Shock Index Calculator (HR ÷ SBP, modified and age shock index)

Understanding the shock index calculator — concept and principle

The shock index (SI) divides heart rate by systolic blood pressure. It is a quick bedside sign of hidden blood loss or circulatory stress, often abnormal before blood pressure itself falls. The calculator also gives the modified shock index (using mean arterial pressure) and the age shock index.

Pulse ÷ systolic: with the default inputs the result is SI 1.14 (1.0 or more → higher risk).

How it is calculated

Shock index = heart rate ÷ systolic blood pressure

Mean arterial pressure (MAP) ≈ (systolic + 2 × diastolic) ÷ 3

Modified shock index = heart rate ÷ MAP

Age shock index = age × shock index

The principle

When blood volume falls, the body first speeds up the heart to keep blood pressure up. So heart rate rises while systolic pressure stays nearly normal, and their ratio rises early. In healthy adults it is usually 0.5–0.7. As compensation fails, pressure falls and the ratio rises further. The age version helps in older adults, whose heart rate response is often blunted.

Worked example

Default inputs: HR 112, BP 98/62 mmHg, age 58.

Shock index = 112 ÷ 98 = 1.14 → 1.0 or more: higher risk.
MAP = (98 + 2 × 62) ÷ 3 = 222 ÷ 3 = 74 mmHg.
Modified shock index = 112 ÷ 74 = 1.51.
Age shock index = 58 × 1.143 = 66.3.

Each point is the calculator re-run with only “Heart rate (beats/min)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Heart rate (beats/min)	Shock index (HR ÷ SBP)
28	0.29
56	0.57
84	0.86
112	1.14
140	1.43
168	1.71
224	2.29

How to read the result

Where does your result fall?

In emergency department studies an SI above 0.9 was linked with needing massive transfusion after injury, and 1.0 or more with higher mortality. A raised SI should prompt a search for bleeding, sepsis or other causes of shock. A normal SI does not rule them out.

Common mistakes

Ignoring beta-blockers or pacemakers, which blunt the heart rate rise.
Using adult cut-offs for children, who have higher normal heart rates.
Measuring during pain or anxiety without repeating.

Keep in mind: This is a screening sign for clinicians, not a diagnosis. Signs of shock such as cold clammy skin, confusion or fainting need emergency care.

Frequently asked questions

What is a normal shock index?

About 0.5–0.7 in healthy adults.

Why use MAP instead of systolic pressure?

MAP reflects diastolic pressure too, which may make the modified index more sensitive in some studies.

Is the shock index useful in pregnancy?

Normal values differ in pregnancy; obstetric studies use their own thresholds.

Sources: Allgöwer M, Burri C, "Schockindex", Dtsch Med Wochenschr 1967;92(43):1947–1950 · Koch E, Lovett S, Nghiem T, et al., "Shock index in the emergency department: utility and limitations", Open Access Emerg Med 2019;11:179–199

Checked 9 Nov 2026. Koch E, Lovett S, Nghiem T, Riggs RA, Rech MA, “Shock index in the emergency department: utility and limitations”, Open Access Emerg Med 2019;11:179–199 (definition, normal range 0.5–0.7, thresholds, modified and age shock index); original index: Allgöwer M, Burri C, Dtsch Med Wochenschr 1967.

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