HVAC Calculators
Start with the load, the heat your house gains in summer and loses in winter, then pick equipment, compare efficiency and estimate the cost.
HVAC calculators
- AC Size Calculator
Cooling load and central AC size in tons from your state's climate, windows and insulation.
- Duct Size Calculator
Round and rectangular duct sizes from CFM or tons, with friction rate and velocity checks.
- Furnace Size Calculator
Heat loss and gas furnace input size in BTU from your state's NOAA winter design temperature and AFUE.
- Heat Pump Size Calculator
Heat pump tons, balance point and backup heat in kW from your state's NOAA design temperatures.
- HVAC Replacement Cost
Installed price for an AC, heat pump, furnace or mini-split, using HVAC wages from your state.
- SEER Savings Calculator
Annual cooling cost and savings for a higher SEER2 unit, from your state's electricity price and climate.
Why the load comes first
Every heating and cooling decision starts with one number: how many BTUs per hour your home gains on a hot design day and loses on a cold one. Rules of thumb like “one ton per 500 square feet” ignore your climate, your windows and your insulation, and they usually oversize equipment. An oversized air conditioner cools the air quickly and shuts off before it has removed much humidity. An oversized furnace cycles on and off, wastes fuel and wears out sooner.
Our calculators use a simplified heat-gain and heat-loss model: conduction through walls, windows and ceiling, sun through glass, air leakage, people and ductwork. The design temperatures come from NOAA’s 1991–2020 climate normals for a major city in your state. That’s not a Manual J calculation, which is what the building code requires for permits, but it’s a lot closer to one than a square-foot rule. It also shows you where the load comes from, so you can see what an upgrade would change.
How the tools connect
- AC size gives the cooling load and a nominal size in tons.
- Heat pump size adds the heating side: the balance point and how much backup heat you need.
- Furnace size turns the heat loss into the input rating you’d see on a gas furnace.
- The cost and efficiency tools take those sizes and put dollar figures on them.
The sizing calculators share the same inputs, so the link at the bottom of each one carries your house to the next tool.
Which HVAC calculator do I need?
- “What size air conditioner do I need?” The AC size calculator estimates the cooling load in BTU per hour and the size in tons from your state’s climate, your windows and your insulation.
- “Will a heat pump keep my house warm?” The heat pump size calculator shows the balance point, the outdoor temperature where the unit stops keeping up, and how many kW of backup heat to plan for.
- “What size furnace do I need?” The furnace size calculator turns your home’s heat loss into the input rating you’d see on a gas furnace at your AFUE.
- “Are my ducts big enough?” The duct size calculator gives round and rectangular duct sizes from airflow, with friction rate and velocity checks for noise.
- “Is a higher SEER2 worth the extra price?” The SEER savings calculator compares yearly cooling costs and the payback on a more efficient unit, using your state’s electricity price.
- “What will a new system cost?” The HVAC replacement cost calculator prices an AC, heat pump, furnace or mini-split with HVAC wages from your state.
A note on accuracy
These are planning numbers. A contractor should run a full Manual J load calculation before you buy equipment, and a Manual D design before ducts are changed. Use our results to understand the numbers and to ask better questions when you compare quotes.