Heat Index Health Risk Calculator
Calculate the heat index (feels-like temperature) from air temperature and humidity. Get NWS health risk classification and outdoor activity safety guidelines.
Car Heat Calculator
Estimate interior car temperature based on outside temperature, sun exposure, window status, and vehicle color. Includes temperature timeline, heatstroke warning signs, and safety guidance.
🚨 NEVER leave children or pets in a parked vehicle. On a 70°F day, a car interior can reach 104°F in 30 minutes. On a 90°F day, interior temperatures can exceed 130°F. Over 900 children have died from vehicular heatstroke in the US since 1998. Call 911 immediately if you see a child or pet in a hot car.
°F
minutes
Estimated Interior Temperature after 15 minutes
106.2°F / 41.2°C
Temperature rise: +21.2°F (max possible: +47°F)
Interior Temperature⧉
106.2°F (41.2°C)
After 15 minutes with full sun, windows closed.
Dashboard Temperature⧉
136.2°F
Dashboard/steering wheel can be 20-40°F hotter than air temperature due to direct solar absorption.
Temperature Rise⧉
+21.2°F
Maximum possible rise: +47°F at thermal equilibrium (~60 min).
Time to 120°F (49°C)⧉
~34 min
Time for interior to reach 120°F — lethal heatstroke zone for children and pets.
Time to 104°F⧉
~13 min
Human heatstroke begins at core body temperature of 104°F (40°C). This is the ambient threshold.
Cracked Windows?⧉
No ventilation
Cracking windows provides minimal temperature reduction. Studies show only a 3-5°F difference after 30 minutes.
Temperature Timeline
| Minutes | Interior °F | Interior °C | Level |
|---|---|---|---|
| 0 | 85 | 29.4 | |
| 5 | 93.5 | 34.2 | |
| 10 | 100.5 | 38.1 | |
| 15 | 106.2 | 41.2 | |
| 20 | 110.9 | 43.8 | |
| 25 | 114.7 | 45.9 | |
| 30 | 117.8 | 47.7 | |
| 35 | 120.4 | 49.1 | |
| 40 | 122.5 | 50.3 | |
| 45 | 124.2 | 51.2 | |
| 50 | 125.6 | 52 | |
| 55 | 126.8 | 52.7 | |
| 60 | 127.7 | 53.2 |
Heatstroke Warning Signs
| Sign | Severity |
|---|---|
| Red, hot, dry skin | Warning |
| Rapid, strong pulse | Warning |
| Throbbing headache | Warning |
| Dizziness / nausea | Warning |
| Confusion / slurred speech | Emergency |
| Loss of consciousness | Emergency |
| Body temp > 104°F (40°C) | Emergency |
| Seizures | Emergency |
State Laws (Selected)
| State | Law | Penalty |
|---|---|---|
| California | Civil immunity for rescuers breaking into vehicles for children/animals | N/A (protects rescuers) |
| Florida | Felony to leave child <6 unattended in vehicle | Up to 5 years prison |
| Tennessee | First state to allow Good Samaritan vehicle entry for animals | Immunity from liability |
| Arizona | Leaving child in vehicle is class 6 felony | Up to 2 years prison |
| Texas | Leaving child <7 unattended in vehicle for >5 minutes | Class C misdemeanor to state jail felony |
Planning notes, formulas, and examples
About the Car Heat Calculator
Every year, approximately 38 children die from vehicular heatstroke in the United States alone. A car's interior can heat to dangerous levels in just 10-15 minutes, even on seemingly mild days. On a 70°F (21°C) day, a parked car in full sun can reach over 104°F (40°C) in 30 minutes — well into the heatstroke danger zone.
This calculator models the greenhouse effect inside a parked vehicle using data from San Francisco State University's Department of Geosciences hotcar studies. It accounts for outside temperature, sun exposure (full, partial, shade), window status (closed, cracked, half open), and vehicle color (dark colors absorb more heat). The exponential heating model shows how rapidly temperatures rise, with most of the total temperature increase occurring in the first 30 minutes.
Critically, cracking windows provides minimal protection — studies consistently show only a 3-5°F reduction after 30 minutes. No amount of ventilation makes a parked car safe for a child or pet on a warm day. This calculator is designed to raise awareness and provide data-driven evidence of how quickly car interiors become lethal environments.
When This Page Helps
This calculator shows how quickly a parked vehicle can move from uncomfortable to lethal, which is what makes the danger easy to underestimate. It is useful for parents, pet owners, and responders because it translates weather, sun exposure, and vehicle conditions into a concrete temperature timeline instead of a vague warning.
How to Use the Inputs
- Enter the current outside temperature in Fahrenheit or Celsius.
- Select sun exposure: full sun, partial shade, or full shade.
- Choose window status: closed, cracked (1-2 inches), or half open.
- Select vehicle color (dark colors heat faster).
- Enter the duration in minutes to estimate interior temperature.
- Review the temperature timeline and heatstroke warning signs.
Formula used
Interior temperature model:
T(t) = T_ambient + ΔT_max × (1 − e^(−t/τ))
Where:
- ΔT_max = 47°F × sun_factor × window_factor × color_factor
- τ = 25 minutes (time constant)
- Sun factor: full = 1.0, partial = 0.65, shade = 0.35
- Window factor: closed = 1.0, cracked = 0.85, half open = 0.55
- Color factor: dark = 1.15, medium = 1.0, light = 0.88Example Calculation
Result: Interior: ~110°F after 15 minutes.
Starting at 85°F with full sun, closed windows, and a dark vehicle: ΔT_max = 47 × 1.0 × 1.0 × 1.15 = 54°F. After 15 min: rise = 54 × (1 − e^(−15/25)) = 54 × 0.45 = 24.3°F. Interior ≈ 85 + 24 = 109°F — already in the heatstroke danger zone.
Tips & Best Practices
- NEVER leave a child or pet unattended in a vehicle, even for "just a minute."
- Place your phone, purse, or shoe in the back seat as a reminder to check before locking.
- Use windshield sun shades — they reduce interior temperatures by 15-20°F.
- Park in shade when possible, but remember shade shifts during the day.
- If you see a child or animal in distress in a hot car, call 911 immediately.
The Physics of Car Heating
Cars heat through the greenhouse effect: visible sunlight passes through glass, is absorbed by dark interior surfaces (dashboard, seats, steering wheel), and re-emitted as infrared radiation that cannot escape through glass. This traps thermal energy inside the vehicle, causing rapid temperature rise. Dark-colored interiors absorb more radiation than light-colored ones, and dark-colored exterior paint increases overall heat absorption by the vehicle body.
Children and Vehicular Heatstroke
Children are disproportionately affected by hot car deaths for physiological and behavioral reasons. Their body surface area-to-mass ratio is 3× higher than adults, causing them to absorb heat faster. Their thermoregulatory system is less developed, and they cannot exit a vehicle independently. Over 50% of pediatric vehicular heatstroke deaths involve a caregiver who forgot the child was in the car — a phenomenon related to "Forgotten Baby Syndrome," a failure of prospective memory under stress or routine change.
Prevention Technology
Many newer vehicles include rear-seat reminder systems, and modern Euro NCAP protocols also incorporate rear-occupant alerts. Technologies include weight sensors, ultrasonic motion detection, and smartphone alerts when a vehicle is locked with a rear passenger detected. Several aftermarket devices (SensorSafe, Elepho eClip) provide additional protection. Public awareness campaigns like "Look Before You Lock" by NHTSA have helped reduce — but not eliminate — these preventable deaths.
Sources & Methodology
Last updated:
Methodology
This page applies an idealized parked-car heat-rise curve to the entered outdoor temperature, sun exposure, window position, vehicle color, and elapsed time. The model is meant to show how quickly temperature can escalate under common conditions rather than to predict the exact cabin temperature in a specific vehicle.
The result should be read as a safety illustration, not as a reason to test the boundary. Real cabin temperatures depend on vehicle shape, glass area, interior color, humidity, wind, cloud cover, and parking surface, and dangerous heat stress can occur before the estimate reaches its final value.
Sources
- Heatstroke Deaths of Children in Vehicles (National Highway Traffic Safety Administration) — Public safety guidance on pediatric vehicular heatstroke and prevention.
- Temperature change in parked cars in various environmental conditions (PubMed / Pediatrics) — Classic parked-car heat study showing rapid cabin temperature rise.
Frequently Asked Questions
Minimally. Studies show cracking windows 1-2 inches reduces the interior temperature by only 3-5°F after 30 minutes compared to closed windows. The greenhouse effect from sunlight passing through glass and heating interior surfaces is the dominant factor, and a small opening cannot counteract it significantly.
On an 80°F day, a car interior can reach 99°F in 10 minutes and 114°F in 30 minutes. On a 100°F day, interior temperatures can exceed 140°F within 1 hour. The most rapid heating occurs in the first 15-20 minutes. Children regulate body temperature less efficiently than adults and are at higher risk.
Heatstroke occurs when core body temperature exceeds 104°F (40°C). In children, whose thermoregulation is less developed and body surface area-to-mass ratio is higher, heatstroke can occur more rapidly. Death can occur when body temperature reaches 107°F (42°C). A car interior of 120°F+ makes heatstroke nearly inevitable within 15-20 minutes.
If a child appears to be in distress in a hot car, call 911 immediately. Many US states have Good Samaritan laws that provide civil immunity for breaking a vehicle window to rescue a child or animal. Check your state laws. Always call emergency services first, then attempt to remove the child and begin cooling measures.
No. Pets (especially dogs) are more susceptible to heatstroke than humans because they cannot sweat and rely on panting for heat dissipation. Dogs can develop heatstroke within 6-10 minutes in a hot car. Many jurisdictions have laws specifically prohibiting leaving animals in unattended vehicles.
While running climate control can maintain safe temperatures, systems can fail, batteries can deplete, and children may accidentally turn off the system. No vehicle, regardless of climate control capability, should be relied upon to protect an unattended child. The safest practice is to never leave a child alone in any vehicle.
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