Hot Car Temperature Calculator: How Hot Does a Parked Car Get?

How it works

Cabin air temperature = outside temperature + average rise for the time parked · Average rise (closed car in sun): 10 min +19 °F (10.6 °C), 20 min +29 °F (16.1 °C), 30 min +34 °F (18.9 °C), 40 min +38 °F (21.1 °C), 50 min +41 °F (22.8 °C), 60 min +43 °F (23.9 °C), 2 h about +50 °F (27.8 °C), 4 h about +55 °F (30.6 °C); smooth (monotone) interpolation between these points

Sunlight passes through the glass and heats the dashboard, seats and steering wheel, which then heat the air inside. Because the car is closed, that heat builds up quickly. Measurements by meteorologist Jan Null at San Jose State University, published in Pediatrics in 2005 (McLaren, Null and Quinn), found that a closed car in the sun warmed by an average of about 19 °F in 10 minutes, 29 °F in 20, 34 °F in 30 and 43 °F in an hour, with about 80% of the rise in the first half hour. The rate of rise was much the same whatever the outside temperature, so the cabin temperature is roughly the outside temperature plus that rise. This calculator adds the average rise for your parking time to the outside temperature, drawing a smooth curve through the published averages, and tells you how long the cabin takes to pass a warning temperature, 104 °F (40 °C) by default, the body temperature at which heatstroke begins. The table and chart show the build-up over four hours. These are averages from one mid-size sedan on sunny days of 72–96 °F; individual cars and days vary by several degrees either way, and the hottest surfaces get far hotter than the air. Shade and clouds slow the rise but do not prevent it. Unlike the heat index or WBGT calculators, which describe outdoor conditions, this estimates the air inside a parked car.

Worked example

On an 86 °F (30 °C) sunny day, after 30 minutes the cabin air has risen about 34 °F, so it is about 86 + 34 = 120 °F (49 °C) inside. It passes 104 °F (40 °C) after about 10 minutes (the rise needed is 104 − 86 = 18 °F, reached at 9.4 minutes), and the 30-minute temperature is 16 °F above that warning level. After an hour the cabin is about 86 + 43 = 129 °F (54 °C).

Sources: Jan Null, San Jose State University: Vehicle heating dynamics (average rise by elapsed time) · McLaren C, Null J, Quinn J. Heat stress from enclosed vehicles. Pediatrics 2005;116(1):e109–e112 · Jan Null, San Jose State University: Estimated vehicle interior air temperature v. elapsed time (table for 70–95 °F, 10–60 min) · NHTSA: Child heatstroke prevention (body heats 3–5 times faster; heatstroke at about 104 °F)

Frequently asked questions