Correction Factor Calculator
The formula
What the sensitivity factor tells you
The 1800 rule translated to correction factor: how much one unit of insulin drops glucose — the repair term of every bolus.
Calculation: ISF = rule ÷ TDD (mg/dL per unit); ÷18.018 for mmol/L. Enter your inputs and every substitution is shown below the result.
Reading the result
A big CF (250+) means you need whole vials of courage to fix highs; a small CF means half units rule.
Rules of thumb: T1D adult CF 40–60 mg/dL/U on full replacement doses
Limitations and best practice
Exercise, illness and steroid pulses move the CF hour to hour; correct only with a fresh finger or CGM reading.
This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Frequently asked questions
CF vs ISF — same thing?
Why 1800 and not measured?
Source & methodology
Hirsch ISF 1800 rule.
Correction Factor explained: formula, worked example and honest limits
This page was written for one job: turning your own inputs into a defensible figure, with the working shown. Searchers land on a Correction Factor page for three reasons — to plan, to compare options, and to check that somebody else’s arithmetic is honest. This tool serves all three, and everything after the calculator explains the model instead of hiding it.
It helps to separate the mechanics from the interpretation. The mechanics are fixed: ISF = rule ÷ TDD (mg/dL per unit); ÷18.018 for mmol/L. Anyone, anywhere, entering the same inputs will get the same 14.40 mg/dL per U. The interpretation is where experience counts: whether 14.40 mg/dL per U is good, normal or alarming depends on comparisons covered below, and that is the part most articles about Correction Factor never explain.
A calculation like this one is worth roughly as much as a careful sentence: it forces several vague considerations into one comparable quantity. Once Correction Factor is on the table, the argument can stop being about impressions.
What makes a calculation page trustworthy is that its copy is derived from its own arithmetic: here correction factor = 14.40 mg/dL per U, recomputed every time a field changes, and the sensitivity section ranks the inputs that move the result — Total daily insulin first, at ≈11.1 % of the headline for a ten-percent error either way in that input — which is the thread the rest of this guide pulls on.
How Correction Factor is computed, line by line
Here is the complete model this tool runs — ISF = rule ÷ TDD (mg/dL per unit); ÷18.018 for mmol/L — with each input explained below. If a competitor’s page gives you a different answer from the same inputs, this is the text that settles which of you is right.
Total daily insulin (units) (example value 125) is a numeric input whose unit is fixed by the label; a wrong unit is a wrong answer wearing a plausible disguise. Count from the record, not the estimate; whole-number inputs are cheap to verify and expensive to get wrong.
Displayed in (example value mg/dL) is a choice rather than a quantity — pick the option that matches your situation, because the calculation path branches here. The example value is there to demonstrate the mechanics, not to describe your situation — overwrite it.
Constant (example value 1800 (rapid)) is a branching choice rather than a figure you measure; the per-option effects are computed below. If you are unsure what belongs in this field, look at the printed steps: the substitution shows exactly how the engine uses it.
What each “Displayed in” choice actually does: mg/dL → 14.40 mg/dL per U · mmol/L → 0.80 mg/dL per U That table is the honest answer to “does this option matter”, re-derived from the formula rather than hard-coded into page copy.
What each “Constant” choice actually does: 1800 (rapid) → 14.40 mg/dL per U · 1500 (regular) → 12.00 mg/dL per U · 2000 (some experts) → 16.00 mg/dL per U — each value above is the model’s own output for that option, so the choice can be made on evidence rather than on what a category feels like it means.
Correction Factor with real numbers, step by step
Abstract formulas are hard to trust, so here is the complete substitution using the default inputs, written the way an examiner would want it. Starting figures: Total daily insulin (units), Displayed in, Constant. The endpoint of that chain is 14.40 mg/dL per U of correction factor, sitting in the “Each unit should move glucose 14 mg/dL (1,800 ÷ 125 U) — from 200 mg/dL to 110 needs 6.3 U; the number is a starting guess, the 3-hour blank log is the proof” band.
- 1,800 ÷ 125 = 14.4 mg/dL per unit
- ÷ 18.018 = 0.80 mmol/L per unit
- Correction for 200→110 = 90 ÷ 14.4 = 6.3 U
In mg/dL — 14.4 mg/dL/U. This is the number experienced users quote when the headline alone would mislead.
In mmol/L — 0.80 mmol/L/U. Read next to the headline, this is the figure that shows the shape of the trade-off.
High-sensitivity check — CF <25 mg/dL/U = very insulin-sensitive; halve corrections. This is the number experienced users quote when the headline alone would mislead.
Recompute rule — after any >20 % TDD change. Compare it against the main result of 14.40 mg/dL per U before drawing conclusions.
Keep this worked example open while you enter your own figures: matching the shape of the substitution is the fastest way to spot a unit or convention mistake.
The Correction Factor corridor: your result across a realistic band
Read this table as the practical error bar on the headline figure. It answers the question people actually have after seeing a single number: how wrong could this be if the main input is off by ten or twenty percent?
| Total daily insulin setting | Correction factor |
|---|---|
| 80 % of the example value | 18.00 mg/dL per U |
| 90 % of the example value | 16.00 mg/dL per U |
| example inputs | 14.40 mg/dL per U |
| 110 % of the example value | 13.09 mg/dL per U |
| 120 % of the example value | 12.00 mg/dL per U |
That spread — 12.00 mg/dL per U at the low end, 18.00 mg/dL per U at the high — is what a careful read of Correction Factor looks like: a band you can state out loud, with the reason for each edge. Reproduce it in the form above by editing Total daily insulin alone; the substitution steps will follow your figures exactly.
Sensitivity: which inputs change Correction Factor the most
Nobody should have to guess which input matters. The engine was re-run once per field below — ten percent up, ten percent down, everything else frozen — and the results ranked by how far they moved the correction factor. For Correction Factor the ranking is measured rather than guessed, and it belongs to these defaults: shift the starting point and the order can shift with it.
- Nudge Total daily insulin by ±10 % and Correction Factor lands between 13.09 mg/dL per U and 16.00 mg/dL per U (sensitivity ≈11.1 %)
Total daily insulin is the dominant lever here: ten percent either way moves the result by about 11.1 %, from 13.09 mg/dL per U to 16.00 mg/dL per U. That is where a careless figure does the most damage, so check it against a source. The remaining fields can sit at safe defaults without materially changing the answer — knowing which is which is what separates a careful calculation from an anxious one.
Reading the result: bands, zones and thresholds
The value above only becomes useful the moment you know what it is being measured against. The default case lands in the “Each unit should move glucose 14 mg/dL (1,800 ÷ 125 U) — from 200 mg/dL to 110 needs 6.3 U; the number is a starting guess, the 3-hour blank log is the proof” band, and bands are deliberately wide because real situations overlap. The sensitivity list shows how far a modest input error moves you along that ladder: for Total daily insulin, ±10 % covers 13.09 mg/dL per U to 16.00 mg/dL per U. The practical reading rules: read the band as a starting hypothesis, then test it: change the two dominant inputs by ten percent and see whether the verdict survives. Verdicts that survive stress are the ones worth acting on.
Related measures answer related questions, and the distinctions matter: Correction Factor is the quantity, Correction factor is this tool’s reading of it, and the decision that follows belongs to neither of them separately.
Why Correction Factor results go wrong — and how to prevent it
Exercise, illness and steroid pulses move the CF hour to hour; correct only with a fresh finger or CGM reading. This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Reporting the headline and quietly discarding In mg/dL. The companions are where the shape of the trade-off survives the rounding.
This model has its own particular trap: Total daily insulin carries ≈11.1 % of the response per ten percent of error, so effort spent elsewhere is largely decorative.
The subtlest error is answering the wrong question precisely. Confirm that correction factor is actually the quantity your decision needs — the arithmetic is flawless either way, which is what makes the mistake invisible.
Before you trust your Correction Factor: a short checklist
- Compare the output against one published benchmark or a colleague’s independent estimate.
- Re-run the calculation with a ±10 % stress on the two most sensitive inputs.
- Put Total daily insulin under a magnifier; the sensitivity table above is the reason why.
- Check that every field the formula needs actually arrived from a document, not from recollection.
- Recompute once with the dominant input deliberately set low, and see whether the decision changes.
- Run the ±10 % stress on the top two levers and check that your conclusion survives both.
Five steps from raw inputs to a figure you can defend
The tool does the arithmetic; the routine below is what makes the arithmetic trustworthy. Five steps, a minute total — and afterwards you will know not just what the number is, but which assumption produced it.
Step 1 — one document, one pass. Open the statement, label or record that holds total daily insulin (units), displayed in, constant and transcribe straight into the form. Transcription from a single source removes the mismatch errors that appear when figures are assembled from three different places. A result worth acting on survives having its two shakiest inputs halved.
Step 2 — compute the version you would not be embarrassed by. If a stranger audited your inputs, which set would you hand over? Enter that set, read the result, and treat everything else as upside. When in doubt, prefer the boring input: the interesting one is usually the one you misread.
Step 3 — play with the inputs on purpose. Ten percent up, ten percent down, one field at a time. Within a few minutes you will know this calculation better than most people who quote its results. A figure with a date beside it becomes evidence; without one it stays an anecdote.
Step 4 — run the scenario trio. Expected, pessimistic, optimistic. The gap between pessimistic and expected is the number to plan against; the optimistic result is useful only as a ceiling. The input you reach for first is usually the one you measured longest ago.
Step 5 — save the story with the figure. A number without its inputs and its date is unusable within a quarter. Record 14.40 mg/dL per U, the assumptions behind it, and today’s date; your future self will be auditing a stranger’s arithmetic. The test of a good estimate is not that it feels right but that it changes what you do.
Who actually uses Correction Factor — and when
Health & Medical arguments end at arithmetic or they do not end. Correction Factor is the arithmetic: 14.40 mg/dL per U on the example inputs, reproducible by anyone who types the same numbers. Most surprises in numbers are surprises about inputs, not about formulas.
The audience for this page is people who need one trustworthy number before their next decision. In professional Medications & Dosing practice the pattern is always the same — screen with Correction Factor, then invest slow analysis only in whatever survives the screen. Suspiciously tidy answers deserve the same scrutiny as untidy ones.
Nothing here is stored or transmitted, so the habit of checking is free: open the page, re-enter current figures, compare with the number you saved last time. Re-running a number costs seconds; retracting a decision costs considerably more.
People also ask about Correction Factor
What affects Correction Factor the most?
The short answer: Total daily insulin. The longer answer is the sensitivity table on this page, where every numeric input was probed by ten percent and the correction factor recalculated — Total daily insulin moved it furthest, by roughly 11.1 % (13.09 mg/dL per U to 16.00 mg/dL per U).
How much does the “Displayed in” choice change the answer?
A great deal, on these inputs: Correction Factor comes out at 0.80 mg/dL per U on the lowest option (“mmol/L”) and 14.40 mg/dL per U on the highest (“mg/dL”). Each option is listed with its computed value so you can see what the choice is worth before you make it.
What is a typical Correction Factor?
On the worked example above Correction Factor comes out at 14.40 mg/dL per U. Treat that as one point on a distribution rather than a norm: the bands, the companion outputs and the sensitivity ranking tell you how wide the plausible range really is.
How do I calculate Correction Factor by hand?
Three steps: write the formula (ISF = rule ÷ TDD (mg/dL per unit); ÷18.018 for mmol/L), insert your values, and simplify. The worked example section shows exactly that chain for the default inputs; replicating it once on paper teaches you more about Correction Factor than any number of calculator runs.
What happens to Correction Factor if total daily insulin changes by 10 %?
The engine was re-run with that field moved by ten percent while everything else stayed fixed: the correction factor goes across 13.09 mg/dL per U to 16.00 mg/dL per U, a swing of roughly 11.1 % around the 14.40 mg/dL per U baseline. That ratio is the practical definition of how much this input matters.
Is the Correction Factor result of 14.40 mg/dL per U good or bad?
Neither, on its own — the label “Each unit should move glucose 14 mg/dL (1,800 ÷ 125 U) — from 200 mg/dL to 110 needs 6.3 U; the number is a starting guess, the 3-hour blank log is the proof” is the band this figure falls in for the example inputs, and your own inputs will place you elsewhere on the same ladder. Judge the number against your history and your alternatives, and remember that a ten-percent error in Total daily insulin alone moves it by ≈11.1 %.
How do I calculate Correction Factor manually?
Write down the inputs, then follow the printed steps in order — ISF = rule ÷ TDD (mg/dL per unit); ÷18.018 for mmol/L. A spreadsheet replicates it with two or three cells. The manual route is worth doing once to build intuition; after that the calculator saves time and eliminates transcription slips.
Can Correction Factor be negative or empty?
Yes, and that is informative rather than broken: a negative Correction Factor usually means the inputs encode an outflow or a shortfall, and the verdict band above labels exactly that situation instead of hiding it.
CF vs ISF — same thing?
Yes; insulin sensitivity factor and correction factor are the same number under two names.
Why 1800 and not measured?
It is an estimate; the measured version is a dose-only 3-hour glucose drop test, which the ISF engine computes.
Do I need an account to use this Correction Factor calculator?
No, and no data leaves the tab. The engine runs in the page itself, which is why the worked example, the bands and the sensitivity ranking all respond instantly to your keystrokes.
The vocabulary of Correction Factor
- In mg/dL
- 14.4 mg/dL/U for the Correction Factor worked case; it sits next to the main result because it carries context the headline leaves out.
- In mmol/L
- 0.80 mmol/L/U in the default Correction Factor calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
- High-sensitivity check
- CF <25 mg/dL/U = very insulin-sensitive; halve corrections in the default Correction Factor calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
- Correction factor
- The primary output of this tool: 14.40 mg/dL per U for the default inputs, computed exactly from ISF = rule ÷ TDD (mg/dL per unit); ÷18.018 for mmol/L.
- Worked example
- The calculation written out in full so that nothing on this page has to be taken on trust.
- Verdict band
- The plain-language label — here “Each unit should move glucose 14 mg/dL (1,800 ÷ 125 U) — from 200 mg/dL to 110 needs 6.3 U; the number is a starting guess, the 3-hour blank log is the proof” — mapping the computed figure onto experience-based ranges for Health & Medical.
- Sensitivity (±10 %)
- How far the Correction Factor moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.
Methodology, sources and limits
There is no black box on this page — the engine evaluates ISF = rule ÷ TDD (mg/dL per unit); ÷18.018 for mmol/L. Source of record: Hirsch ISF 1800 rule. Where a professional version of this calculation differs, it differs in inputs and jurisdiction-specific coefficients, not in the algebra — which is why re-checking your inputs beats hunting for a second calculator.
This page is an estimation aid, not professional advice. The design rule is simple: two people entering identical figures must get identical answers. The sensitivity table above is part of that promise rather than a bonus: it lets you see how much of the result depends on how carefully you measured.
What to do with your Correction Factor next
Re-run the calculation with your own figures, stress the two most sensitive inputs by ±10 %, and only then act on the result.
This page was written for people who need one trustworthy number before their next decision, and the promise is that nothing above required trust. Bookmark the tool, re-run it whenever an input moves by more than a few percent, and keep the date with the number.