HVAC Calculators
Air Changes Per Hour Calculator
Calculate air changes per hour from airflow and room dimensions, and compare the result against typical ACH targets by space type.
Airflow CFM Calculator
Calculate the airflow in CFM required to deliver a given heat load at a chosen supply-to-room temperature difference using the sensible heat formula.
BTU Calculator
Estimate the BTU output a single room needs for a window unit or mini-split, based on area, sun exposure, occupancy, and kitchen use.
Duct Static Pressure Calculator
Estimate total external static pressure from duct length, fittings, friction rate, and component losses to check against blower capacity.
Furnace Size Calculator
Estimate the required furnace input BTU rating from heat loss and AFUE efficiency, accounting for combustion losses at a given efficiency tier.
Heat Pump Sizing Calculator
Calculate both cooling and heating load for a heat pump and size equipment to whichever load governs, with the design driver identified.
HVAC Duct Sizer Calculator
Calculate the required round duct size from airflow and design velocity using the velocity method, rounded to standard duct sizes.
HVAC Load Calculator
Estimate total cooling and heating load in BTU/hr and tonnage from area, climate zone, insulation, occupants, and windows using a Manual-J-style method.
Refrigerant Charge Calculator
Calculate total refrigerant charge for a split system using the manufacturer factory-charge method, adjusted for actual line set length.
SEER Rating Savings Calculator
Compare annual energy cost between an old and new SEER-rated cooling system based on cooling load, run hours, and electricity rate.
Why an Oversized HVAC System Isn't a Safe Bet
It's tempting to assume a bigger furnace or AC unit is a safer choice, but an oversized HVAC system actually performs worse — it short-cycles, fails to properly dehumidify, and wastes energy, while an undersized one can't keep up on the coldest or hottest days. Ten calculators here work through the load, airflow, and duct sizing math that gets HVAC sizing right the first time.
Popular HVAC Calculators
Ten tools span load calculation, airflow, and equipment sizing:
- HVAC Load Calculator — estimates heating and cooling load from square footage, insulation, climate, and window exposure.
- BTU Calculator — converts a space's characteristics into the BTU output needed to heat or cool it.
- Airflow CFM Calculator — calculates required air volume flow rate for a given space and air change target.
- Duct Static Pressure Calculator — checks whether ductwork resistance falls within a system's design limits.
- SEER Rating Savings Calculator — projects energy cost savings from upgrading to a higher-efficiency unit.
Why Square Footage Alone Doesn't Size a System
Rule-of-thumb sizing based purely on square footage ignores factors that meaningfully change actual heating and cooling load — ceiling height, insulation quality, window count and orientation, local climate, and even the number of occupants all shift the real BTU requirement for the same floor area. This is why a proper HVAC load calculation (commonly following the Manual J methodology in the U.S.) accounts for all of these factors together, and why two homes of identical square footage in different climates or with different insulation can need very differently sized systems.
Frequently Asked Questions
Is a higher SEER rating always worth the extra upfront cost?
It depends on climate and usage — homes in hot climates running AC heavily see faster payback from a higher SEER unit than homes in mild climates with limited cooling season use.
Why does duct static pressure matter if airflow volume is already correct?
Excessive static pressure from undersized or restrictive ductwork forces the blower to work harder, reducing actual delivered airflow and system efficiency even when the equipment itself is properly sized.
Explore More
Working on the building envelope too? See the Home Improvement Calculators, or check electrical load for HVAC equipment in the Electrical Calculators.