LDL Cholesterol Calculator
Estimate LDL cholesterol from a standard lipid panel using the Friedewald equation, alongside non-HDL cholesterol — the number that stays valid when triglycerides are high.
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The Friedewald Equation
Most lipid panels report total cholesterol, HDL cholesterol and triglycerides, and derive LDL cholesterol arithmetically:
- mg/dL: LDL = total cholesterol − HDL − (triglycerides ÷ 5)
- mmol/L: LDL = total cholesterol − HDL − (triglycerides ÷ 2.2)
- Non-HDL cholesterol: total cholesterol − HDL (no triglyceride assumption)
The divisor 5 (or 2.2 in SI units) comes from the assumption that very-low-density lipoprotein contains roughly one fifth of its mass as cholesterol. That holds reasonably well at ordinary triglyceride levels and breaks down as triglycerides rise.
When the Estimate Fails
Above a triglyceride level of about 400 mg/dL (4.5 mmol/L) the Friedewald estimate becomes unreliable and systematically underestimates LDL cholesterol. This calculator therefore returns no LDL value above that threshold and shows non-HDL cholesterol instead. Other situations where the estimate drifts include:
- Type III hyperlipoproteinaemia (dysbetalipoproteinaemia), where the VLDL composition is atypical
- Chylomicronaemia and severe hypertriglyceridaemia
- Non-fasting samples, where triglycerides are transiently elevated
- Very low LDL cholesterol values (below about 70 mg/dL), where the arithmetic error becomes proportionally large
Alternatives include the Martin-Hopkins adjustment, the Sampson equation, or direct homogeneous LDL assays. Many laboratories now report non-HDL cholesterol routinely for exactly this reason.
What the Numbers Mean
There is no single “normal” LDL value, because the target depends on a person’s total cardiovascular risk. Guidelines generally treat LDL cholesterol as the primary target and non-HDL cholesterol as a secondary target, especially when triglycerides are raised or diabetes is present. Reference points such as “below 100 mg/dL” or “below 70 mg/dL” describe different risk groups rather than universal thresholds, and decisions about treatment belong with a clinician who can see the whole risk profile.
Frequently Asked Questions
Why is my laboratory’s LDL value different from this one?
Many laboratories use a direct assay, the Martin-Hopkins equation or the Sampson equation instead of Friedewald. Differences of a few mg/dL are expected, and larger differences appear at high triglyceride levels.
Should I use non-HDL cholesterol instead?
When triglycerides are above 400 mg/dL, non-HDL cholesterol is the more dependable figure from the same panel. Many guidelines use it as a secondary target even at ordinary triglyceride levels.
Does this calculator work on a non-fasting sample?
It will produce a number, but triglycerides rise for several hours after eating, which inflates the Friedewald subtraction. A fasting sample gives a more comparable result.