How this instrument works
Cholesterol is measured in two different unit systems depending on where the test is run. The United States reports it in milligrams per deciliter, mg/dL, a mass-per-volume measure. Most other countries, following SI convention, report the same reading in millimoles per litre, mmol/L, a count of molecules per volume instead. The same blood sample produces a different-looking number purely because of which unit the lab used — nothing about the patient's actual level has changed.
The conversion factor, 38.67, comes from chemistry rather than medicine: it is derived from this compound's molar mass, roughly 386.65 grams per mole, adjusted for the deciliter-to-litre and milligram-to-millimole scaling. Dividing a mg/dL reading by 38.67 gives the equivalent value in mmol/L; multiplying a mmol/L reading by 38.67 reverses it. That part of the calculation is a fixed physical constant, not subject to updated guidelines or debate.
What does need outside context is what a given number means clinically, and that comes from health authorities, not chemistry. The American Heart Association and the National Cholesterol Education Program's ATP III guidelines describe 200 mg/dL as the traditional threshold for 'borderline high' total readings and 240 mg/dL for 'high,' with an LDL target near 100 mg/dL commonly used for patients at higher cardiovascular risk. This tool converts the number; it does not interpret where a given reading falls for a specific patient — that is a conversation for whoever ordered the test.
Two other unit quirks are worth knowing. First, the conversion is linear, so doubling a mg/dL figure exactly doubles its mmol/L equivalent — there is no separate scaling for high versus low readings. Second, some countries that otherwise use mmol/L for most lab values still occasionally see mg/dL on older equipment or imported reports, so recognizing which unit is printed on a given result matters more than assuming it from geography alone.
- Enter Cholesterol (mg/dL) using the number from a US-style lab report.
- Read Cholesterol (mmol/L), the same value expressed in the units used across most of the rest of the world.
- Use the result to compare a US report against international reference ranges, or to translate a reading for a clinician trained under a different unit system.
- Re-enter a different mg/dL figure any time — total, LDL, or HDL readings all convert through the same division by 38.67.
Worked example — three common thresholds converted
200 mg/dL, a commonly cited threshold for 'borderline high' total cholesterol, divided by 38.67 gives approximately 5.17 mmol/L. A US lab report reading 200 and an international one reading 5.17 describe the exact same measurement.
240 mg/dL, a commonly cited threshold for 'high' total cholesterol, divided by 38.67 gives approximately 6.21 mmol/L — the same fixed division applied to a different starting number.
100 mg/dL, a commonly cited LDL target for patients at higher cardiovascular risk, divided by 38.67 gives approximately 2.59 mmol/L. The same arithmetic applies regardless of which measure — total, LDL, or HDL — is being converted.
Questions
Why does cholesterol use two different units around the world?
Largely by historical convention: the United States reports many blood tests, including this one, in mass per volume (mg/dL), while most other countries adopted the SI system's molar approach (mmol/L) for lab reporting. Both describe the same underlying quantity in the blood — they just count it differently.
Where does the 38.67 conversion factor come from?
From this compound's molar mass, which is roughly 386.65 grams per mole, combined with the unit scaling between deciliters and litres and between milligrams and millimoles. It is a fixed chemistry constant, not a rounded clinical approximation, so the same factor applies whether you are converting a total, LDL, or HDL reading.
Is 200 mg/dL a dangerous level?
It is the traditional threshold where the total reading moves from 'desirable' into 'borderline high' territory under National Cholesterol Education Program guidelines, but a single number rarely tells the whole story. Overall cardiovascular risk depends on LDL, HDL, triglycerides, and personal risk factors together, which is why a clinician interprets the full lipid panel rather than one figure alone.
Does this converter work for LDL and HDL readings too?
Yes — the mg/dL-to-mmol/L conversion factor of 38.67 is the same across total, LDL, and HDL readings, since it comes from their shared molar mass. Only the clinical thresholds used to interpret the resulting number differ between the three.
Why is the same conversion factor not used for triglycerides?
Because triglycerides are chemically different molecules, with a different molar mass, so they convert between mg/dL and mmol/L using their own separate factor, roughly 0.0113. Applying the 38.67 factor meant for this compound to a triglyceride reading would give an incorrect result.
Should I use mg/dL or mmol/L when talking to my doctor?
Use whichever unit appears on your actual lab report — that is the number your clinician's records and reference ranges are built around. This converter is useful when comparing a reading against guidance written in the other unit system, or when moving between healthcare systems that report this measurement differently.
Why do the mg/dL numbers look so much bigger than the mmol/L numbers?
Because dividing by 38.67 shrinks the figure substantially — a reading in the low hundreds on the mg/dL scale lands somewhere between about 2 and 7 on the mmol/L scale. Neither number is more or less serious for being larger or smaller; they are simply two different rulers measuring the same underlying quantity, and clinical thresholds are defined separately within each scale.
Can I convert HDL the same way I convert total or LDL readings?
Yes, the arithmetic is identical, since the conversion factor comes from the shared molar mass rather than anything specific to which fraction is being measured. What changes between total, LDL, and HDL is only how a given converted number gets interpreted, since higher HDL is generally viewed favorably while higher LDL and higher total readings are generally viewed as raising risk.
References
- NHLBI — ATP III Cholesterol Guidelines Executive Summary
- CDC — About Cholesterol
- American Heart Association — Cholesterol
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.