Hot Car Calculator: Interior Temperature and Heatstroke Risk
Enter the outside air temperature, how long the vehicle has been parked, the sun exposure, and the car color. This calculator shows the estimated interior cabin temperature using a field-validated exponential model and flags when conditions become dangerous for a child or a dog left inside. Results update as you type. This page is for educational purposes only; never leave a child or pet unattended in a vehicle under any conditions.
Formula
Worked example
Outside temperature 85 F (29.4 C), full sun, light car, windows closed, 30 minutes parked. Adjusted delta = 24 C. k = 0.038. Interior = 29.4 + 24 * (1 - e^(-0.038*30)) = 29.4 + 24 * (1 - 0.321) = 29.4 + 16.3 = 45.7 C (114.3 F). After a 5-minute lag, child body temp = 37.0 + 0.111 * 25 = 39.8 C (103.6 F), approaching life-threatening levels.
How hot does a car get in the sun?
A parked car in direct sunlight acts like a greenhouse. Glass lets shortwave solar radiation in but traps longwave heat radiation inside. Even at a mild outside temperature of 70 F (21 C), interior temperatures can climb past 104 F (40 C) within 30 minutes on a sunny day. At 90 F (32 C) outside, the cabin can reach 120 F (49 C) or more within the same window. Interior air temperature follows an exponential saturation curve: it rises fastest in the first 15 to 20 minutes, then slows as the car approaches its thermal equilibrium for those conditions. About 80 percent of the total temperature rise occurs in the first 30 minutes, which is why leaving a car to "cool down" after a brief errand is so dangerous.
Why children and pets are at extreme risk
Children's bodies heat up three to five times faster than adult bodies because they have a higher surface-area-to-mass ratio and a less efficient sweating system. A 2-year-old's body temperature can rise roughly 1 degree Fahrenheit every 5 minutes once the interior air exceeds 100.4 F (38 C). Hyperthermia begins at 100.4 F (38 C) body temperature, life-threatening damage occurs above 104 F (40 C), and irreversible brain damage and death can occur at 107.1 F (41.7 C). This progression can happen in as little as 15 to 20 minutes on a hot day. Dogs face a similar crisis: they regulate body temperature almost entirely through panting (evaporative cooling from the tongue and respiratory tract) rather than sweating. Once the cabin air temperature approaches or exceeds a dog's body temperature of about 101 to 102.5 F (38.3 to 39.2 C), panting becomes ineffective and heatstroke sets in rapidly.
Does cracking windows or parking in shade help?
Cracking the windows by a few inches reduces peak interior temperature by only 3 to 5 degrees Fahrenheit in peer-reviewed field studies. This modest reduction does not create a safe environment for a child or pet. The windows-open option in this calculator reflects that small reduction. Parking in shade is more effective: a fully shaded car on an 85 F day may peak 10 to 12 F lower than the same car in full sun, with a slower rate of rise. However, on very hot days (90 F and above), even a fully shaded car can still reach temperatures hazardous to children within 40 to 60 minutes. There is no combination of cracked windows and shade that makes an unattended child or pet safe in a parked vehicle.
The formula: exponential saturation model
Interior temperature at time t follows: T_inside(t) = T_ambient + delta_max * (1 - e^(-k * t)), where T_ambient is the outside air temperature, delta_max is the maximum possible temperature rise (modulated by sun exposure, car color, and window position), k is the heat absorption rate constant, and t is time in minutes. Delta_max ranges from about 6 C (11 F) in full shade to 24 C (43 F) in full sun for a light-colored car. Dark cars absorb roughly 10 percent more heat. The rate constant k is 0.038 per minute in full sun, giving a characteristic time constant of about 26 minutes (meaning 63 percent of the rise happens within 26 minutes in full sun). These values are calibrated to match field measurements published by McLaren et al. (2005) and NHTSA.
Interior car temperature by outside temperature and time (full sun, light car)
| Outside temp | 10 min | 20 min | 30 min | 60 min |
|---|---|---|---|---|
| 70 F (21 C) | 89 F (32 C) | 99 F (37 C) | 104 F (40 C) | 112 F (44 C) |
| 80 F (27 C) | 99 F (37 C) | 109 F (43 C) | 114 F (46 C) | 122 F (50 C) |
| 85 F (29 C) | 104 F (40 C) | 114 F (46 C) | 119 F (48 C) | 127 F (53 C) |
| 90 F (32 C) | 109 F (43 C) | 119 F (48 C) | 124 F (51 C) | 132 F (56 C) |
| 95 F (35 C) | 114 F (46 C) | 124 F (51 C) | 129 F (54 C) | 137 F (58 C) |
| 100 F (38 C) | 119 F (48 C) | 129 F (54 C) | 134 F (57 C) | 142 F (61 C) |
Based on exponential saturation model calibrated against NHTSA field studies. Windows closed.
Frequently asked questions
How hot does a car get in 10 minutes?
At 85 F (29 C) outside in direct sun, the interior of a closed light-colored car reaches approximately 104 to 109 F (40 to 43 C) within 10 minutes. About 40 to 50 percent of the total temperature rise happens in the first 10 minutes, making even very short stops risky for children and pets.
Can a car get hot enough to kill a child on a mild day?
Yes. Studies have recorded life-threatening interior temperatures on days as mild as 72 F (22 C). When the outside temperature is 72 F and there is direct sun, the interior can reach 110 F (43 C) within 60 minutes, far above the threshold for fatal heatstroke in a child. Heatstroke kills children every year on days many people consider comfortable.
Does the color of a car matter?
Yes, but less than many people expect. Dark-colored cars absorb more solar radiation through their body panels and roof, reaching peak temperatures roughly 10 percent higher than light-colored cars under the same conditions. The difference is real but not large enough to make any car safe for an unattended child or pet.
Is it safe to leave a dog in the car if I crack the windows?
No. Cracked windows reduce peak temperature by only 3 to 5 degrees Fahrenheit, which is not enough to protect a dog. Dogs cool themselves almost entirely through panting, and once the cabin air temperature approaches the dog's own body temperature (around 101 to 102.5 F), panting no longer removes heat effectively and heatstroke can develop within minutes.
How long can a child survive in a hot car?
This depends on the outside temperature, sun exposure, and how quickly the interior heats. At 90 F (32 C) outside in direct sun, a 2-year-old can reach a life-threatening body temperature of 104 F (40 C) within 20 to 30 minutes. At 80 F (27 C), the same child may reach that threshold within 45 to 60 minutes. There is no safe time to leave a child unattended in a vehicle.
What should I do if I see a child or pet in a hot car?
Call 911 immediately if the child or pet appears to be in distress or unresponsive. In many jurisdictions, law allows bystanders to break a window to rescue a child or pet in imminent danger, but calling 911 first is always the recommended first step. If the child or pet is conscious but clearly overheating, try to locate the owner through a nearby store while someone stays with the vehicle.
Why does interior temperature rise so fast in the first 15 minutes?
Solar radiation heats the dashboard, seats, and interior surfaces very quickly, and those surfaces then radiate heat back into the cabin air. The process follows an exponential curve where the rate of heating is proportional to the gap between the current interior temperature and the maximum equilibrium temperature. When that gap is large (at the start), heating is fastest. As the interior approaches its maximum, the rate slows. This is why 80 percent of the total temperature rise can occur in the first 30 minutes.