Insulin-to-C-Peptide Ratio Calculator
Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM — September 2026
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.
This ratio answers a question that no other blood test answers cleanly: where did this insulin come from?
When the pancreas makes insulin, it makes proinsulin first, then cleaves it into two fragments — insulin and C-peptide — releasing them into the blood in equal molar amounts. Manufactured insulin contains no C-peptide at all. So the relationship between the two in a blood sample carries information that neither figure carries alone.
A person with high insulin and proportionally high C-peptide is producing it themselves, whether from an insulinoma, from insulin resistance, or from a sulfonylurea driving the pancreas. A person with high insulin and low or absent C-peptide is receiving it from outside — which is the finding that distinguishes exogenous insulin administration from an endogenous cause of hypoglycemia.
Because insulin and C-peptide are reported in different units and have very different molecular weights and clearance rates, the comparison has to be made in molar terms rather than by dividing the raw lab values.
The calculator below performs that conversion. Below it you will find what the ratio means in practice, how it is used in hypoglycemia workup and diabetes classification, and the significant limitations that mean it is never interpreted in isolation.
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.
Written by Ryan Mitchell · Last updated: September 2026