Size it right before you buy it.

Roughly 20–30 BTU per sq ft for cooling in most of the US. Use the low end for shaded, well-insulated rooms in mild climates and the high end for sunny rooms, high ceilings, or hot climates like the Southwest and Gulf Coast.

One ton equals 12,000 BTU, so 9,000 BTU is ¾ ton, 12,000 is 1 ton, and 18,000 is 1½ tons. Window units and mini-splits are sold in BTU; central systems are sold in tons.

You’re cooling volume, not floor area. A 12 ft ceiling holds 50% more air than an 8 ft ceiling, and hot air stratifies up high while the thermostat sits at 5 ft — the system works longer to satisfy it.

Typically 3 to 4 tons, depending on climate. About 3 tons in northern zones with good insulation, up to 4 tons in the Deep South or in a drafty older home. This calculator’s range is the honest answer — anyone giving one number without asking about your climate and insulation is guessing.

No. Right-sized systems run longer, steadier cycles, which removes more humidity, evens out room temperatures, and usually lasts longer. Bigger only helps if the house was genuinely undersized — chronic inability to hold setpoint on design-temperature days, with verified airflow and charge.

Manual J is the ACCA-standard load calculation that accounts for your actual windows, walls, orientation, insulation, infiltration, ducts, and local weather design data. It’s the calculation a contractor should run before final sizing — these web calculators are for planning and quote-checking.

For most homes, the value sweet spot is the mid-tier (roughly 15.2–17 SEER2 range): noticeably cheaper to run than minimum efficiency without the steep premium of top-tier variable-speed systems. Go higher if you’re in a hot climate, plan to stay in the home 10+ years, or rebates/credits close the price gap.

Not necessarily — SEER is efficiency, not capacity. A 3-ton 20 SEER and a 3-ton 14 SEER both deliver ~36,000 BTU. Higher-end systems are often variable-speed, which does improve comfort and humidity control, but that’s the compressor staging, not the SEER number itself.

Duct leakage, refrigerant charge, airflow, and thermostat habits all move real consumption. A leaky duct system (25–30% losses are common in older homes) can erase a full SEER tier — sealing ducts often pays back faster than upgrading equipment.

About 2,100–3,600 watts depending on efficiency: roughly 3,600 W at 10 SEER, 2,570 W at 14 SEER, and 2,250 W at 16 SEER. Check the nameplate MCA/RLA or measure with a clamp meter for your specific unit — startup surge is much higher but lasts under a second and doesn’t move the bill.

Usually runtime. This calculator uses the hours you enter — a system that’s undersized, low on charge, or fighting leaky ducts may run 14–16 hours on a hot day, not 8. Also confirm your true rate: divide the full bill (including delivery and fees) by kWh used, not just the supply rate printed in ads.

For a conventional single-stage system, no — setback saves energy because the system moves the same heat with less total runtime. The exception is extreme heat, where recovery from a big setback during peak afternoon temps can cost as much as maintaining. A modest 3–4°F setback is the practical middle ground.

A 6″ round duct at standard velocity handles roughly 100–110 CFM comfortably. In flex duct, step up to 7″ for the same airflow. A 6″ is the most common bedroom supply size for exactly this reason.

400 CFM per ton is the standard design point. In humid climates techs often drop to 350 CFM/ton for better moisture removal; in dry climates 450 can make sense. Too little airflow across the coil risks icing; too much hurts dehumidification.

Rarely. If the branch duct itself is undersized or crushed, a bigger register grille changes nothing — the bottleneck is upstream. Check for kinked flex, closed dampers, and long runs first; those are the usual culprits.

In a Zone 4 climate with average insulation, the heat load is roughly 68,000 BTU of output — that’s about an 80,000 BTU input furnace at 96% AFUE, or a 90,000 at 80% AFUE. In the South the same house might need only a 60,000; in northern Minnesota, a 100,000.

On gas use alone, a 96% furnace burns about 17% less gas for the same heat. Whether it pays back depends on your winters and gas price — in cold climates with long heating seasons it usually does; in mild climates the 80% (where code still allows it) can be the smarter buy. Condensing furnaces also need a condensate drain and PVC venting, which adds install cost.

They share a blower and duct system, not a size. It’s common for a home to need 3 tons of cooling but 80,000 BTU of heat — the furnace is picked for the heat load and its blower must still move the AC’s 1,200 CFM. Both calculations matter.