The PR interval is measured from the very beginning of the P wave to the very beginning of the QRS complex on an ECG strip. A normal PR interval falls between 120 and 200 milliseconds, which translates to 3 to 5 small boxes on standard ECG paper. Calculating it yourself requires nothing more than counting boxes and multiplying.
What the PR Interval Represents
Every heartbeat starts with an electrical signal in the upper chambers of the heart. That signal has to travel through the atria, pause briefly at the AV node (a relay station between the upper and lower chambers), then continue down specialized pathways until it reaches the lower chambers and triggers them to contract. The PR interval captures that entire journey. Most of the time consumed during this interval is spent at the AV node itself, where the signal deliberately slows down to give the upper chambers time to finish squeezing blood into the lower chambers before they fire.
How ECG Paper Works
Standard ECG paper moves through the machine at 25 millimeters per second. The grid is made of small boxes (1 mm each) grouped into larger boxes of 5 mm. Each small box represents 0.04 seconds (40 milliseconds). Each large box, containing five small boxes, represents 0.20 seconds (200 milliseconds). These values are consistent across virtually all 12-lead ECGs and telemetry strips, so the math stays the same regardless of the machine.
Step-by-Step Measurement
To calculate the PR interval, follow these steps:
- Find the P wave. It’s the small, rounded bump that comes before the tall QRS complex. Identify where it first lifts off the baseline. This is your starting point.
- Find the start of the QRS complex. This is the first sharp deflection after the P wave, whether it goes up or down. This is your ending point.
- Count the small boxes. Count every small box between your two points. Include any partial boxes as best you can estimate.
- Multiply by 0.04 seconds. If you count 4 small boxes, the PR interval is 4 × 0.04 = 0.16 seconds, or 160 milliseconds. If you count 3.5 small boxes, it’s 3.5 × 0.04 = 0.14 seconds.
You can also count large boxes and multiply by 0.20 seconds, but small boxes give you better precision for something this short.
Which Lead to Use
On a 12-lead ECG, the PR interval can look slightly different across leads because each lead views the heart’s electrical activity from a different angle. Use the lead where the P wave is most clearly visible and upright. Lead II is the most common choice because it typically shows the tallest, most distinct P wave.
Normal and Abnormal Ranges
In adults, a normal PR interval is 120 to 200 milliseconds (0.12 to 0.20 seconds), or 3 to 5 small boxes on the ECG strip. Anything outside this range has clinical significance.
A PR interval longer than 200 milliseconds is classified as first-degree AV block. This means the electrical signal is taking longer than normal to pass through the AV node. It shows up on the strip as an unusually wide gap between the P wave and the QRS complex. While first-degree AV block is often harmless, especially in young athletes and during sleep, it can sometimes signal underlying conduction disease or medication effects.
A PR interval shorter than 120 milliseconds suggests the signal is bypassing the normal AV node pathway entirely or passing through it faster than usual. This pattern is a hallmark of pre-excitation syndromes. In Wolff-Parkinson-White syndrome, the short PR interval appears alongside a slurred upstroke at the beginning of the QRS complex (called a delta wave), indicating that an extra electrical pathway is short-circuiting the normal route. In Lown-Ganong-Levine syndrome, the PR interval is short but the QRS looks normal, suggesting enhanced conduction through or around the AV node without an obvious accessory pathway reaching the ventricles directly. Both conditions can predispose someone to episodes of rapid heart rate.
PR Intervals in Children
Children have shorter PR intervals than adults, and the normal range shifts upward as they grow. According to data from the American Heart Association, a newborn’s average PR interval is around 108 to 110 milliseconds, while a child aged 4 to 5 averages about 123 milliseconds. By the late teenage years, the average reaches roughly 140 milliseconds, and young adults in their 20s and 30s average around 147 to 150 milliseconds. Using the adult cutoff of 200 milliseconds to judge a toddler’s ECG would miss a meaningfully prolonged interval for that age group, so pediatric-specific reference tables are essential.
How Accurate Is Manual Measurement?
Modern ECG machines calculate the PR interval automatically using software algorithms. Research comparing manual and automated measurements found strong agreement for the PR interval, with an average difference of just 1 millisecond between the two methods. The variation ranged from about 25 milliseconds shorter to 23 milliseconds longer when comparing individual readings, but overall the correlation was excellent (an ICC of 0.94). This makes the PR interval one of the more reliably measured ECG values, whether you read it yourself or trust the machine’s printout. That said, if the P wave is low-amplitude or the baseline is noisy, manual measurement lets you use clinical judgment about where the wave truly begins, something the algorithm may misjudge.
Factors That Affect the PR Interval
Heart rate has a direct relationship with the PR interval. As your heart rate increases, the AV node conducts faster, and the PR interval shortens. At rest, the same person might have a PR interval of 180 milliseconds that drops to 140 milliseconds during exercise. This is why a single PR measurement should always be interpreted in the context of the heart rate recorded on that same strip.
Certain medications slow AV node conduction and lengthen the PR interval, particularly beta-blockers, calcium channel blockers, and digoxin. Electrolyte imbalances, increased vagal tone (common in well-trained athletes), and age-related fibrosis of the conduction system can also push the PR interval beyond 200 milliseconds. Conversely, stimulants, anxiety, and anything that activates the sympathetic nervous system can shorten it.