Free inside car temperature calculator
Estimate the inside car temperature of a parked car from the outside temperature and minutes parked — with a heatstroke-risk flag, using the McLaren (2005) heating curve, updated live, as you type.
On this page12 sections
Never leave a child or pet in a parked car — cracking the windows barely helps. An estimate, not safety advice. How accurate is this?
Results are estimates. Consult a professional.
How fast does the inside of a car heat up?
A parked car turns into an oven faster than almost anyone expects. Sunlight pours through the glass, the seats and dashboard soak it up, and the trapped air has nowhere to go. The interior climbs roughly 19 °F in the first 10 minutes, about 29 °F by 20 minutes, and 34 °F by half an hour — most of the rise is over before you would think it had even started. The National Weather Service sums it up as roughly 20 degrees in 10 minutes and 50 degrees in an hour.
This calculator takes the two numbers that decide the danger — the outside temperature and how long the car has been parked — and estimates the temperature inside, along with a clear heatstroke-risk band. It is built for a single grim purpose: to show how quickly a "quick errand" on a mild day becomes life-threatening for a child or a pet left behind.
How the inside car temperature calculator works
The estimate uses the heating curve measured by Catherine McLaren, Jan Null, and James Quinn in their 2005 study in the journal Pediatrics. They parked cars in the sun and logged the interior temperature minute by minute. The key finding is that the interior temperature is the outside air temperature plus a rise that depends almost entirely on time, not on how hot it is outside.
Why outside temperature barely changes the rise — and cracked windows barely help
The most important — and most counterintuitive — point is this: a parked car heats up by roughly the same amount whether it is 70 °F or 100 °F outside. After 30 minutes the interior is about 34 °F hotter than the air, full stop. That is why even a mild spring day is deadly: at 70 °F outside, the inside still reaches about 104 °F in an hour, right at the heatstroke threshold.
It is worth being precise here. The final interior temperature is not independent of the outside air — a hot day still ends hotter inside than a cool one, because the rise is added on top of the ambient temperature. What is roughly constant is the size of the rise. So a higher starting temperature shifts the whole danger curve earlier; it never makes the car safe.
Cracking the windows is the reflex many people reach for, and it does almost nothing. The heating is driven by sunlight entering through the glass and being trapped as heat, not by a lack of fresh air. The National Weather Service is blunt about it: leaving the windows slightly open does not significantly decrease the heating rate of a parked vehicle.
Car interior temperature rise by time and outside temperature
This chart shows the estimated interior temperature (°F) for common outside temperatures and parking times. Read down to your outside temperature, across to the minutes parked. Notice how every column to the right is dangerous, and how fast the first three columns climb.
| Outside | 10 min | 20 min | 30 min | 60 min |
|---|---|---|---|---|
| 70 °F (21 °C) | 89 | 99 | 104 | 113 |
| 80 °F (27 °C) | 99 | 109 | 114 | 123 |
| 90 °F (32 °C) | 109 | 119 | 124 | 133 |
| 100 °F (38 °C) | 119 | 129 | 134 | 143 |
Interior temperature in °F = outside + rise (McLaren 2005). The 104 °F column at 30 minutes for a 70 °F day shows that even mild weather reaches the heatstroke threshold. Values of 120 °F and above are life-threatening within minutes.
A worked example using the inside car temperature calculator
It is a warm summer afternoon, 90 °F (32 °C) outside. A driver runs into a shop "for just half an hour" and leaves the car in the sun. How hot is it inside after 30 minutes?
Step 1 — Find the rise for the time parked
At 30 minutes the McLaren curve gives a rise of about 34 °F above the outside air. Most of that heating already happened in the first 10–20 minutes.
Step 2 — Add the rise to the outside temperature
Interior = 90 °F + 34 °F = 124 °F, which is about 51 °C.
Step 3 — Read the risk band
124 °F is past the 120 °F mark, so the calculator flags it extreme — life-threatening. It is well above the 104 °F core-temperature threshold for heat stroke, and a child or pet could be killed in minutes. "Half an hour" was never safe.
What interior temperatures cause heatstroke?
Heat stroke is medically defined by a core body temperature above 104 °F (40 °C), and mortality rises steeply once the body passes about 106 °F. A parked car drives a person's core temperature up far faster than the open air, and a small body has little margin. The bands below key the interior air temperature to that medical threshold.
| Interior temp | Risk band | What it means |
|---|---|---|
| Below 90 °F | Below heatstroke level | Below the danger threshold — but the interior is still climbing fast. |
| 90–103 °F | Caution | Hot enough to harm a child or pet on prolonged exposure; rising toward danger. |
| 104–119 °F | Danger — heatstroke risk | At or above the 104 °F core-temperature threshold for heat stroke; harm within minutes. |
| 120 °F and above | Extreme — life-threatening | Interiors routinely exceed 120–130 °F on hot days; heat stroke and death can occur very quickly. |
Bands match the calculator's risk output and are anchored to the medical heatstroke threshold. They describe interior air temperature; a body, especially a child's, can overheat well below these figures.
Never leave a child or pet in a parked car
- Look before you lock. Open the back door every time you park, every trip, to be certain no one is left behind.
- Keep cars locked at home. Children climb into unlocked, unattended cars and cannot get out — many deaths happen this way, not just from being left.
- Cracking the windows does not help. The heating comes from trapped sunlight, not stale air. A gap in the window barely changes the rise.
- If you see a child or pet alone in a hot car, act. Call emergency services immediately; minutes matter.
How accurate is this inside car temperature calculator?
The calculator uses the published McLaren, Null & Quinn (2005) heating curve, the most widely cited measurement of how fast enclosed vehicles heat up. Their finding — that the rise depends mainly on time and is largely independent of the starting temperature — is what makes a simple "outside plus rise" model reliable enough for safety awareness. The figures match the National Weather Service's public guidance closely.
Real cars vary. A dark interior, direct overhead sun, dark paint, and a closed cabin all push the temperature higher than the average curve; light colours, shade, and tinted glass push it lower. The curve was measured on warm, sunny days, so it is least reliable in deep shade or cold weather — though tragically, deaths still occur at outside temperatures as mild as the 60s. Treat the result as a realistic estimate of the danger, never as a guarantee of safety. For related heat tools, see the heat index calculator and the UV index calculator.
Frequently asked questions about the free inside car temperature calculator
About this Inside car temperature calculator
This calculator runs entirely in your browser. Nothing you type is sent anywhere — the interior temperature and heatstroke-risk band are computed locally from the McLaren (2005) heating curve as you adjust the outside temperature and minutes parked. It is an estimate, not a safety device: never leave a child or pet in a parked car.
It is one of our weather calculators. Browse the full set of free calculators for more heat, climate, and everyday tools.