ECG criteria
- Locate the calibration signal: a rectangular deflection on the tracing (usually on the left or at the end of the recording)
- Check the height of the calibration pulse: 10 mm corresponds to 1 mV (0.1 mV per 1 mm)
- Check the recording speed via the device's printed label or time markers on the tracing: 25 mm/s — 1 mm = 0.04 s; 50 mm/s — 1 mm = 0.02 s; speed cannot be assessed from the width of the calibration pulse itself unless its preset duration is known in advance
- At half calibration (5 mm = 1 mV), 1 mm corresponds to 0.2 mV; if the amplitude in mm is substituted into the standard formula (10 mm/mV), the result in mV must be multiplied by 2 — it is safer to divide the amplitude in mm directly by the actual sensitivity in mm/mV
- At double calibration (20 mm = 1 mV), 1 mm corresponds to 0.05 mV; when using the standard formula (10 mm/mV), the result in mV must be divided by 2
- At a speed of 50 mm/s, divide all time intervals (QT, PQ, QRS) by two when calculating in ms, if the measurement is made with a ruler without rescaling the scale
- Match the actual millimeter grid: large box 5 mm = 0.2 s at 25 mm/s, small box 1 mm = 0.04 s
What it looks like on ECG
The calibration signal is a rectangular ("square wave") pulse that the device prints separately from the lead waveforms, usually before the first complex or after the last lead. At standard calibration, the height of this pulse is 10 mm, which corresponds to 1 mV (meaning 1 mm vertically = 0.1 mV).
The paper speed determines the horizontal scale. At a speed of 25 mm/s: 1 mm = 0.04 s, large box (5 mm) = 0.2 s, 5 large boxes = 1 s. At a speed of 50 mm/s, the same distances in millimeters correspond to half the time: 1 mm = 0.02 s, large box = 0.1 s.
| Speed | 1 mm | 1 large box (5 mm) |
|---|---|---|
| 25 mm/s | 0.04 s | 0.2 s |
| 50 mm/s | 0.02 s | 0.1 s |
| Calibration | 1 mm |
|---|---|
| 10 mm/mV (standard) | 0.1 mV |
| 5 mm/mV (half) | 0.2 mV |
| 20 mm/mV (double) | 0.05 mV |
When interpreting voltage criteria (hypertrophy, low voltage) and time intervals (PQ, QRS, QT), the physician must first check the calibration pulse and speed marking rather than counting boxes by default.
Mechanism
The electrocardiograph records the potential difference between electrodes and converts it into a vertical deflection of the stylus or digital beam, and the paper feed rate or time scale horizontally. The calibration signal is a test signal of known amplitude (1 mV) applied by the device to the amplifier input before or at the end of the recording. It demonstrates that the set gain corresponds to the specified level (10, 5, or 20 mm/mV), thereby verifying the vertical scale calibration precisely at that point in the range. It does not replace a technical check of the entire device, does not evaluate the quality of electrode-skin contact, and does not confirm the fidelity of biological signals across the entire amplitude range — a separate service check is required for that. The paper feed speed is set mechanically or programmatically and determines how many millimeters of paper correspond to one second of recording; it is an independent parameter unrelated to signal amplification. Both parameters are purely technical device settings unrelated to cardiac electrophysiology, but without knowing them, no ECG parameter can be measured correctly.
Normal Variants
The standard per the 2009 AHA/ACCF/HRS recommendations for ECG standardization is a speed of 25 mm/s and calibration of 10 mm/mV. Permissible alternatives that the device must explicitly label:
- speed of 50 mm/s — used in tachycardia and narrow complexes when greater temporal resolution is needed (e.g., for analyzing a delta wave or aberrant conduction);
- half calibration of 5 mm/mV — used for very tall waves (e.g., marked hypertrophy, to prevent complexes from overlapping);
- double calibration of 20 mm/mV — used for low-amplitude tracings to distinguish small waves.
Speed and sensitivity must be indicated in text on the recording itself. If there is no text marking, sensitivity (vertical scale) can be estimated from the height of the calibration pulse, but only if its preset value is known (typically 1 mV). Recording speed cannot be determined solely from the shape or height of the calibration pulse — it requires the device's printed label, reliable time markers on the tracing, or data from the device itself. If this information is missing, the speed cannot be reliably reconstructed from the standard calibration rectangle, and this uncertainty must be noted during interpretation.
Pathological changes
Strictly speaking, calibration and speed are not ECG findings but recording parameters, so there is no such thing as a "pathology" of the signal itself. Clinically significant problems arise via two distinct mechanisms and distort different measurements:
- mismatch between the printing speed and the actual paper feed distorts only the measurement of time intervals (PQ, QRS, QT, RR) — the horizontal scale becomes incorrect, while the vertical scale (wave amplitude) remains unchanged;
- malfunctioning or incorrectly set sensitivity (unstable calibration pulse, "floating" or distorted vertical amplitude) distorts amplitude metrics — voltage criteria for hypertrophy, low or high QRS voltage.
These two errors can occur simultaneously on the same recording, but they have different causes and different consequences, so they must be analyzed separately during interpretation.
Common interpretation errors
- Measuring intervals and amplitudes without checking the calibration pulse and speed marking — especially when reading a printout from an unfamiliar device or archival tracing.
- Automatically assuming "25 mm/s, 10 mm/mV" by default when the recording itself shows other values (a frequent cause of misdiagnosing left ventricular hypertrophy or interval shortening/prolongation).
- Confusion when reading a tracing at 50 mm/s: intervals appear twice as long if boxes are counted using the rules for 25 mm/s (e.g., apparent QT or QRS prolongation).
- Ignoring half or double calibration when calculating voltage criteria for hypertrophy (Sokolow-Lyon index, Cornell criterion) — the result is off by a factor of two.
- Comparing serial ECGs of the same patient recorded at different calibrations/speeds without recalculation — creates false dynamics.
1. What is the standard amplitude calibration on an ECG according to AHA/ACCF/HRS recommendations?
The standard is 10 mm corresponding to 1 mV, meaning 1 mm vertically equals 0.1 mV.
2. What is the duration of one millimeter horizontally at a recording speed of 50 mm/s?
At 50 mm/s, the paper moves twice as fast as at 25 mm/s, so 1 mm corresponds to 0.02 s instead of 0.04 s.
3. The ECG was recorded with a calibration of 5 mm = 1 mV. How to correctly determine the true wave amplitude in mV?
At a sensitivity of 5 mm/mV, 1 mm corresponds to 0.2 mV, so the amplitude in mm must be divided by 5 (the actual scale in mm/mV) rather than the standard 10.
Frequently asked questions
How to verify that an ECG was recorded with standard calibration?
You need to find the rectangular calibration pulse on the tracing (usually at the beginning or end of the recording) and measure its height. If the height is 10 mm, the calibration is standard (1 mV = 10 mm). If the machine prints a digital speed and sensitivity marker, it duplicates this information.
What changes if an ECG is recorded at a speed of 50 mm/s?
All horizontal distances on the tracing are stretched twofold compared to 25 mm/s, so for time calculations, 1 mm corresponds to 0.02 s rather than 0.04 s. Wave amplitudes (vertical dimensions) do not change, as paper speed does not affect signal gain.
Why is half-calibration (5 mm/mV) sometimes used?
This is done in cases of very tall R or S waves, when at the standard 10 mm/mV complexes from adjacent leads overlap or go beyond the paper edge. With this calibration, 1 mm corresponds to 0.2 mV, meaning the amplitude measured in mm must be divided by 5 instead of 10 to obtain mV.
Is it possible to compare ECGs recorded at different speeds or calibrations?
Yes, but only after converting millimeter measurements into standard units (ms and mV) taking into account the actual speed and sensitivity of each recording. Direct 'box-to-box' comparison without conversion yields false dynamics.
Where on a real ECG should one look for the speed and calibration marker?
Modern machines print these values as text next to the recording (e.g., '25 mm/s, 10 mm/mV') and additionally display a rectangular calibration pulse at the beginning or end of the strip. If there is no text marker, sensitivity can be estimated from the height of the calibration pulse given its known value, while speed can only be determined from time markers or device data; if this is not possible, the uncertainty must be accounted for during interpretation.
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
- Goldberger A.L. Clinical Electrocardiography: A Simplified Approach
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.