Calculate the insulin-to-C-peptide molar ratio from paired results in µIU/mL, ng/mL or pmol/L, solve either value back, or convert C-peptide to pmol/L.
Disclaimer: For educational purposes only; not medical advice and not for diagnosis or medication changes. Unit conversions used here are approximate and can vary by laboratory/assay—use your lab’s reported units and reference ranges, and consult a qualified clinician for interpretation (especially with symptoms of hypoglycemia or concerns about diabetes/endocrine disorders).
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Insulin C-Peptide Ratio Formula
The insulin C-peptide ratio compares the insulin concentration in a blood sample with the C-peptide concentration from the same sample. The calculator solves the molar ratio or either concentration from a supplied molar ratio. The conventional mixed-unit quotient for the same pair is shown in Calculation details.
Conventional ratio
- R = insulin to C-peptide ratio in µIU/mL per ng/mL
- I = insulin converted to µIU/mL
- C = C-peptide converted to ng/mL
Molar ratio
- Rm = molar insulin to C-peptide ratio in pmol/pmol
- Ipmol = insulin converted to pmol/L
- Cpmol = C-peptide converted to pmol/L
Unit conversions used
The conventional mixed-unit quotient is insulin in µIU/mL divided by C-peptide in ng/mL and retains those units. The primary molar ratio divides pmol/L by pmol/L. This calculator assumes an insulin conversion factor of 6.00; if your laboratory uses another factor, enter its molar result directly. C-peptide331.1 is an approximate conversion. Neither quotient provides a diagnosis or universal normal range.
Ratio Definitions and Unit Conversions
| Quantity | Arithmetic definition | Scope |
|---|---|---|
| Conventional ratio | Insulin in µIU/mL ÷ C-peptide in ng/mL | Mixed-unit quotient; not a molecule-to-molecule ratio. |
| Conventional ratio | Not directly comparable with a molar ratio | No universal reference interval is provided here. |
| Conventional ratio | Depends on concentration units | Do not infer insulin source or clearance from a generic cutoff. |
| Molar ratio | Insulin in pmol/L ÷ C-peptide in pmol/L | Dimensionless arithmetic; interpretation requires clinical context. |
| Molar ratio | Uses paired concentrations | Sample timing, assay method, medicines and renal function matter. |
| Molar ratio | No diagnostic classification | A clinician must interpret paired glucose and other findings. |
| Measurement | Conversion |
|---|---|
| Insulin | 1 µIU/mL = 1 mU/L |
| Insulin | 1 µIU/mL ≈ 6.00 pmol/L |
| C-peptide | 1 ng/mL ≈ 331.1 pmol/L |
| C-peptide | 1 nmol/L = 1000 pmol/L |
| C-peptide | 1 nmol/L ≈ 3.0203 ng/mL |
Example Calculations
Example 1: Conventional ratio
You have an insulin result of 10 µIU/mL and a C-peptide result of 2.0 ng/mL.
The conventional insulin to C-peptide ratio is 5.0 µIU/mL per ng/mL.
Example 2: Molar ratio
You have an insulin result of 60 pmol/L and a C-peptide result of 600 pmol/L.
The molar insulin to C-peptide ratio is 0.100 pmol/pmol.
FAQ
What is the insulin C-peptide ratio used for?
The ratio helps compare circulating insulin with C-peptide. Since the pancreas releases insulin and C-peptide together, an unusual mismatch can give clues about insulin source, insulin clearance, or timing of the blood draw. It is not a diagnosis by itself.
How do the conventional quotient and molar ratio differ?
Conventional units use common lab reporting units, such as µIU/mL for insulin and ng/mL for C-peptide. The molar ratio converts both values to pmol/L before dividing, which compares the number of insulin molecules with the number of C-peptide molecules more directly.
Should fasting and non-fasting results be interpreted the same way?
No. Food intake can raise insulin and C-peptide and can change the ratio. Fasting samples are usually easier to compare with reference discussions. If your sample was taken after a meal, during illness, or during insulin treatment, interpretation should account for that context.
