Insulin-to-C-Peptide Ratio Calculator
Calculate the molar insulin-to-C-peptide ratio — a lab marker used to help tell apart insulin your body made from insulin that was injected.
How to Use This Calculator
- Enter serum insulin in µIU/mL from the same blood sample.
- Enter C-peptide in ng/mL.
- Read the molar ratio and its interpretation — below 1 points to the body's own insulin, above 1 raises suspicion of injected insulin.
This is a diagnostic screening estimate, not a dosing tool. The result is only meaningful with the glucose level at the time of sampling and is interpreted by a clinician.
Why the Ratio Matters
When the pancreas secretes insulin, it releases equal molar amounts of insulin and C-peptide (they are cleaved from the same proinsulin molecule). Injected (exogenous) insulin contains no C-peptide. So in a low-blood-sugar workup, comparing the two helps distinguish causes:
To convert: insulin µIU/mL × 6.0 = pmol/L; C-peptide ng/mL × 331 = pmol/L. A ratio well below 1 points to the body's own insulin; a ratio above 1 raises suspicion of injected insulin (which suppresses C-peptide).
Example: insulin 30 µIU/mL → 180 pmol/L; C-peptide 2.0 ng/mL → 662 pmol/L; ratio = 180 ÷ 662 ≈ 0.27 — consistent with endogenous insulin.
Interpreting the Molar Ratio
| Molar ratio | Suggests |
|---|---|
| < 1.0 | Endogenous insulin (e.g., insulinoma, sulfonylurea) — C-peptide preserved |
| > 1.0 | Possible exogenous (injected) insulin — C-peptide suppressed |
Cut-offs and units vary between laboratories. Some labs report a non-molar insulin (µIU/mL) : C-peptide (ng/mL) ratio with different thresholds. Always interpret using your lab's reference and a clinician's judgement.
What C-Peptide Reveals — Diabetes Type & Hypoglycemia
C-peptide is released one-for-one with the body's own insulin, so it is a direct gauge of how much insulin the pancreas is still making — information a glucose or A1c test cannot give.
Telling diabetes types apart
A low or undetectable C-peptide means little endogenous insulin, typical of type 1 diabetes (and long-standing, insulin-deficient type 2). A normal or high C-peptide means insulin production is preserved, pointing toward type 2 diabetes or insulin resistance. C-peptide also helps flag slower-onset autoimmune diabetes in adults (LADA) and inherited forms (MODY), where it is often partly preserved.
Hypoglycemia and insulinoma workup
In an unexplained low-blood-sugar workup, a high insulin with a high C-peptide suggests the body is overproducing insulin — as in an insulinoma or a sulfonylurea effect — whereas high insulin with a low C-peptide points to injected (exogenous) insulin. This is exactly the distinction the ratio above is built for.
Frequently Asked Questions
What does the insulin-to-C-peptide ratio tell you?
It helps distinguish insulin made by the body from injected insulin. The pancreas releases insulin and C-peptide in equal molar amounts, but injected insulin has no C-peptide. A low molar ratio points to endogenous insulin; a high ratio raises suspicion of exogenous insulin.
How do I convert insulin and C-peptide to the same units?
For a molar ratio, convert both to pmol/L: multiply insulin in µIU/mL by 6.0, and C-peptide in ng/mL by 331. Then divide insulin (pmol/L) by C-peptide (pmol/L).
Does a low C-peptide always mean injected insulin?
Not by itself. Low C-peptide with high insulin suggests exogenous insulin, but the full picture — the glucose at sampling, proinsulin, ketones, and a sulfonylurea screen — is needed. A clinician makes the diagnosis.
Is this a dosing calculator?
No. The insulin-to-C-peptide ratio is a diagnostic lab marker, not a way to estimate an insulin dose. For dosing, see our dose, carb-ratio, and sensitivity-factor calculators.
What is a normal C-peptide level?
A typical fasting C-peptide is roughly 0.5–2.0 ng/mL (about 0.17–0.66 nmol/L), though ranges vary by lab and rise after eating. Values must be read against the glucose at the time — a "normal" C-peptide during hypoglycemia can still be inappropriately high. Use your lab's reference range.
Can C-peptide tell whether I have type 1 or type 2 diabetes?
It helps. A low C-peptide suggests the pancreas makes little insulin (type 1, or long-standing insulin-deficient type 2), while a preserved or high level points to type 2. It is interpreted alongside autoantibodies, age, and clinical picture — and can also flag LADA or MODY — so it supports the diagnosis rather than making it alone.
Sources
- Cryer PE, et al. "Evaluation and management of adult hypoglycemic disorders: Endocrine Society Clinical Practice Guideline." J Clin Endocrinol Metab. 2009.
- Standard unit conversions: insulin 1 µIU/mL ≈ 6.0 pmol/L; C-peptide 1 ng/mL ≈ 331 pmol/L.
Last reviewed: June 2025