Measure the conditioned space
Use the room’s clear length, width and ceiling height. Include connected open-plan space that cannot be isolated. A nominal floor area can miss tall ceilings, adjacent rooms and large unshaded glass.
Transparent calculator
How many BTUs do I need?
Start with room volume, then adjust for use, insulation, sun, glazing, occupants and heat-producing devices.
How this estimate works
The baseline uses room volume, then applies documented room-use, insulation, sun, glazing and open-plan factors. Occupants and high-heat devices add a modest allowance. The result is deliberately shown as a range.
CAPACITY IS A RANGE
Room dimensions are the start, not the final selection.
This screening tool is for a room appliance decision. It is not an ACCA Manual J calculation for a whole-home HVAC system.
Record heat gains
Sun exposure, glazing, weak insulation, occupants, cooking and heat-producing electronics can move the planning range. Climate, air leakage and humidity also affect comfort but cannot be captured precisely by a short room calculator.
Avoid automatic upsizing
Bigger is not always safer. Excess capacity can increase cycling, noise and weak humidity control, while insufficient capacity may not maintain the setpoint. Check the manufacturer’s sizing instructions and operating range for the exact model.
Whole-home boundary
Central AC, heat-pump and furnace selection requires address-specific load assumptions, duct capacity and matched equipment evidence. Do not use this room result to approve contractor tonnage.