Total cholesterol is the sum of all the cholesterol carried in your blood, and the formula is straightforward: Total Cholesterol = LDL + HDL + VLDL. On a standard lipid panel, your lab measures total cholesterol, HDL, and triglycerides directly, then uses those numbers to calculate LDL. So in practice, you rarely need to calculate total cholesterol yourself, but understanding how these pieces fit together helps you make sense of your results.
The Basic Formula
Total cholesterol is made up of three components:
- LDL (low-density lipoprotein), often called “bad” cholesterol
- HDL (high-density lipoprotein), often called “good” cholesterol
- VLDL (very-low-density lipoprotein), a type of cholesterol linked to triglycerides
The equation is: Total Cholesterol = LDL + HDL + VLDL
VLDL isn’t measured directly on most lab tests. Instead, it’s estimated by dividing your triglycerides by 5 (when using mg/dL). So if your triglycerides are 150 mg/dL, your estimated VLDL is 30 mg/dL. If you’re working in mmol/L (common outside the U.S.), you divide triglycerides by 2.2 instead.
How Labs Actually Use This Math
Here’s where it gets slightly counterintuitive. Labs measure your total cholesterol directly with a blood test. What they calculate is your LDL, not your total. The formula gets rearranged to solve for the piece that’s hardest to measure:
LDL = Total Cholesterol − HDL − (Triglycerides ÷ 5)
This is the Friedewald equation, introduced decades ago and still widely used. It works by subtracting your HDL and your estimated VLDL from the total, leaving LDL as the remainder. If you already have your LDL, HDL, and triglycerides from a lab report and want to verify your total cholesterol, just reverse the equation: add LDL + HDL + (Triglycerides ÷ 5).
A Worked Example
Say your lipid panel shows:
- LDL: 120 mg/dL
- HDL: 55 mg/dL
- Triglycerides: 140 mg/dL
First, estimate VLDL: 140 ÷ 5 = 28 mg/dL. Then add everything up: 120 + 55 + 28 = 203 mg/dL. That’s your total cholesterol. The CDC considers an optimal total cholesterol around 150 mg/dL, and levels above 200 mg/dL are generally considered high for both adults and children.
When the Standard Formula Loses Accuracy
The Friedewald equation assumes a fixed relationship between triglycerides and VLDL, and that assumption breaks down when triglycerides are elevated. At triglyceride levels between 150 and 199 mg/dL, the formula underestimates LDL by about 9 mg/dL. Between 200 and 399 mg/dL, the error grows to roughly 18 mg/dL. Above 400 mg/dL, the equation is considered unreliable and most labs won’t use it at all.
This matters because an underestimated LDL can make your cholesterol picture look better than it actually is. A newer method called the Martin/Hopkins equation, developed at Johns Hopkins in 2013, replaces that fixed divisor of 5 with a variable number based on your specific triglyceride and cholesterol levels. In a large comparison study, the Martin/Hopkins method correctly classified 89.6% of patients’ LDL values, compared to 83.2% for the Friedewald equation. The American College of Cardiology and the American Heart Association now recommend it, and many U.S. labs have adopted it.
If your triglycerides are below 150 mg/dL, all the major equations agree more than 95% of the time, so the method your lab uses makes little practical difference.
The Cholesterol Ratio
Beyond the raw total, many people encounter the total cholesterol-to-HDL ratio. You calculate it by dividing your total cholesterol by your HDL. If your total is 200 mg/dL and your HDL is 50 mg/dL, your ratio is 4:1. Higher ratios signal higher cardiovascular risk because they indicate a larger share of your cholesterol is the harmful type relative to the protective type. This ratio can be a more useful snapshot than total cholesterol alone, since a high total driven mainly by high HDL is very different from one driven by high LDL.
Converting Between Units
The U.S. reports cholesterol in mg/dL, while most other countries use mmol/L. To convert cholesterol values (total, LDL, or HDL) from mg/dL to mmol/L, multiply by 0.02586. To go the other direction, multiply by 38.67. Triglycerides use a different conversion factor: multiply mg/dL by 0.01129 to get mmol/L, or multiply mmol/L by 88.57 to get mg/dL. This is why the Friedewald equation uses a divisor of 5 in mg/dL but 2.2 in mmol/L for the triglyceride-to-VLDL estimate.
Fasting and Its Effect on Your Numbers
You may have been told to fast before a cholesterol test. Eating a meal with fat raises triglyceride levels by roughly 15%, which in turn affects any calculation that uses triglycerides. Total cholesterol, LDL, and HDL themselves change minimally, typically less than 5%, whether you’ve eaten or not. The main concern is that inflated post-meal triglycerides will throw off the VLDL estimate and, by extension, the calculated LDL. If your triglycerides tend to run high, a fasting test gives a more reliable baseline for the math to work correctly.