Heat Index Calculator · Practical guide
When Is It Too Hot to Exercise Outside?
Use the heat index, not the thermometer, to decide whether your outdoor workout is safe today.
At a glance
The short answer
Sports medicine guidance based on National Weather Service heat index levels is straightforward: below 90°F heat index, exercise is generally safe with normal hydration. From 90 to 103°F, shorten workouts, take breaks every 15-20 minutes, and watch for warning signs. From 103 to 124°F, move your workout indoors. Above 124°F, avoid outdoor exertion entirely — heat stroke can strike in minutes.
Interactive tool
Heat Index Calculator
Enter your own values, then compare the result with the examples below.
Heat Index Results
Check the humidity, not just the temperature
A 90°F afternoon sounds manageable until you check the humidity. At 70% relative humidity, that 90°F day has a heat index of about 106°F — firmly in the Danger zone where heat exhaustion is likely during exercise. Meanwhile 95°F in dry air at 25% humidity feels like about 94°F and is far safer. Enter both numbers in the calculator above before you head out.
| Heat Index (°F) | Exercise Guidance |
|---|---|
| Below 90 | Safe for most people; hydrate normally |
| 90 - 103 | Shorten sessions, add rest breaks, watch for cramps |
| 103 - 124 | Move indoors; high risk of heat exhaustion |
| Above 124 | No outdoor exercise; heat stroke highly likely |
Warning signs to stop immediately
Stop exercising and get to air conditioning if you notice dizziness, headache, nausea, muscle cramps, or unusually heavy sweating that suddenly stops. Confusion, slurred speech, or hot dry skin signals heat stroke — call 911 and cool the person with whatever is available while waiting for help.
Practical adjustments for hot days
If the heat index is in the 90-103°F range and you still want to train, shift your workout to before 10 a.m. or after 6 p.m., cut intensity by about a third, drink 16-24 ounces of water in the two hours beforehand, and wear light-colored breathable clothing. Give yourself 10-14 days to acclimate to summer heat before returning to full-intensity sessions.
Frequently Asked Questions
Is it safe to run in 90-degree weather?
It depends on humidity. At 90°F with 40% humidity the heat index is about 91°F, so a shortened, easy run with water is reasonable. At 70% humidity the heat index is about 106°F and you should move indoors.
How long does it take to acclimate to heat?
Most people adapt to exercising in heat over 10 to 14 days of gradually increasing exposure. Until acclimated, cut your usual intensity and duration significantly.
How much water should I drink when exercising in heat?
A common guideline is 16-24 ounces in the two hours before exercise, then 6-12 ounces every 15-20 minutes during the workout. For sessions over an hour, include electrolytes.
Are some people at higher risk in hot weather?
Yes. Children, adults over 65, people with heart conditions, and those taking certain medications (like diuretics or antihistamines) should use more conservative heat index limits.