HVAC Calculator

Manual J Simplified — cooling BTU/hr, AC tonnage and furnace input rating from square footage, ASHRAE climate zone, insulation and sun exposure.

Space dimensions

ft²
ft
8 ft is standard residential. Vaulted: average to ridge.

Climate & envelope

Discrete loads

Each adds 1,000 BTU (conduction + sensible).
Each adds 1,000 BTU.
Each adds 600 BTU (ACCA Manual J).
W
Permanent electronics / appliances always on. 1 W ≈ 3.41 BTU/hr.

HVAC sizing

Recommended cooling system

— ton AC

Cooling BTU/hr

— BTU/hr

Heating BTU/hr

— BTU/hr

Furnace input @ 80 % AFUE

— BTU/hr

Furnace input @ 95 % AFUE

— BTU/hr

Standard residential cooling sizes: 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 tons. The calculator rounds up the raw cooling load to the next standard size.

Informational only. This is a simplified approximation of ACCA Manual J for early-stage sizing. For new construction or major remodels, request a full ACCA-certified Manual J ($100-300) before purchasing equipment. Oversizing is a real problem — an oversized AC short-cycles, does not dehumidify, and wastes 15-30 % more energy. Trust the rounded-up number; do not add an extra ton for safety.

How HVAC sizing works (Manual J Simplified)

base = sqft × ceiling_height × BTUper ft³ (by climate zone)
adjusted = base × insulation × sun × window_type
cooling = adjusted + occupants × 600 + windows × 1,000 + doors × 1,000 + equipment × 3.41 + room_adj
heating = cooling × climate_zone_heating_factor
tons = cooling / 12,000  ·  round up to nearest standard size

Climate zone base BTU per cubic foot

ZoneCooling baseHeating factor
1 — Very Hot4.5 BTU/ft³×0.6
2 — Hot4.5 BTU/ft³×0.8
3 — Warm3.7 BTU/ft³×1.1
4 — Mixed3.2 BTU/ft³×1.3
5 — Cool2.7 BTU/ft³×1.5
6 — Cold2.7 BTU/ft³×1.7
7 — Very Cold2.2 BTU/ft³×2.0
8 — Subarctic2.2 BTU/ft³×2.2

Worked example — 1,200 ft² home, Zone 4, average insulation

  • Base = 1,200 × 8 × 3.2 = 30,720 BTU/hr
  • Insulation ×1.00, sun ×1.00, double-pane ×1.00 → adjusted = 30,720
  • + 2 occupants × 600 = 1,200
  • + 6 windows × 1,000 = 6,000
  • + 2 doors × 1,000 = 2,000
  • + room adj = 0 (baseline)
  • Total cooling ≈ 39,920 BTU/hr
  • Tons = 39,920 / 12,000 ≈ 3.33 → round up to 3.5 ton system
  • Heating = 39,920 × 1.3 = 51,896 BTU/hr
  • Furnace input @ 80 % AFUE = 51,896 / 0.80 ≈ 64,870 BTU/hr

Why oversizing is a real problem

A common contractor habit is to size HVAC generously — “just to be safe”. The result is short-cycling: the system turns on, blasts the air for 4-5 minutes, satisfies the thermostat, and shuts off before it has time to dehumidify the air. Inside feels cold but clammy. Long-term, short-cycling adds wear on the compressor (the most expensive component), wastes electricity at start-up, and reduces equipment life by 30-50 %.

The Manual J right-sized system runs longer cycles at lower output, removes more humidity, and uses less energy per unit of cooling delivered. Trust the calculator and round up only to the next standard size, never two sizes up.

Frequently asked questions

What size AC do I need for a 1,500 sq ft house?
In ASHRAE Zone 4 (mixed climate) with average insulation, 2 occupants and a typical window count: about 45,000-50,000 BTU/hr cooling, which is a 4-ton system. Zone 1-2 hot climates push to 4.5-5 tons; well-insulated Zone 5-6 homes drop to 3-3.5 tons.
What size AC for a 2,000 sq ft house?
Zone 4 with average insulation: about 55,000-60,000 BTU/hr cooling, calling for a 4.5 to 5-ton system. With excellent insulation and shaded exposure, this can drop to a 4-ton; with poor insulation and sunny exposure, 5-ton is needed.
How many BTUs do I need per square foot?
Rules of thumb run from 20 BTU/ft² in very cold climates to 30 BTU/ft² in very hot. But the rule of thumb is often wrong — insulation, sun exposure, window count and ceiling height matter more than raw square footage. Use the calculator, not the rule.
Is it bad to oversize an AC unit?
Yes. An oversized unit short-cycles (turns on/off frequently), fails to dehumidify properly (rooms feel cold and damp), uses more electricity per cooling delivered, and wears out the compressor 30-50 % faster. Right-sized always beats oversized.
What is the difference between cooling and heating BTU?
Cooling BTU/hr measures heat removed; heating BTU/hr measures heat added. They are not equal — cooling load is driven by sun and humidity in summer; heating load is driven by cold air infiltration and temperature differential in winter. Heating typically runs 1.3-2.0× the cooling load in mixed-to-cold climates.
What is AFUE on a furnace?
Annual Fuel Utilization Efficiency — the ratio of heat delivered to fuel consumed, as a percentage. 80 % AFUE is the residential minimum (standard "single-stage" furnace). 95-97 % AFUE is "high-efficiency condensing" with sealed combustion. Required input rating: heating_BTU ÷ AFUE.
How many tons is 36,000 BTU/hr?
Exactly 3 tons. 1 ton = 12,000 BTU/hr by definition (the energy to melt 1 ton of ice in 24 hours). 36,000 ÷ 12,000 = 3 tons. Standard residential systems come in 0.5-ton increments: 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 tons.
Should I get a heat pump or a furnace?
In ASHRAE Zone 1-4, a heat pump can handle 100 % of heating efficiently. In Zone 5-6, a heat pump with electric strip backup (or dual-fuel with a gas furnace) covers cold snaps. In Zone 7-8, dedicated furnaces still outperform heat pumps below 20°F outdoor temperatures.