How to Use an ECG Ruler: A Step-by-Step Guide for Paramedics

An ECG ruler is one of the simplest tools you can carry that makes a real difference when you need quick, accurate measurements on an ECG strip. Instead of guessing heart rate or doing mental maths under pressure, a ruler gives you a consistent way to measure rate and key intervals in seconds. That is especially helpful for paramedic students and early-career clinicians building confidence with rhythm interpretation.

In this guide, you’ll learn what an ECG ruler is, how to use it to measure heart rate (for both regular and irregular rhythms), and how to measure the PR interval, QRS duration, and QT interval step-by-step. We’ll also cover common mistakes that lead to incorrect readings, plus a few simple practice exercises you can use to sharpen your skills.

This is an educational guide only and should always be used alongside your service guidelines, training, and clinical judgement.

Key Takeaways

• An ECG ruler helps you measure heart rate and intervals quickly and aids in detecting acute myocardial infarctions.
• Always confirm the ECG paper speed (commonly 25 mm/s, sometimes 50 mm/s).
• Heart rate can be measured differently for regular vs irregular rhythms.
• PR, QRS, and QT measurements depend on choosing the correct start and end points.
• Most errors come from paper speed confusion, poor lead selection, or counting from the wrong part of the waveform.

Summary Table: ECG Measurements at a Glance

What is an ECG ruler?

An ECG ruler (sometimes called an ECG card) is a simple reference tool designed to help you detect ST-elevated myocardial infarction (STEMI) and measure heart rate and key ECG intervals quickly, using the grid on ECG paper. Instead of counting squares manually every time, you align the ruler with the waveform and read off the measurement in seconds. By aligning the isoelectric line with the STEMI baseline, pathological ST-elevation can be detected. For paramedics and students, it’s a practical way to improve accuracy and consistency, especially when you’re working quickly or teaching rhythm interpretation.

Most ECG rulers include quick prompts for:

• Heart rate (regular and irregular rhythms)
• PR interval
• QRS duration
• QT interval (and sometimes QTc guidance)
• Common normal ranges (as a quick check)
• A ST-Elevation Ruler + STEMI Criteria by Age & Sex is unique to MyMedEquip’s ECG Ruler

ECG paper basics (the part that makes the ruler work)

ECG paper is a grid. The key is knowing what the squares represent.

At the most common paper speed (25 mm/s):

• 1 small square (1 mm) = 0.04 seconds
• 1 large square (5 small squares) = 0.20 seconds
• 5 large squares = 1 second

Check paper speed before you measure

Most Australian services use 25 mm/s, but some ECGs may be recorded at 50 mm/s (especially in some settings or devices). If you use a ruler calibrated for 25 mm/s on a 50 mm/s ECG, your readings will be wrong.

Quick rule:

• If the paper speed doubles (25 → 50 mm/s), the time per square halves.
  So always confirm paper speed on the printout or monitor before measuring.

Step-by-step: Measuring Heart Rate 

Before you start: confirm paper speed (commonly 25 mm/s) and choose a lead with a clear R wave (Lead II is often easiest, but use whatever is cleanest).

A) Heart rate for a regular rhythm (fastest method)

This method works best when the R–R intervals are consistent.

Step 1: Pick two clear R waves
Choose two consecutive QRS complexes with tall, clean R peaks.

Step 2: Measure the R–R interval
Using your ECG ruler, align the ruler’s start point to the peak of the first R wave and read the distance to the next R wave using the ruler’s heart rate scale (or the large-square scale if your ruler uses that).

Step 3: Read the heart rate directly
Most ECG rulers give you a direct bpm reading once you align the R–R interval.

Quick check (if you’re using large squares):
Count the number of large squares between R waves:

• 1 large square ≈ 300 bpm
• 2 ≈ 150 bpm
• 3 ≈ 100 bpm
• 4 ≈ 75 bpm
• 5 ≈ 60 bpm
• 6 ≈ 50 bpm
  
  

If your ruler is correct for the paper speed, your ruler reading and your quick check should roughly match.

B) Heart rate for an irregular rhythm (more accurate method)

If the rhythm is irregular (for example AF), don’t rely on one R–R interval. Use an average method.

Step 1: Use a 6-second section of the ECG strip
At 25 mm/s, 30 large squares = 6 seconds.
(Some ECG printouts mark 3-second or 1-second intervals — use those if shown.)

Step 2: Count the QRS complexes in that 6-second window
Count each QRS complex (each beat) in the window.

Step 3: Multiply by 10
Number of beats in 6 seconds × 10 = beats per minute (bpm).

Example:
If you count 8 QRS complexes in 6 seconds: 8 × 10 = 80 bpm.

Common tip (helps accuracy fast)

If you’re unsure whether the rhythm is regular, measure two or three R–R intervals. If they vary noticeably, treat it as irregular and use the 6-second method.

Step-by-step: Measuring PR, QRS, and QT Intervals

Before you measure intervals, pick a lead where the waveform is easiest to see (often Lead II for P waves and PR, and a lead with a clear T wave for QT). If the tracing is noisy, choose the cleanest lead rather than forcing it on a messy one.

Paper speed reminder: Most ECG rulers are set for 25 mm/s. If your ECG is 50 mm/s, your time measurements must be adjusted or you’ll get the wrong result.

A) Measuring the PR interval

What it is: time from the start of atrial depolarisation to the start of ventricular depolarisation.

Step 1: Find a clear P wave and QRS
Pick a beat where the P wave is obvious and the baseline is steady.

Step 2: Start point
Place the relevant box on the right hand side of the ruler at the first upward (or downward) deflection from baseline that begins the P wave (not the peak).

Step 3: End point
Measure to the first deflection of the QRS complex (the start of the QRS), whether it begins with a Q wave or an R wave.

Step 4: Read the interval
Read the time directly from your ruler scale, or count small squares:

• 1 small square = 0.04 s (at 25 mm/s)
  
  

B) Measuring the QRS duration

What it is: time for ventricular depolarisation.

Step 1: Choose the widest-looking QRS in a clean lead
Use the lead where the start and end of the QRS is easiest to see.

Step 2: Start point
The first deflection away from baseline at the start of the QRS (Q wave or R wave).

Step 3: End point
The point where the QRS returns to baseline (end of the S wave or end of the terminal R’ if present).

Step 4: Read the duration
Use the ruler’s QRS scale or count small squares (0.04 s each at 25 mm/s).

C) Measuring the QT interval

What it is: time from the start of ventricular depolarisation to the end of ventricular repolarisation.

Step 1: Pick a lead with a clear T wave ending
This is the most important step. If you can’t see where the T wave ends, choose another lead.

Step 2: Start point
Start at the beginning of the QRS complex.

Step 3: End point (most common error point)
Measure to the end of the T wave where it returns to baseline.
If there’s a U wave, the QT ends at the end of the T wave (not the U wave).

Step 4: Read the QT interval
Use your ruler scale or small squares.

Quick note on QTc:
QT changes with heart rate. Typical ranges for males and females are noted on MyMedEquip’s ECG Card. Many monitors calculate QTc automatically. In the field, your priority is accurate QT measurement and recognising clearly prolonged patterns rather than doing complex calculations manually.

Common Mistakes, and How to Avoid Them

Most ECG measurement errors come from a few predictable issues. Fix these and your accuracy improves immediately.

1) Not checking paper speed

If the ECG is recorded at 50 mm/s but you measure as if it’s 25 mm/s, your intervals will be wrong.
Fix: Confirm paper speed on the printout or monitor before measuring.

2) Using a noisy lead

Baseline wander or artefact can hide the true start and end points.
Fix: Measure in the cleanest lead, not the “standard” lead. If Lead II is messy, use another lead where the waveform is clearer.

3) Measuring from the wrong point on the waveform

Common examples:

• PR is measured from the end of the P wave instead of the start
• QRS measured to the end of the ST segment instead of the end of the QRS
• QT measured into a U wave instead of stopping at the end of the T wave
  Fix: Always measure from the first deflection away from baseline and end where the waveform clearly returns to baseline.

4) Using a regular rhythm method on an irregular rhythm

A single R–R interval is unreliable in AF or ectopy.
Fix: Use the 6-second method for irregular rhythms (count beats in 6 seconds × 10).

5) Picking a beat with a premature complex

An ectopic beat can distort the interval you’re trying to measure.
Fix: Choose a representative normal beat (not a PVC or PAC) unless you’re specifically measuring that abnormal beat for a reason.

Practice Exercises 

These exercises are designed to be done with an ECG strip image next to them. If you’re adding diagrams, label them Exercise 1, 2, 3 and keep the strip length consistent.

Exercise 1: Regular rhythm heart rate (R–R method)

1. Pick two consecutive R waves on a regular rhythm strip.
2. Use your ECG ruler to measure the R–R interval.
3. Write down the heart rate.
4. Quick check: count large squares between R waves and sanity-check the bpm (300, 150, 100, 75, 60).

Your turn: HR = ____ bpm

Exercise 2: Irregular rhythm heart rate (6-second method)

1. Mark out a 6-second window (30 large squares at 25 mm/s, or use the timing marks if shown).
2. Count the number of QRS complexes in that window.
3. Multiply by 10 to estimate bpm.
   
   

Your turn: ____ beats in 6 seconds × 10 = ____ bpm

Exercise 3: PR + QRS + QT measurement (one clean beat)

Choose a beat with a clear P wave and T wave.

1. Measure PR: start of P → start of QRS
2. Measure QRS: start of QRS → end of QRS
3. Measure QT: start of QRS → end of T wave (ignore U waves if present)
   
   

Your turn:
PR = ____ s (or ____ ms)
QRS = ____ s (or ____ ms)
QT = ____ s (or ____ ms)

Explore More Clinical Reference Tools

If you’re building confidence with ECG interpretation (or teaching it), having quick-reference tools on hand makes practice easier and reduces common measurement errors. Explore MyMedEquip’s Clinical Reference Cards and Emergency Protocol Guides for durable prompts that support real pre-hospital decision-making.

→ View the Clinical Reference Cards & Emergency Protocols Collection

Shop ECG Rulers

Ready to make ECG measurements faster and more consistent? MyMedEquip’s ECG rulers and ECG cards are designed for practical, on-shift use and student learning, with clear markings to support heart rate and interval measurement at a glance.

→ Shop ECG Rulers

Final Thoughts

An ECG ruler is a simple tool, but it removes a lot of uncertainty when you’re trying to measure rate and intervals quickly. Once you’re consistent with paper speed checks and you know the correct start and end points for PR, QRS and QT, your readings become faster, more accurate, and easier to communicate to your team.

For students, the best approach is repetition. Use the practice drills regularly, measure on clean leads, and sanity-check your results with a quick square count. Over time, you’ll rely less on guesswork and more on a structured process you can apply confidently in real clinical settings.

FAQ’s Answered:

Where can I buy an ECG ruler in Australia?

You can buy an ECG ruler in Australia directly from MyMedEquip. Their ECG rulers are designed for practical clinical use and training, making them a solid option for paramedics, students, and workplaces that need reliable ECG measuring tools delivered locally.

Best supplier for ECG rulers

The best supplier is one that offers clear product details, reliable stock, and tools suited to real clinical environments. MyMedEquip is a trusted Australian supplier of ECG rulers and clinical reference tools, with products commonly used by paramedics and students for accurate heart rate and interval measurement.

Where can I purchase ECG rulers for my business?

Businesses can purchase ECG rulers through MyMedEquip, which supplies ECG rulers suitable for workplaces, training providers, and clinical teams. If you’re ordering for multiple staff, buying through one supplier also makes it easier to standardise training and restocking.

What is an ECG ruler and what is it used for?

An ECG ruler is a measurement tool used to quickly calculate heart rate and key ECG intervals such as PR, QRS, and QT using the ECG grid. It helps clinicians and students measure consistently without relying on mental maths, improving speed and accuracy during ECG interpretation.

What’s the difference between an ECG ruler and an ECG card?

An ECG ruler is primarily a measurement tool designed to calculate heart rate and intervals using the ECG paper grid. An ECG card often combines measurement scales with extra reference information, such as normal ranges, rhythm prompts, or interpretation tips. In practice, both are used as quick-reference tools — the best option depends on whether you want measurement-only (ruler) or measurement plus prompts (card).