LDL Cholesterol Calculator (Friedewald)
LDL Cholesterol Calculator (Friedewald)
Estimate LDL cholesterol from total cholesterol, HDL and triglycerides using the Friedewald equation — and see the four situations in which the estimate should not be reported.
LDL Cholesterol (Friedewald)
3 inputs → mg/dLTotal cholesterol 210 mg/dL, HDL 48 mg/dL, triglycerides 150 mg/dL
The Friedewald equation
In SI units: LDL-C = total cholesterol − HDL-C − (triglycerides ÷ 2.2)
- TG ÷ 5
- the estimate of VLDL cholesterol, assuming a fixed VLDL cholesterol to triglyceride ratio of 1:5 in mg/dL
- ÷ 2.2
- the same assumption expressed in mmol/L, where every term is molar; 5 × 0.0113 ÷ 0.0259 = 2.18, conventionally rounded to 2.2
- fasting
- the equation was derived on fasting samples; chylomicrons in a non-fasting sample inflate the triglyceride term and depress the calculated LDL
Worked example
Total cholesterol 210 mg/dL, HDL 48 mg/dL, triglycerides 150 mg/dL
VLDL-C estimate = 150 ÷ 5 = 30 mg/dL
210 − 48 − 30 = 132 mg/dL
Between 130 and 160 → borderline high
ATP III LDL cholesterol categories
| LDL-C (mg/dL) | LDL-C (mmol/L) | Category |
|---|---|---|
| < 100 | < 2.6 | Optimal |
| 100 – 129 | 2.6 – 3.3 | Near optimal / above optimal |
| 130 – 159 | 3.4 – 4.1 | Borderline high |
| 160 – 189 | 4.1 – 4.9 | High |
| ≥ 190 | ≥ 4.9 | Very high |
When Friedewald should not be used
| Situation | What goes wrong | Use instead |
|---|---|---|
| Triglycerides ≥ 400 mg/dL (4.5 mmol/L) | The fixed 1:5 ratio no longer holds | Direct LDL, or non-HDL cholesterol |
| Type III dysbetalipoproteinaemia | Remnant particles carry far more cholesterol per unit triglyceride | Beta-quantification, lipoprotein electrophoresis |
| Non-fasting sample | Chylomicron triglyceride inflates the VLDL term | Non-HDL cholesterol, or apolipoprotein B |
| LDL below about 70 mg/dL | Systematic under-estimation, largest where accuracy matters most | Martin-Hopkins, Sampson-NIH, or direct LDL |
What the equation assumes, and where that fails
The Friedewald equation estimates LDL cholesterol rather than measuring it. Total cholesterol and HDL cholesterol are assayed directly, while the triglyceride term stands in for the cholesterol carried in very-low-density lipoprotein: dividing triglyceride by five assumes a fixed VLDL cholesterol to triglyceride ratio in a fasting sample. That one assumption is the source of every limitation the equation has, and it explains why a calculated and a directly measured LDL on the same sample can disagree.
The assumption fails in predictable ways. Above a triglyceride of 400 mg/dL the ratio no longer holds and the result should not be reported at all, which is why most laboratories suppress it automatically. It is also invalid in type III dysbetalipoproteinaemia, where cholesterol-rich remnant particles carry much more cholesterol per unit of triglyceride than the divisor allows for. In a non-fasting sample chylomicron triglyceride inflates the VLDL term, and at low LDL concentrations — below roughly 70 mg/dL, precisely the range that matters in secondary prevention — Friedewald under-estimates systematically, so a target can appear to have been met when it has not.
The SI form of the equation divides triglycerides by 2.2 rather than 5, and the change is arithmetic rather than clinical. It follows from the two conversion factors: cholesterol converts at 0.0259 mmol/L per mg/dL and triglyceride at 0.0113, so the mg/dL divisor of 5 becomes 5 × 0.0113 ÷ 0.0259, which is 2.18 and is conventionally rounded to 2.2. Applying the wrong divisor to the wrong units is a common error and produces an LDL that is wrong by tens of milligrams per decilitre.
Two modern estimating equations address the fixed-factor problem directly. Martin-Hopkins replaces the single divisor with a factor selected from a large table according to the patient’s own triglyceride and non-HDL cholesterol, and is more accurate at low LDL and higher triglycerides. Sampson-NIH was derived against beta-quantification and extends the usable triglyceride range further still. Where either is available, it should be preferred to Friedewald; where neither is, non-HDL cholesterol needs no assumption about VLDL at all.
Frequently asked questions
How is LDL cholesterol calculated?
By the Friedewald equation: LDL equals total cholesterol minus HDL cholesterol minus triglycerides divided by five, with every value in mg/dL. In mmol/L the triglyceride term is divided by 2.2 instead.
Why is Friedewald invalid when triglycerides are 400 mg/dL or above?
The equation assumes VLDL cholesterol is a fixed fifth of the triglyceride concentration. That relationship breaks down in marked hypertriglyceridaemia, so the calculated LDL becomes unreliable and most laboratories stop reporting it.
Do I need to fast for a calculated LDL?
Yes, if the LDL is to be calculated by Friedewald. Chylomicrons in a non-fasting sample raise the triglyceride term and lower the calculated LDL. Non-HDL cholesterol can be read from a non-fasting sample without this problem.
Why does the equation divide by 2.2 in mmol/L?
Because cholesterol and triglyceride convert between mg/dL and mmol/L with different factors — 0.0259 and 0.0113 respectively. Carrying the mg/dL divisor of 5 through those factors gives 2.18, rounded to 2.2.
What are Martin-Hopkins and Sampson-NIH?
Two newer LDL estimating equations that replace Friedewald’s fixed divisor with an adjustable factor. Both are more accurate at low LDL and at higher triglycerides, which is where Friedewald performs worst.
Related calculators
References
- Friedewald WT, Levy RI, Fredrickson DS. Estimation of the concentration of low-density lipoprotein cholesterol in plasma, without use of the preparative ultracentrifuge. Clin Chem. 1972;18(6):499–502.
- Martin SS, Blaha MJ, Elshazly MB, et al. Comparison of a novel method vs the Friedewald equation for estimating low-density lipoprotein cholesterol levels from the standard lipid profile. JAMA. 2013;310(19):2061–2068.
- Grundy SM, Stone NJ, Bailey AL, et al. 2018 AHA/ACC multisociety guideline on the management of blood cholesterol. Circulation. 2019;139(25):e1082–e1143.