Heart Rate from RR Interval Calculator
Heart Rate from RR Interval Calculator
Convert an RR interval into a heart rate from whatever you measured it in — millimetres at 25 or 50 mm/s, large boxes, milliseconds or seconds — and convert any other interval measured the same way into milliseconds.
Heart rate from the RR interval
mm, boxes, ms or s → bpmRR interval 20 mm at 25 mm/s, and a second interval of 4 mm
Formula
At 25 mm/s: RR (s) = millimetres × 0.04, so heart rate = 1500 / millimetres
At 50 mm/s: RR (s) = millimetres × 0.02, so heart rate = 3000 / millimetres
- 25 mm/s
- the standard paper speed. One small box is 1 mm and 0.04 s; one large box 5 mm and 0.2 s. The familiar shortcuts follow: 1500 divided by small boxes, or 300 divided by large boxes
- 50 mm/s
- used to separate closely spaced deflections — a delta wave, the components of a wide complex. Every interval looks twice as long, so reading such a trace with the 25 mm/s rule halves the heart rate and doubles the PR, QRS and QT
- 10 mm/mV
- the standard calibration, marked by a 10 mm deflection at the start of the trace. It affects no interval here, but it decides every voltage criterion for hypertrophy: at half standard, a QRS that meets a 35 mm criterion measures 17.5 mm
- one interval is not the rate
- 60 divided by one RR interval is the instantaneous rate for that cycle, which is the correct input for a QT rate correction. For the rate of an irregular rhythm, count the complexes on a 10-second strip and multiply by six. Normal intervals: PR 120 to 200 ms, QRS 60 to 110 ms
Worked example
RR interval 20 mm at 25 mm/s, and a second interval of 4 mm
RR = 20 mm × 0.04 s/mm = 0.800 s, which is 800 ms
Heart rate = 60 / 0.800 = 75 bpm
By the shortcuts: 1500 / 20 mm = 75, and 300 / 4 large boxes = 75 bpm — three forms of one identity
The second interval: 4 mm × 0.04 = 160 ms. As a PR interval that is within the usual 120 to 200 ms
The same 20 mm on a 50 mm/s trace is only 0.400 s, giving 150 bpm. Reading the paper speed wrong is a factor-of-two error in both directions
The same RR interval in every unit this page accepts
| Measured as | At 25 mm/s | RR (ms) | Heart rate (bpm) |
|---|---|---|---|
| 10 mm | 2 large boxes | 400 | 150 |
| 15 mm | 3 large boxes | 600 | 100 |
| 20 mm | 4 large boxes | 800 | 75 |
| 25 mm | 5 large boxes | 1,000 | 60 |
| 30 mm | 6 large boxes | 1,200 | 50 |
| 37.5 mm | 7.5 large boxes | 1,500 | 40 |
Normal intervals, and what they are in millimetres
| Interval | Normal range | At 25 mm/s | Why it matters here |
|---|---|---|---|
| PR | 120 to 200 ms | 3 to 5 mm | Over 200 ms is first-degree atrioventricular block; under 120 ms with a delta wave suggests pre-excitation |
| QRS | 60 to 110 ms | 1.5 to 2.75 mm | 120 ms or more defines a complete bundle branch block or a paced complex, and a QT measured from it needs a wide-QRS adjustment |
| QT | Rate-dependent | Varies | Meaningful only after rate correction, and interpretable only with a narrow QRS and a T wave separate from any U wave |
| RR | 600 to 1,000 ms at 60 to 100 bpm | 15 to 25 mm | The input to every QT correction on this site, and the quantity those formulas need in SECONDS |
Four ways to the same number, and the one that goes wrong
Heart rate is 60 divided by the RR interval in seconds, and every bedside shortcut is that identity with the paper speed folded in. At the standard 25 mm/s one millimetre is 0.04 s, so 1500 divided by millimetres gives the rate; one large box is 5 mm and 0.2 s, so 300 divided by large boxes gives it too. A 20 mm RR interval is 800 ms, 4 large boxes, and 75 bpm by all three routes.
The error this page exists to prevent is the paper speed. A trace recorded at 50 mm/s — used to separate closely spaced deflections, to look at a delta wave or to pick apart a wide complex — looks exactly like a 25 mm/s trace of a much slower heart. Every interval on it is twice as wide. Read it with the usual rule and the rate halves, the PR doubles and a normal QRS becomes a bundle branch block. The speed is printed at the foot of the trace, and it is worth looking at before measuring anything.
The second distinction is between the instantaneous rate of one cycle and the rate of a rhythm. Sixty divided by one RR interval is the former. For a QT rate correction that is exactly the right quantity — the correction asks what cycle length preceded this beat — which is why the QT correction pages on this site take an RR interval rather than a rate printed at the top of the ECG. For describing an irregular rhythm it is the wrong quantity, and the answer is to count the complexes on a 10-second strip and multiply by six.
The same conversion handles any other interval. A PR of 4 mm at 25 mm/s is 160 ms, inside the usual 120 to 200 ms. A QRS of 3 mm is 120 ms, the threshold for a complete bundle branch block — worth noticing, because the entire normal QRS range of 60 to 110 ms fits inside three small boxes, so a complex that looks only slightly broad can already be wide enough to invalidate an ordinary QT correction. One thing the paper speed does not affect is voltage: that depends on the calibration, conventionally a 10 mm deflection for 1 mV, and a trace recorded at half standard will fail every hypertrophy voltage criterion by construction. This supports a clinician’s judgement rather than replacing it. An ECG interval is read alongside the patient, the drug chart and the electrolytes, and never instead of them.
Frequently asked questions
How do you calculate heart rate from the RR interval?
Heart rate equals 60 divided by the RR interval in seconds. An RR interval of 800 ms is 0.8 s, so the rate is 75 bpm. At the standard 25 mm/s paper speed the same answer comes from 1500 divided by millimetres, or 300 divided by large boxes.
What is one small box on an ECG?
At the standard paper speed of 25 mm/s a small box is 1 mm wide and represents 0.04 s, and a large box is 5 mm and 0.2 s. Vertically, at the standard calibration of 10 mm per millivolt, a small box is 0.1 mV.
What happens if the ECG was recorded at 50 mm/s?
Every interval looks twice as wide. One millimetre is 20 ms rather than 40, so the rate is 3000 divided by millimetres rather than 1500. Reading a 50 mm/s trace with the 25 mm/s rule halves the heart rate and doubles the PR, QRS and QT intervals.
Can I use one RR interval for the heart rate in atrial fibrillation?
Not for the rate of the rhythm. One RR interval gives the instantaneous rate of that cycle; in an irregular rhythm, count the complexes on a 10-second strip and multiply by six. The instantaneous value is still the right input for rate-correcting the QT of that particular beat.
What are the normal PR and QRS durations?
PR 120 to 200 ms and QRS 60 to 110 ms. At 25 mm/s that is 3 to 5 millimetres for the PR interval and 1.5 to 2.75 for the QRS. A QRS of 120 ms or more — three millimetres — defines a complete bundle branch block or a paced complex.
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References
- Kligfield P, Gettes LS, Bailey JJ, et al. Recommendations for the standardization and interpretation of the electrocardiogram: part I — the electrocardiogram and its technology. J Am Coll Cardiol. 2007;49(10):1109–1127.
- Standard Electrocardiography. McMaster Textbook of Internal Medicine (Empendium), Medycyna Praktyczna, Kraków. Accessed October 2026.
- Rautaharju PM, Surawicz B, Gettes LS, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV — the ST segment, T and U waves, and the QT interval. J Am Coll Cardiol. 2009;53(11):982–991.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/