PSI Calculator — Physiological Strain Index
Calculate the Physiological Strain Index (PSI) — a 0–10 scale combining core body temperature and heart rate to quantify physiological strain during exercise or heat exposure. Enter your current and resting values to see the strain level.
°C
bpm
°C
bpm
High strain
- 1
Thermal component
5 × (38.2 − 36.5) ÷ (39.5 − 36.5) = 2.83Scales the core temperature rise against the maximum tolerable rise (to 39.5 °C). - 2
Cardiovascular component
5 × (140 − 60) ÷ (180 − 60) = 3.33 - 3
Physiological Strain Index
2.83 + 3.33 = 6.17
How does this calculator work?
PSI = 5 × (Tc − Tc₀)/(39.5 − Tc₀) + 5 × (HR − HR₀)/(180 − HR₀). A score of 0–3 is low strain, 4–5 moderate, 6–7 high, and 8–10 very high to maximal strain during heat exposure. Reference: Moran et al., European Journal of Applied Physiology, 1998.
Formula
How this is calculated
The Physiological Strain Index (PSI) was developed by Moran et al. (1998) to provide a single, easy-to-interpret measure of physiological strain during heat stress. It combines two components that each contribute up to 5 points: a thermal component based on the rise in core body temperature (Tc) above its resting value (Tc₀ ≈ 36.5 °C), scaled to a maximum expected core temperature of 39.5 °C; and a cardiovascular component based on the rise in heart rate (HR) above resting (HR₀), scaled to a maximum expected HR of 180 bpm. PSI therefore ranges from 0 (no strain relative to rest) to 10 (maximal strain at both limits simultaneously).
Core temperature is most accurately measured rectally or with an oesophageal probe in research settings; ingestible gut-temperature pills are a practical field alternative. Tympanic (ear) and axillary (armpit) measurements are less accurate and may underestimate true core temperature, especially during exercise. PSI assumes a typical healthy adult; athlete populations may have lower resting values and higher exercise tolerances, shifting the practical interpretation of each PSI band.
PSI levels of 7–8 signal high physiological strain requiring attention, while 9–10 indicate potential medical risk; guidelines from occupational health bodies generally recommend work-rest schedules that keep workers below PSI 7. The index is validated for moderate-intensity exercise in temperate and hot environments and has been used in military, athletic and industrial settings. It is descriptive, not diagnostic — always combine it with clinical judgment and direct observation.
Frequently asked questions
PSI values of 0–3 indicate low physiological strain and are generally safe for continued activity. Values of 4–5 indicate moderate strain — normal during vigorous exercise in warm conditions. Values of 6–7 indicate high strain; activity intensity or heat load should be reduced. Values of 8–10 indicate very high or maximal strain, with increasing risk of heat illness; rest and cooling are recommended.
Rectal temperature is the clinical gold standard but impractical in field settings. Ingestible temperature-sensing pills (telemetry capsules) are the most accurate non-invasive alternative. Oesophageal sensors are used in lab research. Tympanic (ear) measurements are convenient but underestimate core temperature during exercise by 0.5–1 °C or more, leading to an underestimated PSI.
PSI was validated primarily in young, healthy adults. Older adults, people with cardiovascular conditions, and those unaccustomed to heat stress may experience higher physiological strain at lower PSI values than the scale suggests. Athletes with strong cardiovascular fitness may show lower heart-rate responses and therefore lower PSI despite significant thermal load. Use PSI as a guide rather than an absolute threshold.
Also known as
TG we-Calculate Editorial Team. (2026). PSI Calculator — Physiological Strain Index [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/psi-calculator
TG we-Calculate Editorial Team. "PSI Calculator — Physiological Strain Index." TG we-Calculate. 2026. https://we-calculate.com/calculator/psi-calculator.
TG we-Calculate Editorial Team, "PSI Calculator — Physiological Strain Index," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/psi-calculator
@misc{wecalculate_psi_calculator, title = {PSI Calculator — Physiological Strain Index}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/psi-calculator}}, year = {2026}, note = {TG we-Calculate} }
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