How fast a parked car heats up
These are the average rises measured in a closed car parked in the sun, added to the outside temperature:
| Time parked | Rise inside | On a 70°F day | On an 80°F day | On a 90°F day |
|---|---|---|---|---|
| 10 minutes | +19°F | 89°F | 99°F | 109°F |
| 20 minutes | +29°F | 99°F | 109°F | 119°F |
| 30 minutes | +34°F | 104°F | 114°F | 124°F |
| 40 minutes | +38°F | 108°F | 118°F | 128°F |
| 50 minutes | +41°F | 111°F | 121°F | 131°F |
| 60 minutes | +43°F | 113°F | 123°F | 133°F |
| 1 to 2 hours | +45 to 50°F | 115 to 120°F | 125 to 130°F | 135 to 140°F |
The figures come from Jan Null of San José State University, who measured a closed, dark sedan with Catherine McLaren and James Quinn on 16 clear days in 2002, with outside temperatures from 72°F to 96°F (published in Pediatrics in 2005). The calculator applies the same rise at any outside temperature, so treat results outside that range as an estimate.
Most of the heat builds in the first half hour
About 80% of the rise happened in the first 30 minutes. Even on the coolest day measured, the inside of the car reached 117°F. A quick errand is long enough: after 10 minutes the car is already about 19°F hotter than the air outside. Cracking the windows open 1.5 inches made no difference to how fast it heated or how hot it got.
Why children are most at risk
According to NHTSA, a child's body heats up 3 to 5 times faster than an adult's. Heatstroke begins when core body temperature reaches about 104°F, and about 107°F is lethal. Since 1998, an average of 37 children a year have died of vehicular heatstroke in the US, more than half of them under 2.
Most were not left on purpose. From 1998 to 2025, 52.9% were forgotten by a caregiver, 23.8% got into an unlocked car on their own, and 21.9% were knowingly left in the car.
How to make sure it never happens
The American Academy of Pediatrics recommends:
- Never leave a child alone in a car, not for a minute, not with the windows down.
- Always check the back seat and make sure every child is out before you lock the car and walk away.
- Put your phone, bag or purse in the back seat, so you look there when you arrive.
- Be extra alert when your routine changes, like when someone else drives your child or you take a different route to child care.
- Ask your child care provider to call you if your child is more than 10 minutes late.
- Keep your car locked when it's parked and the keys out of children's reach, so a child can't climb in on their own.
A rear-facing baby is out of sight from the driver's seat, and a quiet, sleeping one is easy to overlook. A baby car mirror keeps them in view while you drive; the habit of checking the back seat is what protects them when you park.