Indoor Temperature Calculator
Estimated indoor temperature
Likely range: —
Why?
How Does the Indoor Temperature Estimator Work?
The estimator starts from the current outdoor temperature and estimates a likely starting indoor temperature based on your windows and room size. From there, it models the room moving toward an equilibrium temperature over time — the same "approach curve" principle used in Newton's law of cooling, rather than a fixed offset like "outdoor minus 10 degrees." When the AC or heater is running, that equilibrium is pulled toward the set temperature you choose, but heat continually leaking in or out through the building keeps the room from ever landing exactly on the setpoint.
What Affects Indoor Temperature?
Outdoor Temperature
Outdoor temperature is the starting point for every estimate. A closed, unconditioned room will still drift toward a neutral indoor baseline over time, but the outdoor temperature has a strong pull on that baseline.
Open vs. Closed Windows
Open windows let outdoor air exchange freely with the room, so indoor temperature moves much closer to outdoor temperature. Closed windows insulate the room somewhat and also make AC or heating far more effective, since conditioned air isn't constantly escaping.
Air Conditioning
Running AC pulls the estimate toward the set temperature you choose — but only part of the way, since outdoor heat keeps working against it. The longer it runs, the closer the room gets to that effective equilibrium, quickly at first and more slowly as it settles in. Open windows significantly reduce how effective the AC can be, so the equilibrium sits much closer to outdoor temperature.
Heating
Heating works the same way in reverse: the room is pulled toward the heater's set temperature, but not all the way there, and the longer the heater runs, the closer it gets. Open windows let heat escape and reduce the heater's effectiveness.
Fan
A fan doesn't cool or heat air on its own, but it circulates it. Running with the AC or heater, it helps the room reach its equilibrium faster and more evenly — it doesn't change the equilibrium itself. Running alone, it has a much smaller effect — mostly clearing hot or cold pockets rather than changing the overall temperature.
Room Size
Room size mainly affects how quickly the room changes temperature, not how far it ultimately goes. Smaller rooms respond to AC or heating faster; larger rooms take longer to reach the same equilibrium.
How Accurate Is the Estimate?
This calculator estimates indoor temperature. It does not measure the actual air temperature. It's a simplified model, not a scientific-grade HVAC simulator — it doesn't account for insulation quality, sun exposure, humidity, occupancy, or building materials, so treat the result as a reasonable, transparent ballpark rather than an exact reading. The displayed range and confidence level describe how much uncertainty the model itself has, not measurement precision. For an accurate number, use an indoor thermometer.
Frequently Asked Questions
How can I estimate my room temperature?
Enter (or auto-detect) the outdoor temperature, then tell the calculator whether your windows are open, whether AC or heating is running (and at what set temperature and for how long), and your approximate room size. It combines these into a single estimate.
Can I estimate indoor temperature from outdoor temperature?
Outdoor temperature alone only gives a rough starting point. Windows, AC or heating use and their set temperature, and room size all shift the indoor temperature meaningfully away from the outdoor reading.
Does opening windows change indoor temperature?
Yes. Open windows move indoor temperature much closer to outdoor temperature and reduce how effective AC or heating can be.
How much can AC lower room temperature?
It depends on the set temperature you choose, how long it has run, and the room size. With windows closed, the room will move toward your AC's set temperature but typically settle a little above it, since outdoor heat keeps working against the AC.
Will my room reach the exact temperature I set the AC or heater to?
Usually not exactly. The set temperature strongly pulls the estimate, but outdoor conditions, open windows, and the simplified nature of the model mean the room typically settles close to — rather than precisely at — the setpoint.
How long does AC take to cool a room?
Most of the change happens within the first one to two hours, with smaller further gains after that as the room approaches its cooling equilibrium. Smaller rooms cool faster than larger ones.
Does room size affect cooling?
Yes. Smaller rooms have less air to condition and respond faster to AC or heating than larger rooms, given the same running time — though the size of the room doesn't change the equilibrium it's heading toward.
How does a heater affect indoor temperature?
A running heater gradually pulls the room toward its set temperature. Effectiveness builds with running time and is reduced if windows are open.
Does a fan change the room temperature?
Only a little on its own — it mainly moves air around rather than cooling or heating it. Its biggest effect is helping AC or heating reach their equilibrium faster by improving air circulation.
Can this calculator measure my actual room temperature?
No. This calculator estimates indoor temperature — it does not measure the actual air temperature. For an exact reading, use an indoor thermometer.