Hot Car Calculator (how hot a parked car gets)

Understanding the hot car temperature calculator — concept and principle

This calculator estimates how hot the air inside a closed car parked in the sun can get after a given number of minutes, from the outside temperature. It is meant to show how quickly a parked car becomes dangerous for children and pets, even on a mild day.

A parked car heats fast: with the default inputs the result is ≈ 43 °C inside (≈ 110 °F).

How it is calculated

The temperature rise comes from measurements in a closed dark sedan on clear, sunny days (McLaren, Null and Quinn, 2005):

after 10 min +19 °F (11 °C), 20 min +29 °F (16 °C), 30 min +34 °F (19 °C), 40 min +38 °F (21 °C), 50 min +41 °F (23 °C), 60 min +43 °F (24 °C).
Between those points the calculator interpolates in a straight line.
Inside temperature = outside temperature + rise; the share of the 60-minute rise = rise ÷ 43 °F.
The study covered outside temperatures from 72 to 96 °F (22–36 °C).

The principle

Sunlight passes through the windows and is absorbed by the dashboard, seats and other surfaces, which heat up and warm the air. The heat cannot escape easily through closed glass, so the cabin acts like a greenhouse. The study found the rise was roughly the same whatever the outside temperature in its range, so even a pleasant day can produce dangerous cabin temperatures. About 80% of the rise happened in the first 30 minutes, and leaving the windows slightly open made little difference. Children heat up faster than adults, which is why the danger is so great for them.

Worked example

Default inputs: outside 27 °C, parked 20 minutes.

27 °C = 80.6 °F, which is within the study's 72–96 °F range: yes.
Rise after 20 minutes = 29 °F = 29 × 5 ÷ 9 = 16.1 °C, shown as 16 °C.
Inside = 27 + 16.1 = 43.1, shown as 43 °C (80.6 + 29 = 110 °F).
Share of the 60-minute rise = 29 ÷ 43 = 67%.

Each point is the calculator re-run with only “Outside air temperature” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Outside air temperature	Estimated temperature inside the car
6.75	23 °C
13.5	30 °C
20.2	36 °C
27	43 °C
33.8	50 °C
40.5	57 °C
54	70 °C

How to read the result

A cabin above 40 °C can quickly cause heatstroke in a child or a pet. The estimate is for a typical case; cloud and shade lower it, while a dark interior, a small car or a very sunny day can raise it. Outside the measured temperature range the result is less certain.

Common mistakes

Thinking a few minutes is safe.
Believing that cracking the windows keeps the car cool.
Assuming a cool morning means no risk; the study saw large rises at 72 °F.
Forgetting that pets cool themselves poorly by panting in hot, still air.

Keep in mind: Never leave a child or a pet in a parked car, not even for a minute. If you see one alone in a hot car, call the emergency number straight away.

Frequently asked questions

How hot does a car get in 10 minutes?

About 11 °C (19 °F) above the outside temperature in the study's sunny conditions.

Does opening the windows a little help?

Very little. The study found cracking the windows did not prevent the rise.

Is it safe on a cool day?

No. Even at about 22 °C outside, the cabin can rise well above 40 °C within an hour.

Sources: McLaren C., Null J., Quinn J. (2005), "Heat Stress From Enclosed Vehicles: Moderate Ambient Temperatures Cause Significant Temperature Rise in Enclosed Vehicles", Pediatrics 116(1):e109–e112 · J. Null, San José State University, "Estimated Vehicle Interior Air Temperature v. Elapsed Time"

Checked 9 Nov 2026. McLaren C., Null J., Quinn J. (2005), “Heat Stress From Enclosed Vehicles: Moderate Ambient Temperatures Cause Significant Temperature Rise in Enclosed Vehicles”, Pediatrics 116(1):e109–e112; time table “Estimated Vehicle Interior Air Temperature v. Elapsed Time”, Jan Null, Department of Meteorology & Climate Science, San José State University.

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