# Hot Car Temperature Calculator

> A closed car parked in the sun heats up about 19°F in 10 minutes and 43°F in an hour, even on a mild day. Enter the temperature outside to see how hot the inside gets, from measured data.

- Interactive calculator: https://www.leoella.com/tools/hot-car-temperature-calculator
- Plain-text version: https://www.leoella.com/tools/hot-car-temperature-calculator.md
- Category: Car Seats & Car Travel
- Publisher: Leo&Ella (https://www.leoella.com)
- Last reviewed: October 19, 2026

## How it works

The calculator adds a measured rise to the outside temperature you enter (60 to 115°F, or 16 to 46°C): +19°F after 10 minutes, +29°F after 20 minutes, +34°F after 30 minutes, +38°F after 40 minutes, +41°F after 50 minutes, +43°F after 60 minutes, with straight-line steps in between. The rises come from Jan Null's chart for a closed car parked in the sun, built on measurements taken on days from 72 to 96°F, so results outside that range are estimates. Past the first hour it shows the summary's ranges: 45 to 50°F after 1 to 2 hours and 50 to 55°F after 2 to 4 hours.

## Key facts

- In measurements by Jan Null of San José State University, a closed car parked in the sun heated up about 19°F in 10 minutes, 29°F in 20 minutes, 34°F in 30 minutes and 43°F in an hour. [2]
- In McLaren, Null and Quinn (Pediatrics, 2005), about 80% of a closed car's temperature rise happened in the first 30 minutes, and even on the coolest day measured (72°F) the inside of the car reached 117°F. [1]
- In the same Pediatrics study, cracking the windows open 1.5 inches did not slow how fast the car heated up or lower the highest temperature it reached. [1]
- According to NHTSA, a child's body heats up 3 to 5 times faster than an adult's; heatstroke begins at a core body temperature of about 104°F, and about 107°F is lethal. [4]
- Since 1998, an average of 37 children a year have died of heatstroke in vehicles in the US; from 1998 to 2025, 52.9% of them had been forgotten by a caregiver, according to NoHeatStroke.org. [3]
- The AAP recommends always checking the back seat before locking the car, and asking your child care provider to call if your child is more than 10 minutes late. [5]

## Reference data

### Inside a closed car parked in the sun

Cabin air temperature by time parked and outside temperature. The study measured days from 72 to 96°F; columns outside that range apply the same rise and are marked as estimates.

| Time parked | 70°F outside (estimate) | 80°F outside | 90°F outside | 100°F outside (estimate) |
| --- | --- | --- | --- | --- |
| 10 minutes | 89°F (32°C) | 99°F (37°C) | 109°F (43°C) | 119°F (48°C) |
| 20 minutes | 99°F (37°C) | 109°F (43°C) | 119°F (48°C) | 129°F (54°C) |
| 30 minutes | 104°F (40°C) | 114°F (46°C) | 124°F (51°C) | 134°F (57°C) |
| 40 minutes | 108°F (42°C) | 118°F (48°C) | 128°F (53°C) | 138°F (59°C) |
| 50 minutes | 111°F (44°C) | 121°F (49°C) | 131°F (55°C) | 141°F (61°C) |
| 60 minutes | 113°F (45°C) | 123°F (51°C) | 133°F (56°C) | 143°F (62°C) |
| 1 to 2 hours | 115 to 120°F | 125 to 130°F | 135 to 140°F | 145 to 150°F |
| 2 to 4 hours | 120 to 125°F | 130 to 135°F | 140 to 145°F | 150 to 155°F |

### Rise over the outside temperature

| Time parked | Rise |
| --- | --- |
| 10 minutes | +19°F |
| 20 minutes | +29°F |
| 30 minutes | +34°F |
| 40 minutes | +38°F |
| 50 minutes | +41°F |
| 60 minutes | +43°F |
| 1 to 2 hours | +45 to 50°F |
| 2 to 4 hours | +50 to 55°F |

## Frequently asked questions

### How hot does a car get inside?

Much hotter than outside, very fast. In measurements by Jan Null of San José State University, a closed car's inside air rose about 19°F in 10 minutes, 29°F in 20 minutes, 34°F in 30 minutes and 43°F in an hour, and 45 to 50°F after one to two hours. On an 80°F day that's about 99°F after 10 minutes and 123°F after an hour.

### Can a car get dangerously hot on a mild day?

Yes. In the Pediatrics study behind these figures, even on the coolest day measured (72°F outside), the inside of the car reached 117°F. Most of the rise, about 80%, happened in the first 30 minutes.

### Does cracking the windows help?

No. In the same study, opening the windows 1.5 inches did not lower the rate at which the car heated up or the highest temperature it reached.

### Why are hot cars so dangerous for children?

A child's body heats up 3 to 5 times faster than an adult's, according to NHTSA. Heatstroke begins at a core body temperature of about 104°F, and about 107°F is lethal. Since 1998, an average of 37 children a year have died of heatstroke in vehicles in the US.

### How do children end up in hot cars?

Of the hot car deaths of children in the US from 1998 to 2025, 52.9% were children forgotten by a caregiver, 23.8% got into a car on their own, and 21.9% were knowingly left in the car. More than half were under 2.

### What should I do if I see a child alone in a hot car?

Call 911 right away. Check whether the child is responsive and follow the dispatcher's instructions. Every minute counts.

## Sources

1. [McLaren C, Null J, Quinn J. Heat stress from enclosed vehicles: moderate ambient temperatures cause significant temperature rise in enclosed vehicles. Pediatrics 2005;116(1):e109-12](https://doi.org/10.1542/peds.2004-2368)
2. [Jan Null, San José State University: Estimated vehicle interior air temperature v. elapsed time](https://noheatstroke.org.ggweather.com/vehicle_heating.htm)
3. [Jan Null, NoHeatStroke.org: Heatstroke deaths of children in vehicles (updated September 28, 2026)](https://www.noheatstroke.org/)
4. [NHTSA: Child heatstroke](https://www.nhtsa.gov/campaign/heatstroke)
5. [American Academy of Pediatrics (HealthyChildren.org): Prevent Child Deaths in Hot Cars](https://www.healthychildren.org/English/safety-prevention/on-the-go/Pages/Prevent-Child-Deaths-in-Hot-Cars.aspx)

## About this page

This is general information for parents; the labels and manuals of the products involved always have the final say. The calculator runs in the visitor's browser and stores nothing they enter. When quoting this page, cite "Hot Car Temperature Calculator, Leo&Ella" and link https://www.leoella.com/tools/hot-car-temperature-calculator.