Cardiology

R-R Interval Heart Rate Calculator

Calculate heart rate from R-R interval duration

Primary source: Calculated as 60000 / RR_ms; standard ECG paper speed 25 mm/s

Aleksandr A. Aulov, MD, internal medicine Last clinical review: April 28, 2026

Online calculation

R-R interval (ms)

If you have the number of small squares (1 mm each), multiply by 40 at a paper speed of 25 mm/s. For example, 21.4 small squares = 857 ms

About this score

Heart rate from the R-R interval is a basic calculation used in automated ECG interpretation and in manual tracing review. Formula: HR (bpm) = 60,000 / RR (ms), or 1,500 / number of small squares between two adjacent R waves at a standard paper speed of 25 mm/s.

An alternative quick method divides 300 by the number of large squares (5 mm each) between R waves: 1 square = 300 bpm, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50.

The accuracy of manual calculation depends on rhythm regularity and tracing quality. In atrial fibrillation, HR is averaged over a 6-second strip (30 large squares × 10), which is needed to assess rate control on medical therapy (beta-blockers, calcium channel antagonists, digoxin).

When to use

Manual ECG interpretation, both as a teaching skill and in routine practice.
Verifying automated monitor interpretation when an artifact is suspected.
Calculating average HR in atrial fibrillation from a 6-second strip.
Assessing adequacy of rate control in patients on rate-lowering therapy.

Clinical example

Case

Patient with paroxysmal atrial fibrillation on digoxin. Resting ECG: 9 R waves over a 6-second strip. Rhythm irregular.

Calculation

9 × 10 = 90 bpm (average heart rate per minute).

Interpretation and management

Average HR of 90 bpm is above the target range for a rate control strategy in atrial fibrillation (target <80 at rest for strict control; <110 for lenient control). Management: increase the beta-blocker dose or add a non-dihydropyridine calcium channel antagonist (verapamil, diltiazem). Reassess in 2 weeks with Holter monitoring.

Limitations and cautions

In irregular rhythm, a single R-R interval is unreliable; a 6- or 10-second strip is needed.
A paper speed other than 25 mm/s distorts the calculation.
Rapid breathing in children causes sinus arrhythmia, which complicates counting.
Motion artifact and baseline drift can throw off automated calculation.

Frequently asked questions

Which method is more accurate: counting squares or using the RR interval?

The RR interval in milliseconds (60,000/RR) is more accurate, but "300 / number of large squares" is more practical for regular rhythm.

How is heart rate calculated in atrial fibrillation?

Only as an average: count R waves over 6 or 10 seconds and multiply by 10 or 6, respectively.

What about sinus arrhythmia in children?

Calculate the average HR over a 30-second strip. Variation of up to 10-20 bpm with inspiration/expiration is normal.

What is normal heart rate in adults?

60-100 bpm at rest (sinus rhythm). In trained athletes, <60 is normal.

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Aleksandr A. Aulov Physician, internal medicine, Sechenov School — independent medical education platform. Calculators are compiled from the original publications and current clinical guidelines. Interpretation thresholds follow the source study unless stated otherwise.

This calculator is intended for healthcare professionals. Do not use it for self-diagnosis or self-treatment. Management decisions are made by the treating clinician based on the full clinical picture of the individual patient.