How do you calculate heart rate on an electrocardiogram (EKG)?

How to Calculate Heart Rate on an EKG

The most reliable method to calculate heart rate on an EKG is to count the number of large boxes (0.20 seconds each) between consecutive R waves and divide 300 by this number, or for irregular rhythms, count the number of QRS complexes in a 6-second strip (30 large boxes) and multiply by 10.

Standard Methods for Heart Rate Calculation

Method 1: The 300 Rule (Regular Rhythms)

• Count the number of large boxes (5 mm or 0.20 seconds) between two consecutive R waves and divide 300 by this number to obtain beats per minute 1
• This works because at standard paper speed (25 mm/sec), there are 300 large boxes per minute 1
• For example: if there are 4 large boxes between R waves, the heart rate is 300 ÷ 4 = 75 bpm
• This method is most accurate for regular rhythms where RR intervals are consistent 1

Method 2: The 1500 Rule (More Precise for Regular Rhythms)

• Count the number of small boxes (1 mm or 0.04 seconds) between consecutive R waves and divide 1500 by this number 1
• This provides greater precision since there are 1500 small boxes per minute at standard paper speed
• Use this when you need exact heart rate measurements, particularly for bradycardia

Method 3: The 6-Second Method (Irregular Rhythms)

• For irregular rhythms such as atrial fibrillation, count the number of QRS complexes in a 6-second strip (30 large boxes) and multiply by 10 1
• This is the preferred method when RR interval variability is large 1
• Most EKG paper has 3-second markers at the top; use two consecutive markers for a 6-second interval

Important Technical Considerations

When to Use Each Method

• Use the 300 or 1500 rule for regular sinus rhythm or any rhythm with consistent RR intervals 1
• Switch to the 6-second method when RR intervals vary significantly beat-to-beat, as occurs with atrial fibrillation or frequent ectopy 1
• If sinus arrhythmia is present with beat-to-beat variation, calculate an average RR interval from multiple consecutive beats 1

Verification of Computer-Derived Heart Rates

• Always manually verify computer-calculated heart rates, as automated measurements have approximately 90-95% accuracy 1
• The computer derives heart rate from the RR interval using the formula: HR = 60,000 ÷ RR interval (in milliseconds) 1
• Examine lead II or V5 to confirm the computer's RR interval measurement matches your manual measurement within approximately 10 ms 1

Common Pitfalls to Avoid

Measurement Errors

• Do not attempt heart rate calculation when artifact obscures QRS complexes or creates false R waves 1
• Muscle artifact and power-line interference can create high-frequency noise that mimics QRS complexes 1
• Movement artifact and baseline wander can obscure true R waves, leading to undercounting 1

Rate-Dependent Considerations

• For heart rates below 50 bpm, consider having the patient perform mild aerobic activity to achieve a rate closer to 60 bpm before measurement 1
• This is particularly important when calculating rate-corrected intervals (like QTc), as extreme bradycardia affects correction formulas 1
• For heart rates above 90 bpm, allow additional resting time to achieve a lower, more stable heart rate 1

Lead Selection

• Use lead II as the primary lead for heart rate calculation, as it typically provides the clearest R waves 1
• Lead V5 can serve as an alternative if lead II quality is poor 1
• Avoid using leads where R waves are small or biphasic, as this increases measurement error 1

Special Populations

Pediatric Considerations

• Normal heart rates are significantly higher in children than adults 1
• The same calculation methods apply, but interpret the resulting heart rate using age-appropriate normal ranges 1
• In neonates, heart rates between 100-190 bpm may be normal 1