How Fast a Parked Car Heats Up
A parked car acts like a small greenhouse: sunlight passes through the glass, warms the dashboard, seats, and interior surfaces, and that heat radiates back at wavelengths that don't escape through the glass as easily, so it builds up in the cabin air. Peer-reviewed research (McLaren, Null & Quinn, 2005, Pediatrics) measured this rise directly across a range of outside temperatures from 72–96°F (22–36°C) and found a strikingly consistent pattern: the cabin heats up fast at first, then more slowly, but keeps climbing well past the first hour. This calculator applies that published rise curve to your outside temperature, time parked, and sun exposure to estimate the interior temperature and how long until it crosses the 104°F (40°C) heat-stroke threshold.
How the temperature-rise curve works
The study's measured average temperature increase above outside air was about +19°F at 10 minutes, +29°F at 20 minutes, +34°F at 30 minutes, +38°F at 40 minutes, +41°F at 50 minutes, and +43°F at 60 minutes — roughly 80% of the total rise happens in that first half hour, after which the curve flattens as the cabin approaches equilibrium with incoming solar heat. This calculator interpolates along that published curve, then extends it gently for times beyond 60 minutes. A "partial shade" or "full shade" selection scales the modeled rise down, since a shaded cabin still heats up but more slowly than one in direct sun — this shade adjustment is an approximation, since the original study measured vehicles in direct sun.
Why cracking a window doesn't help enough
The same research tested vehicles with windows cracked open roughly 1.5 inches and found no statistically significant reduction in how hot the interior got or how fast it heated up. A small gap lets some air exchange but does almost nothing against direct solar heating of the dashboard and seats, which is the dominant driver of cabin temperature. That is why cracking a window is never treated as a safety measure for a person or pet left in a vehicle, regardless of the outside temperature.
Why the danger threshold is lower than it feels
A core body temperature of 104°F (40°C) marks the clinical onset of heat stroke, and temperatures above about 107°F (41.7°C) are frequently fatal. Because a child's body warms 3 to 5 times faster than an adult's, a cabin that "only" feels warm to an adult stepping back in can already be dangerous for an infant, young child, elderly adult, or pet who has been inside the whole time. This is why pediatric and vehicle-safety organizations describe every parked-car interval as unsafe, not just intervals above a specific number on this calculator.
Frequently Asked Questions
How hot can a car get inside in just 10 minutes?
Does cracking the windows help keep a car cooler?
What interior temperature is dangerous for a child or pet?
Why do children overheat faster than adults in hot cars?
Educational tool, not a safety plan: this calculator gives an estimate based on published averages, not a measurement of your specific vehicle. Never leave a child, pet, or dependent adult alone in a parked vehicle for any length of time, in any weather, with or without windows cracked — if you see someone else's child or pet alone in a hot car, call 911 or your local emergency number immediately.