Most lab reports do not measure your LDL cholesterol directly — they calculate it, with a 1972 equation that needs only the three values every standard panel does measure: total cholesterol, HDL, and triglycerides. This LDL calculator runs the same Friedewald equation your lab uses, in mg/dL or mmol/L, and tells you when the estimate can be trusted and when only a direct measurement will do.
The Friedewald Equation
LDL = total cholesterol − HDL − (triglycerides ÷ 5) in mg/dL. The trick is the last term: VLDL cholesterol — the third cholesterol fraction — is almost never measured, but in a fasting sample VLDL carries most triglyceride in a fairly fixed ratio, so VLDL ≈ TG/5. Everything else in the panel is measured directly, so the equation assembles LDL from three measurements and one well-tested approximation. In mmol/L the same logic divides triglycerides by 2.2 instead — the divisor changes because the TG-to-VLDL mass ratio converts differently between unit systems, and mixing the two divisors is a classic spreadsheet error.
Worked Example
A panel reads: total cholesterol 240 mg/dL, HDL 50, triglycerides 150. VLDL ≈ 150 ÷ 5 = 30. LDL = 240 − 50 − 30 = 160 mg/dL. The calculator also reports non-HDL cholesterol (240 − 50 = 190), the figure many guidelines now prefer because it captures every atherogenic fraction and needs no triglyceride assumption. In mmol/L the twin example: total 6.2, HDL 1.3, triglycerides 1.7 → VLDL ≈ 1.7 ÷ 2.2 ≈ 0.77, LDL = 6.2 − 1.3 − 0.77 ≈ 4.13 mmol/L.
When Friedewald Fails
Three situations break the estimate, and honest calculators must say so. Triglycerides above 400 mg/dL (4.5 mmol/L): the TG/5 ratio no longer holds and LDL comes out understated — labs flag it and clinicians order a direct LDL. Non-fasting samples: a recent meal floods the blood with chylomicrons carrying extra triglyceride, distorting the same ratio (one reason panels were traditionally fasting). Very low LDL with high triglycerides — increasingly common in treated patients — is where the newer Martin-Hopkins adjustable-factor and Sampson equations outperform Friedewald; several large labs have switched. If your panel’s LDL looks surprisingly low next to everything else, ask which equation produced it.
Reading the Result in Context
Guideline categories in mg/dL describe LDL as optimal under 100, near-optimal 100–129, borderline high 130–159, high 160–189, and very high at 190 and above — but modern practice does not treat the number in isolation. Targets are set against total cardiovascular risk (age, blood pressure, smoking, diabetes, family history), and two people with the same 160 can be advised entirely differently. This calculator computes the estimate; the interpretation belongs to a clinician who knows the rest of the chart. The same restraint applies to the friendly “good” and “bad” cholesterol shorthand — HDL’s role is transport, not virtue.
Why Not Just Measure LDL Directly?
Direct (homogeneous) LDL assays exist and are used when Friedewald’s assumptions fail, but for routine panels the calculation is free, instant, and — within its validity range — closely concordant with direct measurement. Reference-method ultracentrifugation, the true gold standard, is a research procedure, not a clinic one. The fifty-year survival of a three-term equation in everyday medicine is a case study in an approximation being exactly good enough, provided its limits are printed on the label, as they are here.
Common Mistakes
Using the mg/dL divisor (5) with mmol/L values, or the reverse, corrupts the result quietly. Entering non-fasting triglycerides without noting it does the same, louder. Treating calculated LDL as exact to the digit forgets it inherits error from three measurements plus an approximation. And comparing LDL values across labs without checking the equation — Friedewald versus Sampson can differ by 10 to 20 mg/dL in high-triglyceride samples, enough to look like progress or decline that never happened. The habit worth keeping through all of it: compare every new panel against your own previous ones, ask whether the LDL was calculated or directly measured, and read the triglycerides beside it — a climbing calculated LDL on a panel where triglycerides doubled may be estimation error wearing a lab coat, and a repeat fasting panel costs far less than medicating a mirage.
Related Tools
The lipid set continues with the VLDL Calculator from the earlier build, and the wider metabolic family lives in the Health Calculators hub — see the Estimated Average Glucose Calculator for the blood-sugar counterpart.
Frequently Asked Questions
How is LDL cholesterol calculated?
By the Friedewald equation: LDL = total cholesterol − HDL − triglycerides/5 (in mg/dL). For a panel of 240 total, 50 HDL and 150 triglycerides, LDL = 240 − 50 − 30 = 160 mg/dL.
Why does the calculator need triglycerides?
Because the equation estimates VLDL cholesterol as triglycerides divided by 5 (or 2.2 in mmol/L). VLDL is the third cholesterol fraction, rarely measured directly, and the triglyceride ratio is what stands in for it.
When is calculated LDL unreliable?
When triglycerides exceed 400 mg/dL (4.5 mmol/L), when the sample was not fasting, and in some treated patients with very low LDL plus high triglycerides. In those cases a direct LDL measurement or a newer equation (Martin-Hopkins, Sampson) is preferred.
What is a healthy LDL level?
Guidelines call under 100 mg/dL optimal and 160 to 189 high, but treatment targets are individualized against overall cardiovascular risk. The same number can mean watchful waiting for one person and medication for another — a clinician’s call.
What is non-HDL cholesterol?
Total cholesterol minus HDL — every atherogenic fraction in one figure, needing no triglyceride assumption. Many guidelines treat it as a secondary target 30 mg/dL above the LDL target. This calculator reports it alongside LDL.
Is LDL in mmol/L calculated differently?
Only the triglyceride divisor changes: divide by 2.2 instead of 5, because the TG-to-VLDL ratio converts differently between mass and molar units. The worked example: 6.2 total, 1.3 HDL, 1.7 TG gives LDL about 4.13 mmol/L.
Why did my lab report a different LDL than my home calculation?
Possibly a different equation (many labs now use Martin-Hopkins or Sampson), a non-fasting sample, or direct measurement. Differences of 10 to 20 mg/dL between methods are documented in high-triglyceride samples — ask the lab which method produced the number.