Convert ECG/EKG paper grid boxes to seconds, milliseconds, and millimetres — at the standard 25 mm/s paper speed or 50 mm/s. Works in both directions: boxes → time and time → boxes.
Enter a measurement and get the exact time value. Standard calibration: 1 small box (1 mm) = 0.04 s at 25 mm/s.
Reference tool — not medical advice. Double-check the paper speed printed at the bottom of the ECG trace; a 50 mm/s trace halves every value above.
The values clinicians memorize. Verified against the converter engine.
| Small boxes | Seconds | Milliseconds |
|---|---|---|
| 1 | 0.04 s | 40 ms |
| 5 | 0.2 s | 200 ms |
| 10 | 0.4 s | 400 ms |
| 15 | 0.6 s | 600 ms |
| 20 | 0.8 s | 800 ms |
| 25 | 1.0 s | 1000 ms |
Memory aid: 5 small boxes = 1 large box = 0.2 s; 25 small boxes = 5 large boxes = 1 second.
Computed by the converter engine above.
For regular rhythms at 25 mm/s: count the large boxes between two consecutive R waves.
| Large boxes (R–R) | Heart rate |
|---|---|
| 1 | 300 bpm |
| 2 | 150 bpm |
| 3 | 100 bpm |
| 4 | 75 bpm |
| 5 | 60 bpm |
| 6 | 50 bpm |
At the standard paper speed of 25 mm/s, one small box (1 mm) equals 0.04 seconds, which is 40 milliseconds. One large box (5 small boxes, 5 mm) equals 0.2 seconds, which is 200 milliseconds.
At 25 mm/s, multiply the box count by 0.04 seconds: 5 × 0.04 = 0.2 seconds = 200 milliseconds. This converter does the same math in both directions for 25 mm/s and 50 mm/s paper speeds.
At 50 mm/s every time value is halved: one small box = 0.02 seconds (20 ms) and one large box = 0.1 seconds (100 ms). For example, 5 small boxes at 50 mm/s = 0.1 seconds = 100 milliseconds.
Divide seconds by the time value of one box. At 25 mm/s: 0.6 seconds ÷ 0.04 = 15 small boxes, or 0.6 ÷ 0.2 = 3 large boxes. Use the Time to Boxes mode above for any value.
It is a memorization shortcut for regular rhythms at 25 mm/s: count the large boxes between two R waves, then 1 box = 300 bpm, 2 = 150 bpm, 3 = 100 bpm, 4 = 75 bpm, 5 = 60 bpm, 6 = 50 bpm. This page shows it only as a static reference table. Full heart-rate calculation (irregular rhythms, the 1500 method, the 6-second strip method) is outside this tool's scope — this converter measures time, it does not diagnose rhythm. Note: the separate heart-bpm-calculator on this site is a pulse tap counter, not an ECG tool.
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