HVAC Sizing Calculator

Last updated: Jul 12, 2026

Determine the right size air conditioner, heat pump, or furnace for your home based on heating and cooling loads.

sq ft?

sq ft

Climate Zone?

Insulation?

Stories?

Sun Exposure?

Cooling Load (BTU/h)?

BTU/h

Heating Load (BTU/h)?

BTU/h

Equipment Type?

Altitude (ft)?

About This Tool

This calculator helps homeowners determine the right size HVAC equipment for their home. It uses ACCA Manual S sizing methodology to match heating and cooling loads to standard equipment capacities, preventing the comfort and efficiency problems caused by oversized or undersized systems.

Sources: ACCA Manual S · ASHRAE · DOE

How to Use

Choose quick estimate or enter loads directly
Select your equipment type and climate zone
Review recommended sizes and sizing status

How to Use

Choose quick estimate or enter loads directly
Select your equipment type and climate zone
Review recommended sizes and sizing status

Methodology

Equipment sizing follows ACCA Manual S methodology. The calculator estimates heating and cooling loads using BTU-per-square-foot factors adjusted for climate zone, insulation quality, building stories, and sun exposure. Loads can also be entered directly from a Manual J calculation. Equipment is selected from standard residential sizes and checked against Manual S oversizing limits: cooling must not exceed 115% of the design total cooling load, and heating must not exceed 140% of the design heating load. Equipment type recommendations are based on climate zone suitability.

Sources: ACCA Manual S · ASHRAE Handbook

Understanding Your Results

The recommended equipment size is the next standard size above your design load that stays within Manual S limits. A green "Right-Sized" badge means the equipment capacity is between 95-115% of cooling load or 100-140% of heating load. Yellow means slightly oversized — still functional but not ideal. Red means significantly over- or undersized and should be reconsidered. The equipment recommendation suggests the most efficient system type for your climate zone.

Sources: ACCA Manual S · DOE Energy Saver

Practical Examples

Example: A 2,000 sq ft home in Zone 4A (Baltimore) with average insulation, 1 story, average sun exposure. Estimated loads: 44,000 BTU/h cooling (3.67 tons), 56,000 BTU/h heating. Cooling: The next standard size above 44,000 BTU/h is a 4.0-ton (48,000 BTU/h) unit. At 109% of the design load, this is well within the Manual S 115% limit — Right-Sized. Heating: A 60,000 BTU/h furnace at 95% AFUE provides 57,000 BTU/h output, which is 102% of the 56,000 BTU/h load — Right-Sized. Recommendation: Heat pump (Zone 4 has moderate winters where heat pumps are efficient).

Equipment Sizing Tips

Always size equipment based on actual load calculations, not rules of thumb. A common mistake is choosing equipment by square footage alone without considering insulation, windows, and climate. If your cooling and heating loads suggest very different equipment sizes, consider a two-stage or variable-speed system that can modulate its output to match actual conditions. When replacing equipment, don't assume the old system was correctly sized. Many existing homes have oversized equipment that was installed based on outdated rules of thumb. For the most accurate sizing, use the Heat Load Calculator for room-by-room loads, then enter those values in the "From Load" mode of this calculator.

Sources: DOE Energy Saver · ACCA Manual S

Sources

All calculations are performed locally in your browser. No data is sent to any server.

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Frequently Asked Questions

How do I know what size AC I need?

AC size is determined by your home's cooling load — the amount of heat that needs to be removed on the hottest design day. This depends on your home's square footage, climate zone, insulation quality, number of windows, and sun exposure. A properly sized AC for a typical 2,000 sq ft home in a moderate climate is usually 3 to 3.5 tons (36,000-42,000 BTU/h). This calculator estimates your load and matches it to standard equipment sizes.

What happens if my HVAC system is oversized?

An oversized cooling system short-cycles — it cools the air quickly but shuts off before adequately dehumidifying, leaving the home feeling clammy. It also costs more to purchase and install, uses more energy due to frequent start-stop cycles, and wears out faster. Per ACCA Manual S, cooling equipment should not exceed 115% of the design cooling load. Moderate heating oversizing (up to 140%) is more acceptable because furnaces modulate better at partial loads.

What are standard residential AC sizes?

Residential central air conditioners and heat pumps come in standard sizes: 1.5 ton (18,000 BTU/h), 2 ton (24,000), 2.5 ton (30,000), 3 ton (36,000), 3.5 ton (42,000), 4 ton (48,000), and 5 ton (60,000 BTU/h). One ton of cooling equals 12,000 BTU/h. Gas furnaces are available in 40,000, 60,000, 80,000, 100,000, and 120,000 BTU/h input ratings, with actual heating output depending on the efficiency (AFUE) rating.

Should I get a heat pump or a furnace with AC?

Heat pumps are most efficient in mild to moderate climates (Zones 1-4) where they can handle both heating and cooling with one system. In cold climates (Zones 5-6), a dual-fuel system — heat pump for mild weather plus gas furnace backup for extreme cold — offers the best efficiency. In very cold climates (Zones 7-8), a gas furnace with central AC is typically recommended because heat pump efficiency drops significantly below 25°F. This calculator provides a recommendation based on your climate zone.

What are SEER2, HSPF2, and AFUE ratings?

SEER2 (Seasonal Energy Efficiency Ratio 2) measures cooling efficiency — higher is better. The federal minimum is 13.4 for central AC and 14.3 for heat pumps. HSPF2 (Heating Seasonal Performance Factor 2) measures heat pump heating efficiency — minimum is 7.5. AFUE (Annual Fuel Utilization Efficiency) measures furnace efficiency as a percentage — 80% means 80 cents of every energy dollar becomes heat. Condensing furnaces achieve 90-98% AFUE. These ratings replaced the older SEER/HSPF metrics on January 1, 2023.

What is the Manual S 115% rule?

ACCA Manual S limits cooling equipment to no more than 115% of the design total cooling load. For example, if your design cooling load is 36,000 BTU/h, the largest acceptable AC is 41,400 BTU/h (36,000 × 1.15). This prevents oversizing problems like short-cycling and poor humidity control. Heating equipment has a more relaxed limit of 140% because furnaces handle oversizing better. The calculator automatically checks these limits and warns if equipment would be over- or undersized.