Enter square footage, ceiling height, climate zone, and insulation quality — get BTU/hr load and equipment tonnage. A Manual J approximation you can run in the driveway.
Free PDF: quick-size table by sq ft and climate zone, common oversizing mistakes and how to explain right-sizing to customers, and a one-page Manual J checklist.
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The industry rule of thumb — 1 ton per 500 sq ft — was never accurate and it's gotten worse as homes became more airtight and energy efficient. Using it in 2026 means oversizing nearly every system you install.
Short cycling: An oversized unit reaches setpoint quickly and shuts off before completing a proper dehumidification cycle. The result is a house that's cool but clammy — and a customer who calls you because "the AC isn't working right."
Humidity problems: Short cycles mean the evaporator coil never fully condenses moisture. In humid climates (Zones 1–4), this produces indoor humidity at 60–70% even with the thermostat satisfied. Mold risk follows.
Equipment wear: More start/stop cycles = more wear on the compressor and contactor. A 4-ton unit in a 2-ton load application may fail 5 years early due to cycling stress.
A proper ACCA Manual J load calculation accounts for window area and U-values, wall and roof U-values, duct location and insulation, infiltration rate (blower door), and internal gains. This calculator uses industry-validated simplified factors and gives you a working estimate accurate to ±15% for typical residential applications. Use it to sanity-check your quote and start the sizing conversation — run a proper Manual J when the job calls for a precision result.
When your calc says 2.5 tons and the homeowner's old unit was 4 tons, you'll get pushback. The explanation: "Your old unit was oversized from day one — that's why you had humidity problems. We're installing the right size, which will actually keep you more comfortable and cost less to run." Have this conversation before they compare your quote to a competitor who's still guessing by square footage.
Equipment sizing tools, maintenance agreement templates, service call rate schedules, and the business forms to run an HVAC operation that competes on expertise, not just price.
BTU sizing is the foundation of every HVAC installation quote. This calculator estimates heating and cooling load based on square footage, climate zone, insulation quality, window area, and occupancy to give you the correct system size in tons and BTU. Use it as a quick field sizing check before your full Manual J, or to verify that a replacement system is the right size for the space.