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C-peptide in two units: the short answer
To convert c-peptide from ng/mL to nmol/L, multiply by 0.331. To go back from nmol/L to ng/mL, multiply by 3.021 (or divide by 0.331). For example, 1.5 ng/mL × 0.331 = 0.497 nmol/L. The converter above works in both directions and shows where the result sits against typical reference ranges.
What the test measures
C-peptide is released from the pancreas in equal amounts with insulin, so it shows how much insulin your own body is making – even if you inject insulin. US labs report it in ng/mL; many other labs use nmol/L or pmol/L. With a molar mass of about 3,020 g/mol, 1 ng/mL = 0.331 nmol/L = 331 pmol/L.
Why the units differ
C-peptide helps tell type 1 from type 2 diabetes, check for MODY or LADA, and investigate low blood sugar. Published cut-offs are often in pmol/L or nmol/L, so a US result in ng/mL needs converting before comparison.
Where the factor comes from
The conversion factor comes from the molar mass (≈3,020 g/mol) and the volume units on each side. Laboratories may round the factor slightly differently, which can change the last digit but never the clinical meaning.
How to convert step by step
- Check the unit printed next to the result on your report – ng/mL or nmol/L.
- ng/mL → nmol/L: multiply by 0.331.
- nmol/L → ng/mL: multiply by 3.021.
- Round to the same number of decimals your lab uses.
Worked example in reverse: 0.265 nmol/L × 3.021 = 0.80 ng/mL.
C-peptide conversion chart
| ng/mL | nmol/L |
|---|---|
| 0.2 | 0.066 |
| 0.5 | 0.166 |
| 0.8 | 0.265 |
| 1 | 0.331 |
| 1.5 | 0.497 |
| 2 | 0.662 |
| 3 | 0.993 |
| 4 | 1.324 |
| 5 | 1.655 |
| nmol/L | ng/mL |
|---|---|
| 0.066 | 0.2 |
| 0.166 | 0.5 |
| 0.265 | 0.8 |
| 0.331 | 1 |
| 0.497 | 1.5 |
| 0.662 | 2 |
| 0.993 | 3 |
| 1.324 | 4 |
| 1.655 | 5 |
C-peptide levels: what the numbers mean
| Category | ng/mL | nmol/L |
|---|---|---|
| Very low (suggests severe insulin deficiency)About <0.2 nmol/L (<200 pmol/L) | < 0.6 | < 0.199 |
| Typical fasting adult rangeAbout 0.26–1.03 nmol/L; lab dependent | 0.8 – 3.1 | 0.265 – 1.026 |
Fasting reference intervals vary by assay. Interpretation depends on the glucose level drawn at the same time.
Fasting reference intervals vary by assay. Interpretation depends on the glucose level drawn at the same time.
Interpreting c-peptide results
C-peptide is most useful when read alongside a simultaneous glucose. A low C-peptide with high glucose points to insulin deficiency, typical of type 1 diabetes or long-standing type 2. A high C-peptide with low glucose suggests the body is over-producing insulin, as with an insulinoma or some medicines. Kidney disease raises C-peptide because the kidneys clear it, so results in people with reduced eGFR can look misleadingly high.
A single result outside the reference range is common and often not significant on its own. Reference ranges are set so that about 1 in 20 healthy people fall outside them, and every laboratory sets its own limits for its method. Use the range printed on your report and discuss results with the clinician who ordered the test.
When to see a doctor
Contact your clinician if a result is flagged as abnormal and you have symptoms, if a value is far outside the range, or if results are changing over time. This page explains units; it cannot diagnose or replace medical advice.
Frequently asked questions
What is 1 ng/mL C-peptide in pmol/L?
1 × 331 = 331 pmol/L (0.331 nmol/L).
Does insulin injection affect C-peptide?
No. Injected insulin contains no C-peptide, which is exactly why the test measures your own insulin production.
Can I use this converter for any lab?
Yes for the unit conversion, which is fixed by chemistry. Reference ranges differ between labs and methods, so compare your converted value with your own lab's range.