The inside of a parked car is a solar collector. Short-wave radiation passes through the glass, is absorbed by the dashboard, seats and trim, and is re-radiated at wavelengths the glass does not let back out. The interior temperature therefore climbs well above the air outside, and it does so within minutes rather than hours. That physics is the reason children and animals die in vehicles every year, including on days that felt unremarkable.
This page exists to show how fast that happens, using published measurements rather than assertion. It is not a tool for deciding how long someone can be left in a car. There is no such duration. Arb Digital publishes it within a free tools library because the numbers are widely misunderstood, and because "it was only a few minutes" and "it was not that hot" are the two beliefs behind most of these deaths.
What This Hot Car Temperature Calculator Does
You enter the outside air temperature and a number of minutes. The tool projects the interior temperature at 10, 30 and 60 minutes, and at the interval you chose, using the temperature-rise curve measured in the paediatric vehicular hyperthermia literature. It reports the rise above ambient separately, because that rise is roughly independent of the starting temperature — which is precisely why mild days are dangerous.
What it deliberately does not do is produce a countdown, a safe window, or a "you have N minutes" figure. The output is a description of how quickly a vehicle becomes lethal, not a permission. If you need to convert between scales outside this page, the Celsius to Fahrenheit converter and the temperature converter handle that, and the heat index calculator covers outdoor heat stress on people who are not enclosed in a vehicle.
How to Use It
- Enter the outside air temperature. Use the actual temperature where the vehicle is parked, in whichever scale you prefer.
- Enter an interval up to 60 minutes. The underlying measurements covered a one-hour period, so anything beyond that is extrapolation the source does not support.
- Read the rise, not just the total. The rise above ambient is the part that stays roughly constant across mild and hot days, and it is what makes a 70 °F afternoon dangerous.
- Compare the 10-minute and 30-minute figures. Most of the heating happens early, which is the finding that contradicts everyday intuition most directly.
The Formula / How It's Calculated
There is no formula in the physics sense here — the curve is empirical. The figures come from the study "Heat Stress From Enclosed Vehicles: Moderate Ambient Temperatures Cause Significant Temperature Rise in Enclosed Vehicles" by McLaren, Null and Quinn, published in Pediatrics in July 2005. The measurement conditions matter and are stated plainly in the paper: a dark-coloured sedan, on 16 different clear sunny days, with ambient temperatures ranging from 72 to 96 °F, recorded continuously over 60 minutes, with additional runs on two days with the windows opened 1.5 inches.
The rise above ambient measured in that work is approximately 19 °F at 10 minutes, 29 °F at 20 minutes, 34 °F at 30 minutes, 38 °F at 40, 41 °F at 50 and 43 °F at 60 minutes. Roughly 80% of the total hour's rise occurs within the first 30 minutes. This calculator interpolates between those published points and adds the result to the outside temperature you entered.
Worked example, which is what the page loads with. Starting from an outside temperature of 80 °F: after 10 minutes the interior is about 80 + 19 = 99 °F; after 30 minutes, 80 + 34 = 114 °F; after an hour, 80 + 43 = 123 °F. Start from 72 °F — a temperature most people would describe as pleasant — and the interior still passes 106 °F within half an hour.
Why Mild Days Are Not Safe Days
The single most important property of that curve is that the rise is largely independent of the starting temperature. A car does not heat by a percentage; it heats by a roughly fixed number of degrees driven by the solar radiation falling on it. That means the difference between a 95 °F day and a 72 °F day is 23 degrees at the start and 23 degrees an hour later — both of them arriving at an interior temperature far above what a body can regulate.
This is why deaths occur in spring and autumn, and why "it was not even hot" appears so often in the accounts afterwards. The National Weather Service's page on children, pets and vehicles makes the same point and notes that hyperthermia is possible on days in the 70s. Cloud cover reduces the effect, but the sun does not have to be intense for the interior to become lethal.
Cracked Windows Do Not Solve It
Leaving windows partly open is the most common intervention and it does not work. The study above measured runs with the windows opened 1.5 inches and found no meaningful reduction in the heating rate, and the National Weather Service states the same conclusion directly. The reason is that the heat is not trapped air waiting to escape; it is continuous solar input being absorbed by dark interior surfaces and re-radiated. A small gap does almost nothing to that energy balance.
The same applies to the other habitual reassurances. Parking in shade helps only while the shade lasts, and shade moves. Leaving water does not help, because a body that cannot shed heat does not benefit from drinking. A cracked sunroof, a windscreen shade or a light-coloured car all shift the numbers slightly and change nothing about the conclusion.
Why Children and Animals Are Affected Faster
A small body has more surface area relative to its mass and a less developed thermoregulatory system, and a child's core temperature rises several times faster than an adult's in the same conditions. Heatstroke begins when core body temperature reaches roughly 104 °F (40 °C), and around 107 °F (42 °C) is generally regarded as lethal. Those thresholds are reached long before an interior reaches the peak temperatures shown above.
Dogs are in a comparable position for a different reason: they cannot sweat effectively and rely on panting, which becomes useless once the surrounding air is hotter than the animal. A sleeping child, a rear-facing car seat and a quiet dog all share the same problem — nothing about the situation looks like an emergency from outside the vehicle until it is one.
What to Do, and What This Page Cannot Tell You
If you see a child or an animal alone in a parked vehicle, call the local emergency number straight away and stay with the vehicle. Emergency services would rather attend a call that turns out to be nothing. Many jurisdictions have specific laws about leaving children or animals unattended in vehicles, and several have provisions covering bystanders who intervene — those rules differ by country and by state, and this page is not the place to interpret them.
For anyone responsible for a child, the routine advice from safety authorities is procedural rather than judgemental: put something you need — a phone, a bag, a shoe — in the back seat, so that leaving the vehicle always means opening the rear door. Lock vehicles and keep keys out of reach when they are parked at home, because a substantial share of these deaths involve children who climbed into an unlocked car unsupervised rather than children who were left there.
Arb Digital maintains a free tools library covering weather, measurement and everyday arithmetic.
Browse Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Treating a mild day as a safe day — the temperature rise is roughly the same in absolute degrees regardless of the starting temperature.
- Believing cracked windows help — measured runs with windows open 1.5 inches showed no meaningful reduction in the heating rate.
- Assuming shade is protection — shade moves with the sun, and a vehicle that starts shaded rarely stays that way.
- Thinking a few minutes is different — roughly 80% of an hour's temperature rise happens within the first 30 minutes, and much of it in the first 10.
- Assuming a locked car at home is not a risk — many of these deaths involve children who got into an unattended vehicle themselves.
Related Free Tools From Arb Digital
Use the heat index calculator for outdoor heat stress, the temperature converter or Celsius to Fahrenheit converter for scales, the dew point calculator for humidity, and the wind chill calculator for the opposite problem in winter. Everything else is in the tools hub.
Frequently Asked Questions
There is no safe length of time and no safe outside temperature. This page does not produce a safe window, because none exists. A vehicle's interior begins climbing immediately, and most of an hour's rise occurs within the first half hour.
Published measurements of a dark sedan on clear sunny days recorded a rise of about 34 °F above the outside air after 30 minutes. From an 80 °F day that means an interior near 114 °F, and from a 72 °F day it still means an interior above 106 °F.
No. The study these figures come from measured runs with the windows opened 1.5 inches and found no meaningful reduction in the heating rate, and the National Weather Service states the same. The heat comes from continuous solar input absorbed by interior surfaces, not from trapped air.
Yes. The temperature rise is roughly a fixed number of degrees rather than a percentage, so a day in the low 70s produces almost the same increase as a hot one. Deaths have occurred in vehicles on days that nobody would have described as hot.
From measurements published in Pediatrics in 2005 by McLaren, Null and Quinn: a dark-coloured sedan monitored continuously over 60 minutes on 16 clear sunny days with ambient temperatures between 72 and 96 °F. Different vehicles, colours and conditions will differ, but the pattern is consistent.
Call your local emergency number immediately — 911 in the United States — and stay with the vehicle until help arrives. Do not wait to see whether someone comes back. Laws about intervening differ by jurisdiction, so emergency services are the right first call.
A smaller body has more surface area relative to its mass and less developed temperature regulation, and a child's core temperature rises several times faster than an adult's. Dogs cannot sweat effectively and rely on panting, which stops working once the surrounding air is hotter than they are.
This page projects vehicle interior temperature from published measurements taken in one study on one type of vehicle, and it is not a safety assessment of any particular situation. It is not medical advice and does not state or imply that any duration, temperature, shade condition or window position is safe. No child or animal should ever be left in a parked vehicle. If someone has been in a hot vehicle and is unwell, call emergency services immediately.
