BTU Calculator

What this BTU calculator does

This calculator estimates how many BTUs per hour of cooling a single room needs based on the four factors that actually move the number: square footage, climate zone, insulation quality, and sun exposure. It then matches that load to the next retail air-conditioner size from the standard US bands — 5000 / 6000 / 8000 / 10000 / 12000 / 14000 / 18000 / 24000 / 36000 BTU. Optional modifiers add +600 BTU per occupant above 2 and +4,000 BTU for kitchens, the two adjustments the DOE recommends including in any room calculation.

How to use the BTU calculator

1. Enter the floor area of the room in square feet.
2. Pick a climate zone from the dropdown — Zone 1 is far-north Canada / Alaska, Zone 5 is Florida / Arizona / west Texas summer. This is the single biggest multiplier, so get it right for your location.
3. Pick an insulation level. Poor = pre-1980 home with single-pane glass and minimal attic insulation. Average = typical 1990s build. Good = newer build with double-pane glass, foam-sealed sills, R-30+ attic.
4. Pick a sun exposure — shaded (north-facing or heavily treed), normal (most rooms), or sunny (west or south wall with full afternoon sun, no shade).
5. Add occupants if more than 2 will routinely be in the room (a home office with one person doesn’t change much; a media room with five people does).
6. Tick the kitchen toggle if this is a kitchen or has a major cooking surface.
7. Read the result. The headline number is BTU/h needed; the recommended size is the next standard retail unit that meets it.

The DOE 20 BTU per sqft rule (and when to break it)

The starting point used by Energy Star, the Department of Energy and nearly every AC manufacturer is 20 BTU per square foot per hour of cooling at typical conditions. So a 300 sqft room needs 6,000 BTU; a 600 sqft open-plan living needs 12,000. Then four multipliers stack:

• Climate: 0.85 (far north) → 1.15 (hot/desert).
• Insulation: 1.15 (poor) → 0.90 (good). Single-pane windows alone push you well into the “poor” band.
• Sun: 0.90 (shaded) → 1.10 (full west/south sun).
• Ceiling height is implicitly included at 8 ft. Add 10 % for every foot above 8.

For a 300 sqft west-facing bedroom in Houston (Zone 5), pre-2000 insulation, 2 occupants: 300 × 20 × 1.15 × 1.15 × 1.10 ≈ 8,700 BTU — pick a 10,000 BTU unit.

The oversizing problem (why bigger isn’t better)

A common mistake: “I’ll get a 14,000 BTU unit because then it’ll cool fast.” It cools faster, then short-cycles. The thermostat hits its target before the AC has had time to pull moisture out, so the compressor shuts off, the room reheats, and the compressor starts again — every 5–10 minutes. Three problems follow:

1. Humidity stays high. Cold + humid feels worse than warm + dry. Your skin notices.
2. Electricity bills go up. Compressor start-up draws 5–8× the running current; short cycles are mostly start-ups.
3. Compressor lifespan drops. Most window-unit failures are compressor failures, almost always from short cycling.

The DOE recommends sizing within 15 % of the actual load — slightly under beats 20 % over every time.

AC size bands and what they fit

Mini-splits typically beat window units on efficiency above 12,000 BTU and below the size where central air becomes economical.

When to skip the window unit and call an HVAC pro

If your calculated load is above 24,000 BTU, or you’re cooling more than two rooms that don’t share airflow, the window-unit / portable path stops being efficient. At that point you want either: (a) a multi-zone mini-split with one outdoor compressor feeding 2–4 indoor heads, or (b) central air added to existing ductwork if you have it. Both require licensed installation, refrigerant handling, and electrical work — not a DIY job. Use this calculator to size each room, then take the per-room numbers to a contractor for a Manual J load calculation on the whole envelope.

Privacy

This calculator runs in JavaScript on your device. There is no fetch call, no analytics on the room measurements you enter, no server-side logging. The page works the same way offline once loaded.