# ECG Boxes to Seconds Calculator

URL: https://speedycalculator.com/calculators/health/ecg-boxes-seconds/
Last updated: 2026-10-30

> The ECG boxes to seconds calculator converts small and large ECG grid boxes into seconds and milliseconds at 25 or 50 mm/s paper speed, so you can measure PR, QRS and QT intervals.

## Inputs

- Paper speed
- Large boxes
- Small boxes

## Results

- Time
- Time in seconds
- Distance on the paper

## 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
```

## Explanation

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

FieldWhat to enterPaper speedChoose one: 25 mm/s, 50 mm/sLarge boxesA numberSmall boxesA 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.*

## 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.

## FAQ

**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).


## Sources

- [Life in the Fast Lane – ECG basics](https://litfl.com/ecg-library/basics/)