What this calculator does
Shock index is one of the simplest vital-sign calculations there is: heart rate over systolic blood pressure. It exists because the two move in opposite directions under circulatory stress, so the ratio shifts earlier than either figure alone.
A resting value normally sits between 0.5 and 0.7. A heart rate of 120 against a systolic of 120 gives exactly 1.00, and values approaching or above that are treated as a warning sign in emergency assessment.
The formula
Divide heart rate in beats per minute by systolic blood pressure in mmHg. Both are routine observations, which is what makes the index quick to apply.
| Term | Meaning |
|---|---|
| Shock index | Heart rate divided by systolic blood pressure. |
| Compensation | The body raising heart rate to maintain output as pressure falls, which the ratio detects. |
| Typical resting range | Around 0.5 to 0.7 in a healthy adult at rest. |
The inputs explained
| Field | What to enter |
|---|---|
| Heart rate (bpm) | Heart rate in beats per minute. |
| Systolic blood pressure (mmHg) | Systolic blood pressure in mmHg, the upper figure. |
When to use it
Triage assessment
The index is used in emergency and trauma settings as a rapid severity marker.
Spotting early compensation
It can shift before blood pressure alone falls outside the normal range.
Understanding observation charts
Seeing how the two vitals combine explains why both are recorded together.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does heart rate move the index?
The same blood pressure at four heart rates.
| Heart rate | Shock index | Reading |
|---|---|---|
| 60 bpm | 0.50 | Within the typical resting range |
| 80 bpm | 0.67 | Within the typical resting range |
| 100 bpm | 0.83 | Slightly above the typical range |
| 120 bpm | 1.00 | Elevated: seek medical attention if this is a real reading |
Questions
Why not just look at blood pressure?
Because blood pressure can stay normal well into circulatory compromise while the heart rate rises to compensate. The ratio picks up that compensation, which is the argument for using it alongside the individual readings.
What value is concerning?
Above 0.9 is commonly cited as a threshold warranting attention, and above 1.0 more so. Thresholds vary between protocols and populations, and the index is used as one input into assessment rather than as a rule.
Does it work in everyone?
Less well in some groups. Beta blockers blunt the heart rate response, older adults compensate differently, and athletes have low resting rates. In all of these the index can reassure when it should not.
Should I act on a reading at home?
A single home reading is not a clinical assessment, and home devices vary in accuracy. If you feel unwell, seek medical attention based on how you feel rather than on a calculated ratio.
For cardiac workload from the same two readings, see the rate pressure product calculator. For average circulatory pressure, see the mean arterial pressure calculator.