HVAC BTU calculator

Calculate a steady-state heating load from entered envelope and ventilation heat-loss coefficients.

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Envelope heat-loss coefficient (W / kelvin)

Indoor minus outdoor design temperature (C)

More inputs and assumptions

Ventilation heat-loss coefficient (W / kelvin)

Internal and solar heat gains (watts)

Answer

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Calculate a steady-state heating load from entered envelope and ventilation heat-loss coefficients.

More results

Precision and calculation record

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Formula and assumptions

Heating-only balance: (envelope W/K + ventilation W/K) times temperature difference, minus entered gains, bounded at zero. Supply coefficients from an appropriate survey or design calculation; this tool does not derive them from floor area. It excludes transient warm-up, humidity, cooling, duct losses and equipment sizing. It is not ACCA Manual J or a code-compliance calculation. One international-table Btu equals 1055.05585262 joules.

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Updated 2026-09-29

Using HVAC BTU calculator

Calculate a steady-state heating load from entered envelope and ventilation heat-loss coefficients.

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A worked example

For envelope heat-loss coefficient (w / kelvin) = 100, indoor minus outdoor design temperature (c) = 20, ventilation heat-loss coefficient (w / kelvin) = 25, internal and solar heat gains (watts) = 0, calculated heating load (btu/hour) = 8530.354083.

Before you use the result

Example results for envelope heat-loss coefficient (w / kelvin)

These examples use Indoor minus outdoor design temperature (C): 20; Ventilation heat-loss coefficient (W / kelvin): 25; Internal and solar heat gains (watts): 0. They are reference calculations, not recommended settings.

Envelope heat-loss coefficient (W / kelvin)	Calculated heating load (BTU/hour)
50	5118.21245
100	8530.354083
200	15354.637349

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