A coronary artery calcium score is calculated using a method called the Agatston score, which multiplies the area of each calcium deposit in your coronary arteries by a density factor based on how bright that deposit appears on a CT scan. The final number is the sum of all those individual lesion scores across all four major coronary arteries. You don’t calculate this yourself; software does it automatically after a cardiac CT scan. But understanding the math behind it helps you make sense of what your score actually means.
How the Agatston Score Works
The Agatston method, developed in the 1990s, remains the standard way calcium scores are calculated worldwide. It starts with a non-contrast CT scan of your heart, meaning no dye is injected. The scanner takes cross-sectional images of your coronary arteries and measures the brightness of any calcium deposits using a unit called the Hounsfield unit (HU), which is simply a scale for how dense something appears on CT.
Any deposit that reaches 130 HU or higher counts as calcified plaque. The software then measures the area of that deposit in square millimeters and assigns it a density weighting factor from 1 to 4:
- Factor 1: 130 to 199 HU (least dense calcium)
- Factor 2: 200 to 299 HU
- Factor 3: 300 to 399 HU
- Factor 4: 400 HU or higher (most dense calcium)
For each individual calcium deposit, the software multiplies the area by the peak density factor. If a deposit covers 8 square millimeters and its brightest point reaches 350 HU, that lesion scores 8 × 3 = 24. The software repeats this for every deposit in every coronary artery, then adds them all together. That total is your calcium score.
What Happens During the Scan
The CT scan itself takes about 10 to 15 minutes, with the actual image acquisition lasting only a few seconds. You lie on a table that slides into the scanner, and electrodes are placed on your chest to sync the images with your heartbeat. No contrast dye is needed, which keeps the process simple and the radiation dose low. The scanner uses a tube voltage of 120 kV, and the current is adjusted based on your body size to balance image quality with radiation exposure.
Once the images are captured, specialized software identifies every bright spot above 130 HU in your coronary arteries, measures it, and runs the Agatston calculation automatically. A radiologist or cardiologist then reviews the results to confirm accuracy before your score is reported.
What the Numbers Mean
Your total score falls into one of several risk categories that guide treatment decisions:
- 0: No detectable plaque. Your risk of a heart attack is very low.
- 1 to 10: A small amount of plaque. Less than a 10% chance of significant heart disease, and heart attack risk remains low.
- 11 to 100: Some plaque is present. This indicates mild heart disease and a moderate chance of a heart attack. Your doctor may recommend treatment beyond lifestyle changes alone.
- 101 to 400: A moderate amount of plaque, and there’s a possibility that plaque is partially blocking an artery. Heart attack risk is moderate to high, and additional testing is common at this stage.
- Over 400: A large plaque burden. There is more than a 90% chance that plaque is blocking at least one artery, and heart attack risk is high. Further testing and treatment are typically started.
A score of zero is particularly powerful. In one large study, a 70-year-old man with mild hypertension had an estimated 10-year risk of coronary heart disease of 9.3% based on traditional risk factors alone. After a calcium score of zero, that estimate dropped to 3.1%. A zero score doesn’t guarantee you’ll never have a heart attack, but it substantially lowers the statistical likelihood over the next decade.
Who Should Get a Calcium Scan
Calcium scoring is most useful for people in the gray zone of cardiovascular risk. The American College of Cardiology and American Heart Association recommend considering a calcium scan for people whose 10-year risk of cardiovascular disease falls between 7.5% and 20%, particularly when the decision about whether to start a statin isn’t clear-cut. This typically applies to adults in their 40s through 70s with one or two risk factors like high blood pressure, elevated cholesterol, or a family history of heart disease.
The scan is not recommended for people who already have known heart disease or who have already had a heart attack or stent, since the answer to “do you have plaque?” is already yes. It’s also less useful for very young, low-risk adults, where the likelihood of finding calcium is extremely small. The scan costs about $150 and is usually not covered by insurance, so it helps to know whether you fall into the group most likely to benefit before ordering one.
What the Score Cannot Detect
The biggest limitation of a calcium score is that it only measures hard, calcified plaque. Coronary arteries can also contain “soft” plaque made of cholesterol and fatty deposits that haven’t yet hardened. This soft plaque doesn’t show up on a non-contrast CT scan because it doesn’t reach the 130 HU brightness threshold. Soft plaque is actually considered more dangerous in some cases because it’s more prone to rupturing and triggering a blood clot.
This means a calcium score of zero doesn’t rule out all coronary artery disease. It rules out calcified disease. For people with symptoms like chest pain or shortness of breath, a coronary CT angiography, which uses contrast dye, can visualize both calcified and non-calcified plaque and show whether any arteries are actually narrowed. The calcium score is a screening tool for people without symptoms; it’s not a diagnostic test for active heart problems.
Percentile Rankings by Age and Sex
Raw calcium scores don’t tell the full story because calcium naturally accumulates with age. A score of 50 in a 75-year-old is far less alarming than a score of 50 in a 45-year-old. That’s why your results typically include a percentile ranking that compares your score to other people of the same age, sex, and ethnicity. Being above the 75th percentile for your demographic group suggests your plaque burden is advancing faster than expected, even if the raw number seems modest.
Your doctor uses both the raw score and the percentile to decide next steps. Someone with a score of 80 who falls in the 95th percentile for their age group may be treated more aggressively than someone with the same score who falls in the 40th percentile for an older age bracket.