How to use this calculator
- Enter Room area in sq ft.
- Enter Climate.
- Enter Insulation.
- Enter Sun exposure.
- Press Calculate. The highlighted row is your order quantity; the note beneath it translates the number into real-world terms.
Formula & constants
Base cooling load ≈ 20–25 BTU per sq ft, adjusted for climate zone, insulation, sun exposure and occupancy. A 400 sq ft room needs roughly a 9,000–10,000 BTU unit in a moderate climate. Every default in the fields above is editable — when your supplier’s spec sheet lists a different density, coverage rate or price, enter that number instead. The waste and compaction allowances follow common trade practice; see our methodology page for the derivation of each formula and the standards it references.
Where this fits in the project
Electrical sizing works backwards from load: total demand sets service size, circuit amperage sets wire gauge, and conductor count sets conduit fill. HVAC equipment is sized from the BTU load, never from floor area alone.
How this connects to your project
No quantity stands alone — each material in the chain depends on the ones around it. These are the planning steps that pair with the btu calculator:
- HVAC Size Calculator — BTU calculation determines HVAC equipment size.
- Insulation Calculator — Better insulation reduces BTU requirements.
- R-Value Calculator — Higher R-values lower BTU heating and cooling needs.
- Window Cost Calculator — Window quality affects BTU heat loss and gain.
- Door Cost Calculator — Door insulation value affects BTU calculations.
- Siding Calculator — Wall insulation behind siding affects BTU needs.
Frequently asked questions
How many BTU per square foot?
20–25 for cooling in average construction — but ceiling height, windows and kitchen heat all shift it.
Is a bigger AC better?
No — oversized units short-cycle: poor dehumidification, temperature swings and early compressor wear.
Last verified: — prices reflect current national averages. See our methodology.
Estimates are for planning. Material densities, coverage rates and prices vary by region and supplier — confirm final quantities before ordering, and check local code where structural or electrical work is involved.