Normal ecg waves interval and segment boxes and time interval

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Here is a complete, structured breakdown of normal ECG waves, segments, and intervals - with the standard ECG grid boxes reference.

ECG Paper - The Grid

Box	Time (horizontal)	Voltage (vertical)
Small box	0.04 sec (40 ms)	0.1 mV
Large box (5 small)	0.20 sec (200 ms)	0.5 mV
5 large boxes	1.0 sec	1 mV (standard calibration)

Standard paper speed: 25 mm/sec

Waves, Segments & Intervals Diagram

ECG waves segments and intervals labeled

Individual Waves

Wave	Represents	Normal Duration	Normal Amplitude
P wave	Atrial depolarization	0.08 - 0.10 sec (2 - 2.5 small boxes)	< 2.5 mm tall, < 0.12 sec wide
Q wave	Septal depolarization (initial)	< 0.04 sec (< 1 small box)	< 25% of R wave height
R wave	Ventricular depolarization (main)	Part of QRS	Varies by lead
S wave	Terminal ventricular depolarization	Part of QRS	Varies by lead
T wave	Ventricular repolarization	0.16 sec approx	Upright in most leads
U wave	Purkinje repolarization (if visible)	Small, follows T	Same direction as T wave

Segments (isoelectric lines between waves)

Segment	From - To	Normal Duration	Significance
PR segment	End of P wave → Start of QRS	0.06 - 0.10 sec	AV nodal conduction delay; normally at baseline
ST segment	End of QRS (J point) → Start of T wave	0.08 - 0.12 sec	Ventricular repolarization start; should be at baseline (±1 mm)
TP segment	End of T wave → Start of next P wave	Varies with heart rate	True isoelectric baseline reference

Intervals (include waves + segments together)

Interval	From - To	Normal Duration	Boxes (small)	Significance
PR interval	Start of P wave → Start of QRS	0.12 - 0.20 sec	3 - 5 small boxes	AV conduction time; prolonged = 1st degree block
QRS duration	Start of Q → End of S	< 0.12 sec	< 3 small boxes	Ventricular depolarization; wide = bundle branch block
QT interval	Start of QRS → End of T wave	0.35 - 0.45 sec (rate-dependent)	9 - 11 small boxes at 60 bpm	Ventricular depolarization + repolarization; prolonged = torsades risk
QTc (corrected)	QT corrected for heart rate (Bazett)	< 0.44 sec (men), < 0.46 sec (women)	-	Normal upper limit
RR interval	R peak to next R peak	0.60 - 1.0 sec (60 - 100 bpm)	3 - 5 large boxes	Heart rate: 300 ÷ number of large boxes
PP interval	P wave to next P wave	Same as RR in sinus rhythm	-	Atrial rate

Quick Box-Counting Rules

What you're measuring	Rule
Heart rate	300 ÷ large boxes between R waves (or 1500 ÷ small boxes)
PR interval normal	3 to 5 small boxes
QRS normal	Less than 3 small boxes
P wave width normal	Less than 3 small boxes
ST elevation/depression	Measured 1 small box after J point; abnormal if > 1 mm

Summary Visual (ECG with grid)

ECG trace on grid showing PR, QT, ST intervals

Key takeaways:

Small box = 0.04 sec / 0.1 mV
PR interval: 3-5 small boxes (0.12-0.20 sec)
QRS: under 3 small boxes (< 0.12 sec)
QT interval: roughly 9-11 small boxes at normal heart rates
ST segment sits at the isoelectric line (baseline)

Sources: CV Physiology ECG reference | Normal ECG Intervals - Medscape | University of Nottingham Cardiology

Wbat about significant height

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ECG wave amplitude height normal values mV boxes P wave R wave T wave

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https://pmc.ncbi.nlm.nih.gov/articles/PMC1122394

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Here is the complete guide to ECG wave heights (amplitude/voltage) - the vertical axis measurements.

ECG Vertical Scale (Voltage)

Box	Voltage
1 small box	0.1 mV (1 mm)
1 large box (5 small)	0.5 mV (5 mm)
2 large boxes	1.0 mV (10 mm) = standard calibration mark

Normal Wave Heights (Amplitude)

P Wave

Parameter	Normal Value	Boxes
Height (amplitude)	≤ 2.5 mm (0.25 mV)	≤ 2.5 small boxes
Width (duration)	< 0.12 sec	< 3 small boxes
Abnormal: tall P (P pulmonale)	> 2.5 mm in lead II	Right atrial enlargement
Abnormal: wide/bifid P (P mitrale)	> 0.12 sec, notched	Left atrial enlargement

Q Wave

Parameter	Normal Value
Depth	< 2 small boxes (< 2 mm / < 0.2 mV)
Width	< 1 small box (< 0.04 sec)
Relative size	< 25% of corresponding R wave height
Pathological Q	≥ 2 mm deep AND ≥ 0.04 sec wide = old MI

R Wave

Lead	Normal Height
Limb leads (I, II, III)	Varies; R in lead I < 13 mm
Precordial (V1-V6)	Progressive increase V1 → V5
V1	Small r wave normal; tall R = RVH
V5/V6	Usually < 27 mm
S wave depth	Should not exceed 30 mm
R + S (Sokolow-Lyon for LVH)	S in V1 + R in V5/V6 > 35 mm = LVH
R wave in aVL	> 11 mm = LVH

R wave progression - R wave gets taller from V1 to V5, then may decrease slightly in V6. Transition (where R = S) usually at V3 or V4.

T Wave

Parameter	Normal Value
Limb leads	Rarely > 6 mm
Chest leads V2-V3 (men)	Up to 10 mm
Chest leads V2-V3 (women)	Up to 8 mm
Relative to R wave	≥ 1/8 and < 2/3 of corresponding R wave amplitude
Direction	Upright in I, II, V3-V6; inverted in aVR; variable in III, aVL, V1
Abnormal: tall peaked T	> 10 mm (men) or > 8 mm (women) - think hyperkalemia
Abnormal: inverted T	Ischemia, strain, or bundle branch block

ST Segment Height

Parameter	Normal Value
ST elevation	≤ 1 mm in limb leads
ST elevation	≤ 2 mm in precordial leads (men), ≤ 1.5 mm in women
ST depression	< 0.5 mm (> 1 mm is significant)
Measured at	1 small box (0.04 sec) after the J point

Quick Summary Table

Wave	Max Normal Height	Abnormal if
P wave	2.5 mm (0.25 mV)	> 2.5 mm = atrial enlargement
Q wave	< 2 mm deep, < 25% of R	≥ 2 mm + wide = pathological Q
R wave (V5/V6)	< 27 mm	> 27 mm or Sokolow > 35 mm = LVH
T wave	< 6 mm (limb), < 10 mm (chest)	Peaked = hyperkalemia; inverted = ischemia
ST segment	≤ 1 mm elevation	> 1-2 mm elevation = MI / > 1 mm depression = ischemia
U wave	< 1.5 mm	> 1.5 mm or inverted = hypokalemia/ischemia

Key rule: On standard ECG paper, 10 mm = 1 mV (calibration pulse). So every millimeter of height = 0.1 mV. Wave height is measured from the isoelectric baseline (TP segment) to the peak of the wave.

Sources: BMJ - ECG Basic Terminology (PMC) | ECG Waves - T wave guide | LITFL P wave library

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