Hello. The traditional formula to calculate LDL-C is as below:
LDL-C = Total cholesterol - HDL-C - Triglycerides/5
This is called the Friedwald equation.
A few years ago, experts from Johns Hopkin's Medicine proposed a new formula for people with a high level of triglycerides, because the traditional formula makes the LDL-C value lower than what it actually is for highly risky people. They proposed a new equation for people with triglycerides above 4.5 mmol/L (174 mg/dl), which is accepted by many labs in the U.S. They've develped an App named LDL Cholesterol Calculator, available in iTune store and Google Play. If you would like to calculate on your own, the formula is very like the below:
LDL-C = 0.8 * (Total cholesterol - HDL-C) - Triglycerides/10