Shock Index Calculator
The shock index (SI) is a rapid bedside assessment of hemodynamic stability, calculated by dividing heart rate by systolic blood pressure. A normal SI is 0.5-0.7 in resting adults. Values above 1.0 indicate significant hemodynamic compromise and correlate with higher mortality, transfusion requirements, and need for emergency intervention. This calculator also computes mean arterial pressure (MAP) from systolic and diastolic blood pressure, which is a key target in septic shock management (MAP above 65 mmHg). Enter heart rate in beats per minute and blood pressure in mmHg.
Shock index and MAP formulas
Shock Index (SI) = HR / SBP
MAP = DBP + (SBP - DBP) / 3
SI is dimensionless. MAP in mmHg. Normal SI is 0.5-0.7 at rest. Normal MAP is 70-100 mmHg. MAP below 65 mmHg is the target for vasopressor intervention in septic shock (Surviving Sepsis Campaign).
Shock index interpretation
- SI below 0.6: Normal. Hemodynamically stable at rest.
- SI 0.6-0.9: Borderline. Monitor closely; may reflect compensated shock or physiological variation.
- SI 1.0-1.4: Moderate concern. Associated with increased blood loss and adverse outcomes in trauma.
- SI 1.4 or above: Severe hemodynamic instability. Immediate resuscitation and investigation required.
Frequently asked questions
What is the shock index?
The shock index (SI) is the ratio of heart rate to systolic blood pressure: SI = HR / SBP. A normal SI is 0.5-0.7 in healthy adults. An SI of 1.0 or above is associated with significant hemodynamic instability, higher blood transfusion requirements, and worse outcomes in trauma and sepsis.
What does a shock index above 1.0 mean?
An SI above 1.0 (e.g., HR 100, SBP 100) indicates that heart rate exceeds or matches systolic BP, a pattern associated with significant blood loss, distributive shock, or impaired cardiac function. In trauma, SI above 1.0 predicts need for massive transfusion and is used to activate massive transfusion protocols.
How is mean arterial pressure calculated?
MAP = diastolic BP + (1/3) x pulse pressure = DBP + (SBP - DBP)/3. Alternatively: MAP = (SBP + 2 x DBP) / 3. Normal MAP is 70-100 mmHg. A MAP below 65 mmHg is the threshold for intervention in septic shock per Surviving Sepsis Campaign guidelines.
What is the modified shock index?
The modified shock index (MSI) divides heart rate by mean arterial pressure instead of systolic BP: MSI = HR / MAP. Some studies suggest MSI may be more sensitive than SI for predicting adverse outcomes. A normal MSI is approximately 0.7-1.3.
In what clinical situations is the shock index used?
Shock index is used in trauma triage (ATLS protocol), obstetric hemorrhage (postpartum hemorrhage), sepsis assessment, and any presentation with suspected hemodynamic compromise. It provides rapid bedside assessment without laboratory results and can be calculated from two commonly measured vital signs.
Official sources
- NIH National Library of Medicine: Shock (StatPearls, NCBI Bookshelf).
- CDC Injury Center: Injury Prevention and Control.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.