Article — LDL Cholesterol Calculator
LDL cholesterol calculator: what the Friedewald number means
LDL cholesterol carries roughly 60-70% of the cholesterol in your bloodstream and is the lipid fraction that drives atherosclerosis. The Friedewald equation, published in 1972, estimates LDL from total cholesterol, HDL, and triglycerides using the formula LDL = TC - HDL - TG/5 in mg/dL. The result lands in one of five NCEP ATP III bands, from optimal under 100 mg/dL up to very high at or above 190 mg/dL.
Most labs print the calculated LDL right on your lipid panel, but the number behind it relies on three measured values and one assumption that breaks down at high triglyceride levels. Understanding the math behind the calculator helps you read your own lab report and recognise when the calculated LDL is reliable and when it is not.
What is LDL cholesterol?
LDL, or low-density lipoprotein, is the particle that ferries cholesterol from the liver out to the rest of the body. Cells use cholesterol for membranes, hormones, and bile acids, so some LDL is essential. The problem is excess. When LDL concentration runs high, particles squeeze past the artery wall lining and lodge in the intimal layer, where they oxidise and trigger plaque formation.
HDL (high-density lipoprotein) moves cholesterol back to the liver for disposal, while triglycerides are the storage form of fat. LDL is the lipid clinicians act on most often because the relationship to cardiovascular events is unusually linear: every 39 mg/dL drop in LDL lowers major vascular event risk by about 22% in the Cholesterol Treatment Trialists meta-analysis of statin trials.
LDL particles vary in size and density. Small dense LDL (pattern B) is more atherogenic than large buoyant LDL (pattern A) at the same total LDL concentration. Standard lipid panels do not distinguish between the two, which is why apolipoprotein B testing is increasingly recommended for finer risk stratification.
The Friedewald LDL formula
The Friedewald equation is the math behind nearly every calculated LDL value reported by clinical labs. Friedewald, Levy, and Fredrickson published it in Clinical Chemistry in 1972 as a way to estimate LDL without the slow ultracentrifugation that direct measurement required at the time.
LDL = TC - HDL - TG/5 (mg/dL)LDL = TC - HDL - TG/2.2 (mmol/L)The TG/5 term is the clever part. Friedewald observed that in fasting samples the mass ratio of triglycerides to VLDL cholesterol holds at about 5:1. Dividing measured triglycerides by 5 gives a workable estimate of VLDL. Subtracting HDL and estimated VLDL from total cholesterol leaves LDL. The calculation works because in a typical fasting sample, total cholesterol breaks down into LDL, HDL, and VLDL with no other major fractions.
Optimal LDL cholesterol levels
The National Cholesterol Education Program (NCEP) Adult Treatment Panel III set the LDL bands that most US labs still report. Under 100 mg/dL is optimal, 100-129 is near optimal, 130-159 is borderline high, 160-189 is high, and 190 mg/dL or above is very high. These thresholds apply to adults without prior cardiovascular events or special risk factors.
Patients with established cardiovascular disease, diabetes, or familial hypercholesterolemia work to lower targets. The 2018 AHA/ACC guideline calls for LDL under 70 mg/dL for high-risk secondary prevention and under 55 mg/dL after a recent major event. Reaching those targets usually requires a high-intensity statin.
- Optimal = under 100 mg/dL (under 2.59 mmol/L)
- Near optimal = 100-129 mg/dL
- Borderline high = 130-159 mg/dL
- High = 160-189 mg/dL
- Very high = 190+ mg/dL (4.91+ mmol/L)
- High-risk target = under 70 mg/dL (diabetes, CHD)
- Very high-risk target = under 55 mg/dL (recent event)
When the Friedewald LDL formula fails
The TG/5 assumption is the formula's weak point. It holds for fasting triglycerides up to about 400 mg/dL. Above that, the composition of VLDL particles changes, the TG-to-VLDL ratio rises, and dividing by 5 overestimates VLDL. Overestimated VLDL means the calculated LDL is too low, which can mask cardiovascular risk in patients who actually have hypertriglyceridemia and elevated LDL together.
If the lab report shows fasting triglycerides above 400 mg/dL (4.52 mmol/L), the calculated LDL on the panel is not reliable. Ask the lab for a direct LDL measurement (ultracentrifugation or homogeneous assay) or for the Martin-Hopkins calculated value, which handles higher TG levels more accurately.
Two other situations break Friedewald. Non-fasting samples push triglycerides 50-100% higher than fasting values, inflating the VLDL term and pulling calculated LDL down artificially; standard guidance is a 9-12 hour fast. Type III hyperlipoproteinemia, a rare genetic condition, raises remnant lipoprotein cholesterol that does not behave like normal VLDL, making Friedewald unreliable at any TG level.
Martin-Hopkins LDL equation
The Martin-Hopkins equation, published in JAMA in 2013, replaces the fixed TG/5 divisor with a table-lookup factor that varies with both triglyceride and non-HDL cholesterol levels. The divisor ranges from about 3.1 to 11.9. The accuracy gain matters most when triglycerides run between 200 and 400 mg/dL, where Friedewald systematically underestimates LDL by 5-15 mg/dL. For most fasting samples with TG under 200 mg/dL, the two equations agree within 5 mg/dL, so Friedewald remains the workhorse.
Non-HDL cholesterol, the LDL companion
Non-HDL cholesterol is total cholesterol minus HDL. It captures every atherogenic particle in one number: LDL plus VLDL plus IDL plus lipoprotein(a). Because it does not involve a triglyceride correction, non-HDL is reliable even when Friedewald LDL is not. The AHA target is non-HDL under 130 mg/dL for low-risk adults and under 100 mg/dL for higher risk profiles.
When triglycerides run high, switch your attention to non-HDL cholesterol. It captures all the atherogenic lipoproteins in one number and does not depend on the TG/5 assumption that breaks Friedewald LDL at high triglyceride levels.
Apolipoprotein B (apoB) goes a step further. Every atherogenic lipoprotein carries exactly one apoB molecule, so apoB concentration counts particles directly. The 2021 ESC guideline lists apoB as an alternative primary target for high-risk patients.
Lowering elevated LDL
Three levers move LDL: diet, exercise, and medication. Diet changes can drop LDL by 10-20% in 8-12 weeks. The biggest effects come from cutting saturated fat to under 7% of calories, eliminating trans fats, adding 10-25 grams of soluble fiber from oats, beans, and apples, and including plant stanols or sterols at 2 grams per day.
Exercise lowers LDL by 5-10% and raises HDL by a similar percentage when sustained at 150 minutes per week of moderate aerobic activity. When LDL stays above target after 3-6 months of lifestyle change, medication enters the picture: statins reduce LDL by 25-55% depending on dose, ezetimibe adds another 15-25%, and PCSK9 inhibitors drop LDL by another 50-60% on top of a statin.
Calculator limits and medical disclaimer
This LDL calculator implements the Friedewald equation as published in 1972. It assumes a fasting sample, triglycerides below 400 mg/dL, and no exotic lipid disorders. Outside those limits the widget shows a red warning. The number is an educational estimate, not a diagnosis, and should not drive medication decisions.
This tool is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always discuss lipid results, treatment targets, and medication changes with a qualified healthcare professional who knows your full medical history. Cardiovascular risk depends on more than LDL alone.