HVAC load calculator
Room-by-room BTU, mini-split size, duct CFM, and baseboard watts — Manual-J-inspired rule-of-thumb math that shows its work. Free, no signup.
Size one room: baseline load from square footage, then adjusted for the things that actually move the number.
Once you know the BTU, matched equipment on Amazon: window ACs up to 12k BTU, portable ACs, and smart thermostats that keep the room on schedule.
Mini-splits come in fixed head sizes: 9k, 12k, 15k, 18k, 24k, 30k, 36k BTU/hr. Enter the space and zones, get the head size to buy.
The 12k BTU single-zone class is the bestseller for a reason — it covers most bedrooms and mid-size living areas. DIY-friendly Mr Cool-style units vs pro-install brands differ mainly on refrigerant line prep.
From equipment size to airflow: CFM = BTU/hr ÷ (ΔT × 1.08). This is the number your trunk duct has to carry.
Checking your own system's airflow? A clamp meter + anemometer kit and a good manometer are the pro tools for measuring what's actually moving.
Electric baseboard is sized in watts: watts = BTU/hr ÷ 3.41. Rule of thumb: 7–10 W per sq ft, adjusted for insulation and climate.
Circuit work belongs to an electrician — find one locally.
Smart thermostats for electric baseboard (Mysa-style line-voltage models) pay for themselves on scheduling alone.
How the sizing math works
Every tab here is built on the same skeleton, borrowed from ACCA Manual J and simplified to the inputs that move the number most. The baseline is the industry rule of thumb: 20–30 BTU/hr per square foot of cooling, with 25 as the middle. From there the calculators apply four published adjustment factors:
| Factor | Adjustment |
|---|---|
| Ceiling height | multiply by height ÷ 8 (a 10 ft ceiling adds 25%) |
| Sun exposure | very sunny ×1.15 · average ×1.0 · shaded ×0.90 |
| Insulation | poor ×1.25 · average ×1.0 · good ×0.85 |
| Windows | +600 BTU/hr each (single- to double-pane typical) |
Then a 10% equipment safety margin, rounded up to the nearest 500 BTU — matching how equipment is actually sold.
Quick reference: BTU by room size
| Room | Sq ft | Typical BTU/hr | Mini-split head |
|---|---|---|---|
| Bedroom | 130–180 | 4,000–6,000 | 9k |
| Large bedroom / office | 200–300 | 6,000–9,000 | 9k |
| Living room, average | 350–500 | 10,000–14,000 | 12k |
| Great room / open plan | 600–800 | 17,000–23,000 | 18k–24k |
| Whole small apartment | 800–1,200 | 23,000–34,000 | 24k–36k multi-zone |
Assumes 8 ft ceilings, average sun and insulation. Real houses vary — that's what the calculators above are for.
Why oversizing is the #1 DIY mistake
Bigger feels safer and isn't. An AC that's too big cools the air fast, shuts off before it has dehumidified anything, and you get a cold clammy room plus compressor short-cycling that kills the equipment years early. Inverter mini-splits modulate and forgive some oversizing, but past ~30% above load you're paying for capacity you can't use. Undersizing, by contrast, just means the unit runs longer on the worst days — cheaper mistake.
Guides
What size mini-split for each room?
Head-by-head sizing from bedrooms to great rooms, multi-zone vs single-zone economics, and the 9k BTU workhorse.
How many watts of electric baseboard per square foot?
The 7–10 W/sq ft rule, the ÷3.41 conversion, placement along exterior walls, and 120 vs 240 V circuits.
Duct CFM: the BTU ÷ (ΔT × 1.08) guide
Worked CFM charts by tonnage, ΔT by equipment type, per-run airflow, and where Manual D takes over.
GeneratorSizer ↗ · SolarSizer ↗ · WeldingCalc ↗ · Battery Bank Sizer ↗
Sister calculators from the same network: backup power, solar sizing, welding duty cycles, and battery banks.