How is LDL cholesterol calculated?

In Australia, most pathology labs calculate LDL cholesterol using the Friedewald equation rather than measuring it directly: LDL = Total Cholesterol − HDL − (Triglycerides ÷ 2.2), with all values in mmol/L. This has been the standard method since 1972 and appears on most standard lipid panel reports as 'LDL Cholesterol (calc.)' or similar.

Why can't my LDL always be calculated?

The Friedewald equation becomes unreliable when triglycerides are above approximately 4.5 mmol/L (400 mg/dL) — it significantly underestimates LDL, and can even produce an impossible negative number. Above this level, many labs will report 'unable to calculate' instead of a misleading figure, and a direct LDL measurement (a different, more expensive lab test) may be needed instead.

Do I need to fast before a cholesterol test?

Not routinely. Fasting is no longer considered necessary for an initial lipid profile in Australia, and non-fasting is preferred by many laboratories. However, your GP may ask you to fast for 8–12 hours before a repeat test if your triglycerides were high or your results need confirming, since a recent meal can temporarily raise triglycerides enough to affect the calculated LDL result.

What is non-HDL cholesterol and why does it matter?

Non-HDL cholesterol is simply Total Cholesterol minus HDL cholesterol — it captures all the 'bad' cholesterol-carrying particles, not just LDL. Unlike calculated LDL, it stays accurate even when triglycerides are high or you haven't fasted, which is why some guidelines now use it as a backup or alternative target alongside LDL.

LDL Cholesterol Calculator

See how your pathology report's "calculated LDL" is worked out from your other cholesterol numbers — and why it sometimes can't be calculated at all.

📋 Source: Friedewald equation (Friedewald WT, Levy RI, Fredrickson DS. Clin Chem 1972) — the calculation method used by most Australian pathology labs. Unreliable-above-triglycerides threshold and fasting guidance cross-checked against Pathology Tests Explained (RCPA/AACB) ↗.
Verified September 2026.
This calculator does the same arithmetic your lab report does — it doesn't replace a lab result, and your GP interprets what your numbers mean for you.

Total Cholesterol

mmol/L

HDL Cholesterol

mmol/L

Triglycerides

mmol/L

All three numbers are on your pathology report's lipid panel — copy them in exactly as shown.

"Calculated" vs "Direct" LDL — what's the difference?

Most standard lipid panels in Australia don't actually measure LDL cholesterol directly — it's worked out mathematically from three numbers the lab does measure directly: Total Cholesterol, HDL cholesterol, and Triglycerides. This calculation is called the Friedewald equation, and it's been the standard method since 1972:

LDL = Total Cholesterol − HDL − (Triglycerides ÷ 2.2)

A direct LDL test measures LDL particles in the blood directly, without relying on this formula. It's a separate, more expensive test that labs generally only run when the calculated version can't be trusted — most importantly, when triglycerides are too high for the formula to work.

The Friedewald formula estimates one part of the equation (VLDL cholesterol) as Triglycerides ÷ 2.2. That approximation only holds up at normal-to-moderately-elevated triglyceride levels. Above roughly 4.5 mmol/L (400 mg/dL), the approximation breaks down badly enough that the formula can understate LDL significantly, or even produce a negative number — which obviously isn't real. Above that point, labs typically report the result as "unable to calculate" rather than show a misleading figure.

This is also why a recent meal can matter: eating raises triglycerides temporarily (sometimes well above 4.5 mmol/L for a few hours), which can push an otherwise-normal triglyceride level over the calculation threshold. Fasting for 8–12 hours removes that effect, which is why your GP might ask for a fasting repeat if your first result came back "uncalculated" or with unexpectedly high triglycerides.

Not routinely, for a first/screening lipid panel — Australian pathology guidance no longer requires fasting for an initial test, and non-fasting is now preferred by many labs since it's more convenient and reflects your "usual" cholesterol levels. Total cholesterol and HDL aren't meaningfully affected by recent food.

Fasting for 8–12 hours may still be requested for a repeat test — most often to confirm an abnormal or borderline result, or if your triglycerides were high enough that a recent meal might have affected the calculated LDL. If your doctor has specifically asked you to fast, follow that instruction over this general information.

Because the 1972 Friedewald formula has known weaknesses — especially at high triglycerides and at very low LDL levels — two newer calculation methods have been developed and are increasingly used by pathology labs internationally, including some in Australia:

Martin-Hopkins replaces Friedewald's fixed "triglycerides ÷ 2.2" step with an adjustable factor that varies depending on your individual triglyceride and non-HDL cholesterol levels (looked up from a reference table, not a simple fixed division). It's recommended in 2019 US cardiology guidelines and performs noticeably better than Friedewald at low LDL levels.

Sampson equation (NIH equation 2) is a more complex formula built from a large reference dataset. Its main advantage is a much wider working range — it remains reasonably accurate at triglyceride levels up to roughly 9.0 mmol/L (800 mg/dL), well beyond where Friedewald stops working.

Both are more complex than something to hand-calculate reliably, so this calculator doesn't attempt to reproduce them — if your lab uses one of these instead of Friedewald, your report should say so. If you're not sure which method your result used, ask your GP or the pathology lab directly.

This calculator only reproduces the arithmetic your lab uses — it doesn't tell you whether your result is high, normal, or what it means for your health. In Australia, LDL targets aren't a single universal number; they're set individually based on your overall cardiovascular risk (age, blood pressure, smoking status, diabetes, family history, and more), and your GP is the one who interprets your result in that context.

For a plain-English explanation of what LDL and HDL cholesterol are and how high cholesterol is assessed in Australia, see What Does High Cholesterol Mean? and What Is a Cholesterol Test? If there's a strong family history of very high cholesterol or early heart disease, the Dutch Lipid Score Calculator can help estimate the likelihood of Familial Hypercholesterolaemia.