Estimating the Heart Rate on an Electrocardiogram

This post will guide you through estimating the heart rate using the small and large squares on EKG paper, especially when the heart rhythm is regular.

As we previously discussed, a standard EKG has several large squares measuring 5mm x 5mm that are divided into smaller squares measuring 1mm x 1mm. The standard EKG records at a rate of 25 mm/ second. Since each large square is 5mm wide, there are a total of 5 large squares per second. So one large square reflects 1sec/5 squares= 0.2 seconds. Since we have 5 small squares per large square, each small square is 0.2/5=0.04 seconds.

Total Squares in One Minute

Large Squares: 60 seconds / 0.20 seconds per large square = 300 large squares per minute.

Small Squares: 60 seconds / 0.04 seconds per small square = 1500 small squares per minute.

Calculating Heart Rate Using Large Squares

Since we know that each large square is 0.2 seconds, if we had a QRS every large square, the rate would be 60 second

Method: Divide 300 by the number of large squares between two consecutive heartbeats.

• Formula: Heart Rate = 300 / Number of Large Squares between R-R intervals
• Examples:
  • 1 Large Square: 300 / 1 = 300 bpm
  • 2 Large Squares: 300 / 2 = 150 bpm
  • 3 Large Squares: 300 / 3 = 100 bpm
  • 4 Large Squares: 300 / 4 = 75 bpm
  • 5 Large Squares: 300 / 5 = 60 bpm
  • 6 Large Squares: 300 / 6 = 50 bpm

Calculating Heart Rate Using Small Squares

Method: Divide 1500 by the number of small squares between two consecutive heartbeats (R-R intervals).

• Formula: Heart Rate = 1500 / Number of Small Squares between R-R intervals
• Example: If there are 25 small squares between beats, Heart Rate = 1500 / 25 = 60 beats per minute

Estimating Heart Rate in Irregular Rhythms

When the heart rhythm is irregular, the usual methods using small or large squares may not be accurate. Instead, use the 10-second rule.

10-Second Rule Method

• Step 1: Count the number of QRS complexes (heartbeats) in a 10-second EKG strip.
• Step 2: Multiply that number by 6 to estimate the beats per minute.

Why It Works

• A standard EKG strip is 10 seconds long.
• Multiplying by 6 extrapolates the 10-second count to a full minute (10 seconds x 6 = 60 seconds).

Example

• If there are 12 QRS complexes in a 10-second strip:
  • Heart Rate = 12 x 6 = 72 beats per minute

Understanding Normal Heart Rates

• Normal Range: 60 to 100 beats per minute for adults.
• Below Normal (Bradycardia): Less than 60 beats per minute.
• Above Normal (Tachycardia): More than 100 beats per minute.