Insulin Unit Converter
Insulin Unit Converter
Convert insulin from µIU/mL to pmol/L using the correct factor of 6.00 — not the 6.945 that most online calculators still use, which is wrong by about 15%.
Insulin converter
µIU/mL → pmol/LInsulin 8.4 µIU/mL
Formula and conversion factor
µIU/mL = pmol/L ÷ 6.00
- 6.00
- the correct modern factor: potency 28.8 IU/mg from the 1986 WHO standard, with the molar mass of human insulin, 5808 g/mol
- 6.945
- the widely propagated wrong factor: potency 24 IU/mg from the 1959 WHO Fourth International Standard, with an assumed molar mass of 6000 g/mol — about 15% too high
- µIU/mL
- numerically identical to mIU/L; insulin is standardised by international unit of biological activity rather than by mass, which is why no molecular-weight conversion alone will do
- assay dependence
- immunoassays differ in their cross-reactivity with proinsulin and insulin analogues, so absolute values are not comparable between laboratories
Worked example
Insulin 8.4 µIU/mL
8.4 × 6.00 = 50.4 pmol/L
The obsolete factor would give 8.4 × 6.945 = 58.3 pmol/L
A difference of 7.9 pmol/L, or about 15%, on the same measurement
The size of the error, unit by unit
| Insulin (µIU/mL) | Correct: × 6.00 (pmol/L) | Obsolete: × 6.945 (pmol/L) | Difference |
|---|---|---|---|
| 3 | 18.0 | 20.8 | 2.8 |
| 8.4 | 50.4 | 58.3 | 7.9 |
| 15 | 90.0 | 104.2 | 14.2 |
| 25 | 150.0 | 173.6 | 23.6 |
| 60 | 360.0 | 416.7 | 56.7 |
Where each factor comes from
| Correct factor | Obsolete factor | |
|---|---|---|
| Potency assumed | 28.8 IU/mg | 24 IU/mg |
| Standard | WHO 1986 standard, anhydrous human insulin | WHO Fourth International Standard, 1959 |
| Molar mass assumed | 5808 g/mol (human insulin) | 6000 g/mol (rounded, incorrect) |
| Result | 6.00 pmol/L per µIU/mL | 6.945 pmol/L per µIU/mL |
The 6.945 problem, and why insulin has no simple mass conversion
Insulin is reported in µIU/mL — numerically identical to mIU/L — in most of the world, and in pmol/L in SI-reporting laboratories and in almost all research. The correct conversion is 6.00 pmol/L per µIU/mL. Most online calculators use 6.945, and that figure is wrong by about 15%. It survives because it was correct once and has been copied forward ever since, into calculators, review articles and laboratory handbooks alike. A fasting insulin quoted without its unit is genuinely ambiguous, and mistaking a µIU/mL threshold for a pmol/L one — as this page’s own interpretive bands originally did — overstates the result sixfold.
Both numbers are internally consistent, which is why the error is so durable. Insulin is standardised by biological potency rather than by mass, so a conversion needs both a potency and a molar mass. The 6.945 figure combines the 24 IU/mg potency of the WHO Fourth International Standard, set in 1959, with an assumed molar mass of 6000 g/mol. Human insulin actually has a molar mass of 5808, and the 1986 WHO standard specifies a potency of 28.8 IU/mg for modern anhydrous insulin. Those two corrections together give 6.00. Knopp, Holder-Pearson and Chase set the history out in full in 2019.
A second limitation applies whichever factor is used. Insulin immunoassays are not standardised between manufacturers, and they differ in how much they cross-react with proinsulin, its split products and insulin analogues. Two laboratories can return meaningfully different values on the same serum. Absolute insulin concentrations therefore travel poorly between laboratories, and any index built on fasting insulin — HOMA-IR among them — inherits that variability. A result is most useful compared against the same laboratory over time.
In anyone receiving insulin, the measurement answers a different question than it appears to. The assay detects injected insulin, so a level in a treated patient reflects the prescription rather than the pancreas. C-peptide is the better measure of endogenous secretion, because it is co-secreted with insulin in equimolar amounts but is not present in insulin preparations. That difference is what makes the pair diagnostic in hypoglycaemia: high insulin with suppressed C-peptide means exogenous insulin, while high insulin with high C-peptide means the pancreas is secreting it.
Frequently asked questions
What is the correct conversion factor for insulin?
Multiply µIU/mL by 6.00 to obtain pmol/L, and divide pmol/L by 6.00 to go back. An insulin of 8.4 µIU/mL is 50.4 pmol/L.
Why do so many calculators use 6.945?
That factor assumes a potency of 24 IU/mg, from the 1959 WHO Fourth International Standard, and a molar mass of 6000 g/mol. Human insulin has a molar mass of 5808, and the 1986 WHO standard gives a potency of 28.8 IU/mg. The obsolete factor is about 15% too high and has simply been copied forward.
Is µIU/mL the same as mIU/L?
Yes, they are numerically identical, and both appear on reports. Neither is a mass unit — insulin is standardised by international unit of biological activity, which is why the conversion depends on an assumed potency as well as a molar mass.
Can I compare insulin results from two different laboratories?
Not reliably. Insulin immunoassays are not standardised between manufacturers and differ in cross-reactivity with proinsulin and insulin analogues, so the same serum can give meaningfully different values. Serial results are only comparable within one laboratory and one assay.
Why measure C-peptide instead of insulin?
C-peptide is co-secreted with insulin in equimolar amounts but is absent from insulin preparations, so it measures endogenous secretion even in a patient receiving insulin. In hypoglycaemia, high insulin with suppressed C-peptide indicates exogenous administration.
Related calculators
References
- Knopp JL, Holder-Pearson L, Chase JG. Insulin units and conversion factors: a story of truth, boots, and faster half-truths. J Diabetes Sci Technol. 2019;13(3):597–600.
- Staten MA, Stern MP, Miller WG, Steffes MW, Campbell SE. Insulin assay standardization: leading to measures of insulin sensitivity and secretion for practical clinical care. Diabetes Care. 2010;33(1):205–206.
- Jones AG, Hattersley AT. The clinical utility of C-peptide measurement in the care of patients with diabetes. Diabet Med. 2013;30(7):803–817.