ECG intervals are measured by counting grid boxes. Normal values: PR 120–200 ms (3–5 small boxes), QRS under 120 ms (3 small boxes), QTc under 450–460 ms.
How to use the ECG Boxes to Seconds Calculator
- Paper speed
- Large boxes
- Small boxes
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the paper speed and enter the number of large and small boxes; decimals are allowed.
Inputs explained
| Field | What to enter |
|---|---|
| Paper speed | Choose one: 25 mm/s, 50 mm/s |
| Large boxes | A number |
| Small boxes | A number |
What each result means
- Time = ([Large boxes]×5 + [Small boxes])÷[Paper speed]×1000
- Time in seconds = [Time]÷1000
- Distance on the paper = [Large boxes]×5 + [Small boxes]
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
At 25 mm/s: 1 small box = 0.04 s (40 ms); 1 large box = 0.2 s (200 ms)
At 50 mm/s: 1 small box = 0.02 s; 1 large box = 0.1 s
Time = small boxes × small-box duration + large boxes × large-box duration
Worked example
Inputs — Paper speed: 25 mm/s; Large boxes: 0; Small boxes: 4.
Results — Time = 160 ms; Time in seconds = 0.16 s; Distance on the paper = 4 mm.
Key terms
- Paper speed
- How fast the ECG paper moves, usually 25 mm/s.
Frequently asked questions
How many seconds is one small ECG box?
0.04 s at the standard 25 mm/s.
How many large boxes are in 6 seconds?
30 large boxes at 25 mm/s.
What is a normal PR interval?
120–200 ms (3–5 small boxes).