LDL Cholesterol Calculator (Friedewald)

How it is calculated

All three inputs are in mg/dL: total cholesterol TC, HDL cholesterol and triglycerides TG. The estimate is

LDL = TC − HDL − TG / 5

The term TG / 5 stands in for the cholesterol carried by VLDL particles, so the formula subtracts everything that is not LDL from the total. The tool needs total cholesterol above 0, and HDL and triglycerides of 0 or more. It deliberately returns nothing when triglycerides are above 400 mg/dL, the limit beyond which this equation is not valid. The result is given in mg/dL with two decimals. It does not check whether the answer is plausible, so inconsistent inputs can produce a zero or negative value. It does not convert from mmol/L.

Common mistakes

• Entering values in mmol/L. The formula and the 400 limit are in mg/dL; mmol/L figures such as 5.2 would give a meaningless answer, so convert first.
• Using triglycerides above 400 mg/dL and expecting an answer. The tool refuses them on purpose, since the TG/5 approximation fails at that level.
• Swapping the fields, for example typing the LDL you already know in the total cholesterol box, or putting triglycerides in the HDL field. The subtraction then gives a nonsense number.
• Dividing triglycerides by 2.2 (the mmol/L version of the divisor) while entering mg/dL values. In mg/dL the divisor is 5.
• Treating the output as a measured LDL. It is calculated from three other values, so any error in them carries through, and a non-fasting sample can make it unreliable.