What is the R-R interval?

The R-R interval is the time between two consecutive R-wave peaks on an ECG. Each R-wave represents ventricular depolarization (one heartbeat). Measuring this interval and dividing it into 60,000 ms gives the heart rate in BPM.

What paper speed does the square-counting method assume?

The standard ECG paper speed is 25 mm/s. At this speed, one large square (5 mm) equals 200 ms and one small square (1 mm) equals 40 ms. If a non-standard paper speed was used (e.g., 50 mm/s), the results will be inaccurate.

When should I use large squares vs. small squares?

Use large squares for a quick estimate, especially when the heart rate is between 40 and 300 BPM. Use small squares for greater precision at normal to fast rates (60–250 BPM). The R-R interval method (ms or s) gives the most accurate result.

What counts as bradycardia, normal, or tachycardia?

The standard clinical thresholds for adults are: bradycardia 100 BPM. These thresholds may differ for children or athletes.

Can I use this for irregular rhythms like atrial fibrillation?

These methods calculate an instantaneous rate from a single R-R interval. For irregular rhythms (AF, frequent ectopics), the rate varies beat to beat, so you should average several R-R intervals or use the 6-second strip method for a mean rate.

Heart Rate From ECG

Name: Heart Rate From ECG
Availability: InStock
Author: Nham Vu

Enter an R-R interval (ms or s) or count ECG grid squares to instantly calculate heart rate in BPM.

Input Method

Choose calculation method

R-R interval (ms)

Time between two consecutive R-wave peaks in milliseconds.

Normal range: 600 – 1000 ms (60–100 BPM)

Enter a value and press Calculate

Quick Reference — Large Square Method

At standard paper speed (25 mm/s), memorize: 300 – 150 – 100 – 75 – 60 – 50 BPM for 1–6 large squares.

Large squares	Small squares	R-R interval (ms)	Heart rate (BPM)	Rhythm

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Summary

Enter an R-R interval (ms or s) or count ECG grid squares to instantly calculate heart rate in BPM.

How it works

Choose an input method: R-R interval in ms, R-R interval in seconds, large-square count, or small-square count.
For R-R interval methods, the formula is BPM = 60,000 / RR(ms) or BPM = 60 / RR(s).
For the large-square method (each square = 200 ms at 25 mm/s), BPM = 300 / large squares.
For the small-square method (each square = 40 ms at 25 mm/s), BPM = 1500 / small squares.
The calculated BPM is interpreted: < 60 = bradycardia, 60–100 = normal sinus rhythm, > 100 = tachycardia.

Use cases

Quickly estimate heart rate while reading an ECG strip at the bedside.
Cross-check automated ECG machine BPM readings.
Study ECG interpretation methods for clinical exams (USMLE, NCLEX).
Teach medical or nursing students the three standard heart rate estimation methods.
Calculate exact BPM from measured R-R intervals during cardiac research.
Convert Holter monitor R-R interval data into BPM values.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu