How to Calculate Heart Rate from an ECG Strip
Standard ECG paper is recorded at a fixed speed, almost always 25 mm/s. At that speed each small square (1 mm) represents 0.04 seconds, and each large square (5 mm, or 5 small squares) represents 0.2 seconds. Because 300 large squares or 1500 small squares add up to exactly one minute at that speed, counting the squares between two consecutive R waves — the tallest spike of each QRS complex — lets you convert distance on the paper directly into a heart rate in beats per minute (bpm). This calculator implements the three standard bedside methods used in nursing and ECG interpretation.
The three standard counting methods
- Large box method (regular rhythm): HR = 300 ÷ (number of large boxes between two R waves). Quick and commonly used at the bedside for a fairly steady rhythm.
- Small box method (regular rhythm, more precise): HR = 1500 ÷ (number of small boxes between two R waves). Because each small box is only 0.04 seconds, this gives a more exact rate than counting whole large boxes, which is useful for faster heart rates where the R-R interval is short.
- 6-second strip method (irregular rhythm): HR = (R waves counted in a 6-second strip) × 10. Most ECG paper has small tick marks along the top margin exactly 3 seconds apart (15 large boxes at 25 mm/s); two of those marks span 6 seconds. Multiplying the count by 10 extrapolates it to a full 60-second rate.
Choosing the right method for the rhythm
The large-box and small-box methods both assume every R-R interval is essentially the same length, so they only work well for a regular rhythm. For an irregular rhythm — atrial fibrillation being the classic example — individual R-R intervals vary beat to beat, so measuring just one interval and scaling it up can be badly wrong. The 6-second method averages across several beats instead, which smooths out that variability and gives a more representative rate. If you're not sure whether a rhythm is regular, mark several consecutive R-R intervals with calipers or tick marks; if the spacing is essentially constant, treat it as regular.
Reading the heart rate classification
A normal adult resting sinus rate is generally accepted as 60-100 bpm. A calculated rate below 60 bpm is classified as bradycardia (slow) and above 100 bpm as tachycardia (fast). These thresholds describe the rate alone — they say nothing about rhythm regularity, P-wave morphology, or QRS width, all of which matter for a full clinical interpretation. This tool is an educational aid for practicing ECG rate calculation and is not a substitute for interpretation of the complete 12-lead ECG by a qualified healthcare professional; always correlate any rate with the patient's clinical context.