ECG Boxes to Seconds Calculator (small and large boxes to ms)

Understanding the ECG boxes to seconds calculator — concept and principle

This calculator converts a distance counted in small and large boxes on ECG paper into milliseconds and seconds. Students, nurses and doctors use it to measure the PR interval, QRS width, QT interval or R–R interval on a printed tracing.

Paper distance = time: with the default inputs the result is 160 ms (= 0.16 s).

How it is calculated

Total length (mm) = large boxes × 5 + small boxes

Duration (s) = total length (mm) ÷ paper speed (mm/s)

One small box = 1,000 ÷ paper speed (ms); one large box = 5,000 ÷ paper speed (ms)

Rate if the interval is R to R (bpm) = 60 ÷ duration (s)

A small box is 1 mm and a large box 5 mm. Decimals are allowed, for example 2.5 small boxes.
Choose 25 mm/s (standard) or 50 mm/s.

The principle

The ECG machine pulls the paper past the pen at a fixed speed, so distance along the paper is a measure of time. At 25 mm/s, 1 mm passes in 1/25 of a second, which is 0.04 s or 40 ms; a 5 mm large box is 0.20 s. At 50 mm/s the paper moves twice as fast, so each box represents half the time: 20 ms and 100 ms. If the interval measured is one full heartbeat (from one R wave to the next), there are 60 ÷ that many beats in a minute.

Worked example

Default inputs: 0 large boxes, 4 small boxes, 25 mm/s.

Total length = 0 × 5 + 4 = 4 mm.
Duration = 4 ÷ 25 = 0.16 s = 160 ms.
One small box = 1,000 ÷ 25 = 40 ms; one large box = 5,000 ÷ 25 = 200 ms.
If this were an R–R interval, rate = 60 ÷ 0.16 = 375 bpm, which is not a realistic R–R value here; 160 ms is typical of a PR interval.

Each point is the calculator re-run with only “Small boxes (1 mm each)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Small boxes (1 mm each)	Duration (ms)
1	40
2	80
3	120
4	160
5	200
6	240
8	320

How to read the result

Common adult reference points: PR interval 120–200 ms (3–5 small boxes at 25 mm/s), QRS under 120 ms (3 small boxes). A long PR can suggest first-degree AV block and a wide QRS a conduction delay, but these need the whole ECG in context. The R–R rate line only makes sense when you actually measured from one R wave to the next.

Common mistakes

Forgetting the paper speed. A tracing at 50 mm/s read as 25 mm/s doubles every interval.
Counting gridlines instead of the spaces between them.
Mixing large and small boxes. One large box is five small boxes.
Reading the rate output for a PR or QRS measurement.

Keep in mind: This is a measurement aid for education, not a diagnosis. ECG interpretation needs the full tracing, the clinical picture and a trained clinician.

Frequently asked questions

How many milliseconds is one small box on an ECG?

40 ms at 25 mm/s and 20 ms at 50 mm/s.

How many small boxes is a normal PR interval?

About 3 to 5 small boxes at 25 mm/s, or 120–200 ms.

Why do some ECGs use 50 mm/s?

The faster speed spreads waves out, which helps measure fast rhythms or children's ECGs.

How many large boxes make one second?

Five large boxes at 25 mm/s.

Sources: Life in the Fast Lane ECG Library, "ECG Rate Interpretation" (litfl.com)

Checked 9 Nov 2026. Life in the Fast Lane ECG Library, “ECG Rate Interpretation” (litfl.com): 25 mm/s standard, small box 1 mm = 0.04 s, large box 5 mm = 0.2 s; at 50 mm/s 0.02 s and 0.1 s. PR and QRS reference intervals are standard values.

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