education, not medical advice. Dose settings should be set with your prescriber, since hypoglycemia can be dangerous. Small changes beat guesses.

What You Need Before You Start

Grab these pieces first. When one of them is off, the math can look right while the dose feels wrong.

Piece	What It Means	Where You Get It
Total daily dose (TDD)	Insulin used in 24 hours: basal + bolus + corrections	Pump total, pen app, or a 3–7 day log
Type of bolus insulin	Rapid-acting analog vs regular insulin changes the rule number	Prescription label or pump settings
Glucose unit	mg/dL and mmol/L use different constants	Meter or CGM settings
Target glucose	The value you’re correcting toward	Your dosing plan from your clinician
Current glucose	The value you’re correcting from	Fingerstick or CGM (check trend arrows too)
Insulin action time	How long a bolus keeps working	Pump duration setting or your clinician
Insulin on board (IOB)	Active insulin still lowering glucose from recent doses	Pump IOB, bolus calculator, or manual timing
Low-glucose plan	What you do if a correction drops you too far	Your treatment plan and fast carbs you keep nearby

How To Calculate Correction Factor For Insulin

The clean way to get a starting correction factor is to use your TDD and a rule number. This gives you “mg/dL per unit” (or “mmol/L per unit”), which estimates how far 1 unit should move you.

Step 1: Find a steady total daily dose

Use days that look normal. Skip sick days, steroid bursts, or stretches with repeated lows. If your doses swing a lot day to day, take an average from several days.

On injections, add basal insulin plus meal boluses plus corrections. On a pump, use the daily total from the report.

Step 2: Pick a rule number that matches your bolus insulin

Two common starting points:

• 1800 rule (mg/dL): used often with rapid-acting analogs.
• 1500 rule (mg/dL): used often with regular insulin.

Think of this as a first draft. Your sensitivity can shift with time of day, activity, hormones, and illness.

Step 3: Divide to get your correction factor

• mg/dL: correction factor = rule number ÷ TDD
• mmol/L: correction factor = (rule number ÷ 18) ÷ TDD

A handy shortcut: if you’re using the 1800 rule, the mmol/L constant is 100, since 1800 ÷ 18 = 100.

Step 4: Turn the factor into a correction dose

Use this structure:

(current glucose − target glucose) ÷ correction factor = correction dose

If your device subtracts insulin on board, it may suggest a smaller correction. If you do the math by hand, avoid stacking doses close together.

Step 5: Round to what you can dose

Pens and syringes may limit you to whole units or half units. Pumps can dose in smaller steps. When you’re unsure, round down and recheck later instead of chasing the number.

Calculating A Correction Factor For Insulin With The 1800 Rule

The 1800 rule is a common starting point for rapid-acting insulin. The UCSF Diabetes Teaching Center correction factor page uses the same “1800 ÷ total daily dose” setup.

Say your TDD is 36 units. 1800 ÷ 36 = 50, so 1 unit is estimated to drop glucose by 50 mg/dL.

To correct, subtract your target from your current reading, then divide by 50. Round to what your pen or pump can deliver, then watch insulin on board and trend arrows.

Units And Targets That Keep The Math Honest

Unit mix-ups cause bad math. A “50” correction factor means one thing in mg/dL and a different thing in mmol/L. Check the unit on your meter or CGM before you plug in numbers.

mg/dL vs mmol/L

Since 18 mg/dL equals 1 mmol/L, the classic rule numbers have metric twins. Many dosing references list sets like 1500/1800/2000 (mg/dL) and 83/100/110 (mmol/L). A peer-reviewed overview of these correction factor constants is Guidelines for insulin dosing in continuous subcutaneous insulin infusion.

Target glucose is a preset, not a guess

Your target comes from your treatment plan. A target that is set too low can push you into extra corrections and extra lows. If you’re not sure what target you should use, ask your prescriber before changing anything.

Safety Checks Before You Act On A Number

The correction factor is math. The dose is still biology. Run a quick check before you correct, especially if your device doesn’t track active insulin.

Check insulin on board and timing

• If you corrected within the last few hours, some insulin is still active, so another correction can stack.
• If you’re on a pump, the bolus calculator usually accounts for IOB.

Check the trend, not only the number

CGM values can lag behind blood glucose during fast rises or drops. If your arrow is slanting down, a full correction based on the raw number may be too much. When in doubt, confirm with a fingerstick.

Watch for cases where a “normal” correction may not work

• Site trouble: A correction won’t land if insulin isn’t getting in.
• Illness: It can raise insulin needs for a stretch.
• Alcohol: It can raise low-glucose risk hours later.
• Exercise: It can make you more sensitive during the activity and after.

How To Test And Tweak Your Correction Factor

A rule-number estimate is step one. The real test is what your glucose does after a correction when food and activity are steady.

Pick a clean test window

Choose a time when you haven’t eaten for several hours, you don’t plan to work out soon, and you’re not treating a low. Corrections test cleanest when the only moving part is insulin.

Watch the full action time

Rapid-acting insulin keeps working for hours. Don’t judge the factor after 60 minutes. Check where you land near the end of your insulin action time, since that’s when most of the dose has done its work.

Use patterns and small changes

One day can be odd. Three clean corrections that all miss in the same direction tell you more. If you keep landing low after a correction, your factor is too aggressive. If you keep landing high, your factor is too mild.

When you adjust, move in small steps and hold it long enough to learn what it does. If you change your correction factor and your basal dose in the same week, it gets hard to tell which change did what.

Worked Correction Math Patterns

This table shows common correction calculations. The numbers are samples so you can see the structure. Your own target and correction factor come from your plan.

Situation	Math	Estimated Dose
mg/dL, moderate high	(210 − 120) ÷ 50	1.8 units
mg/dL, small high	(165 − 120) ÷ 45	1.0 unit
mg/dL, higher reading	(280 − 110) ÷ 60	2.8 units
mmol/L, moderate high	(12.0 − 7.0) ÷ 2.0	2.5 units
mmol/L, small high	(9.8 − 7.0) ÷ 1.8	1.6 units
mmol/L, higher reading	(15.5 − 7.0) ÷ 2.5	3.4 units

Common Reasons A Correction Misses

When a correction “does nothing,” it’s easy to blame the correction factor. Sometimes it is the factor. Other times, the math is fine and something else is in the way.

Food still digesting

If you correct soon after a meal, carbs may still be hitting. It can look like the correction failed, then the insulin catches up later and you drop fast. This is where tracking insulin on board helps.

Injection or infusion issues

• Insulin can leak if a pen needle is pulled out too soon.
• Scar tissue can slow absorption at the same spots.
• On a pump, a bent cannula or blocked tubing can stop delivery.

Basal dose mismatch

If basal insulin is set too low, you may run high in the background all day. You can end up correcting again and again. In that case, the correction factor is trying to patch a basal problem.

Hidden drivers of resistance

Stress, poor sleep, infection, and some medications can raise insulin needs. If a new pattern starts and sticks, write down what else changed that week and bring it to your clinician.

A Quick Worksheet You Can Reuse

If you still feel stuck on how to calculate correction factor for insulin, copy this worksheet and bring it to your next visit. It turns a fuzzy dosing chat into crisp numbers.

Your starting correction factor

• TDD (units per day): ________
• Rule number you’re using (1500 or 1800): ________
• Correction factor (mg/dL per unit): ________
• Or correction factor (mmol/L per unit): ________
• Target glucose: ________

Your correction dose math

• Current glucose: ________
• Target glucose: ________
• Difference (current − target): ________
• Dose before IOB and rounding: ________
• Dose you took: ________
• Glucose near end of action time: ________

After a few clean entries, you’ll have real evidence of how your factor behaves in your body. That’s the piece your prescriber can use to fine-tune settings with you.

Try to change one setting per stretch unless your clinician tells you to do more. A correction factor is only one part of dosing, and it works best when basal doses, meal ratios, and timing line up.