BTU Calculator
Enter your room's details — the estimate updates as you type, and the link in your address bar always reflects your numbers.
How this calculator works
This is a simplified rule-of-thumb estimate, not a substitute for a real HVAC load calculation. Actual system sizing uses a full Manual J calculation that accounts for every window's size and orientation, your climate zone, ductwork, insulation R-values, and more — if you're about to buy a system, get a professional assessment rather than relying on this number alone. What this tool does give you is a fast, honest starting point: it takes a commonly-cited baseline of 20 BTU per square foot of floor area, then adjusts it for insulation quality (poorly insulated rooms leak conditioned air and need more capacity), sun exposure (a sunny, south-facing room heats up more), occupants (each body beyond two adds roughly 600 BTU of heat load, a commonly-cited adjustment), and ceiling height (taller ceilings mean more air volume, so the estimate scales up proportionally once you pass the standard 8 ft).
Worked example
With the defaults — a 300 sq ft room, an 8 ft ceiling, average insulation, average sun exposure, and 2 occupants — the baseline is 300 × 20 = 6,000 BTU. Average insulation and average sun both apply a ×1.0 multiplier, two occupants add no extra load (the adjustment only kicks in above two people), and an 8 ft ceiling needs no height adjustment. The result: look for a unit rated around 6,000 BTU.
Frequently asked questions
Is this exact enough to actually go buy an air conditioner?
No — treat this as a ballpark for browsing, not a purchase spec. The real standard HVAC contractors use is a full Manual J load calculation, which factors in your climate zone, window count and orientation, ductwork, ceiling and wall construction, and more. This calculator simplifies all of that into a handful of inputs, which is great for narrowing down a range but not precise enough to size a system you're about to spend real money on. Before buying, get a professional assessment.
Why does insulation quality matter so much to the result?
A poorly insulated room — think single-pane windows, an older home, or thin exterior walls — constantly leaks the conditioned air you just paid to cool or heat, so the system has to work harder and longer to hold a temperature. A well-insulated room holds onto conditioned air much better, so it needs less raw capacity to do the same job. That's why we apply a multiplier on top of the base square-footage estimate instead of treating every room the same.
Does this same calculation work for sizing a heater too?
The 20 BTU-per-square-foot baseline is commonly cited as a rough starting point for both cooling and heating loads, so you can use this tool either way in a pinch. That said, heating loads in particular tend to be more sensitive to your climate zone and window count than cooling loads are, so the honest answer is: it's a fine first pass for heating, but lean even more toward a professional opinion before buying a furnace or heat pump.
What if my room has a very tall or vaulted ceiling?
Standard sizing guides assume an 8 ft ceiling. Go taller and there's simply more air volume in the room to condition, even though the floor area hasn't changed — so we scale the estimate up proportionally to the ceiling height once it passes 8 ft. A 12 ft vaulted ceiling, for example, adds 50% more air volume than the 8 ft baseline, and the estimate reflects that.