Plumbing & HVAC Calculators
Pipe volume, airflow and equipment sizing basics. 24 free calculators.
Plumbing and HVAC sizing usually starts with a straightforward unit conversion - a pipe's length and diameter into gallons, a room's size into the BTUs needed to heat or cool it - that's easy to get wrong by an order of magnitude with the wrong formula.
These calculators handle the conversion math for common plumbing and HVAC estimates, from pipe volume to basic equipment sizing.
Last reviewed September 2026
Start here
All 24 Plumbing & HVAC calculators
Pipe Volume Calculator
Use this pipe volume calculator to find how many gallons or cubic feet of water a pipe run holds, from its inner diameter, with a 100 ft example.
Furnace BTU Calculator
Use this furnace BTU calculator to estimate heating capacity from square footage, climate, and insulation, with a 2,000 sq ft example and a Manual J note.
CFM Calculator (Air Changes per Hour)
Use this CFM calculator to find the fan airflow a room needs for a target air changes per hour, or the ACH a known fan delivers in that room.
Flow Rate Calculator
Use this flow rate calculator to get a pipe's GPM from its inner diameter and water velocity, with the exact gallons-per-cubic-foot factor and a 2 in. example.
Window AC Size Calculator
Use this window AC size calculator to match a room's area, sun exposure, occupants, and kitchen use to ENERGY STAR's BTU sizing guidance, with a worked example.
Water Velocity Calculator
Use this water velocity calculator to find how fast water moves in a pipe from its flow rate and inner diameter, with a 49 GPM, 2 in. example.
Refrigerant Line Charge Calculator
Use this refrigerant line charge calculator to find the ounces to add for a line set longer than the factory pre-charge, using your unit's own figures.
Pipe Offset Calculator
Use this pipe offset calculator to get the travel and run for a 22.5°, 30°, 45°, or 60° offset, with a worked 12 in. example and a note on fitting takeout.
Pipe Size Calculator
Use this pipe size calculator to find the inner diameter for a target GPM at a chosen velocity, with a 49 GPM example and round-up guidance for stock pipe.
Tons to BTU Calculator
Use this tons to BTU calculator to convert a central AC or heat pump's cooling capacity from tons to BTU per hour, with a 3 ton example and the exact factor.
BTU to Watts Calculator
Use this BTU to watts calculator to convert a furnace, air conditioner, or space heater rating into watts, with a 12,000 BTU/hr example and the factor.
BTU to kW Calculator
Use this BTU to kW calculator to turn any furnace or air conditioner rating into kilowatts with the exact 0.00029307107 factor. Worked example included.
BTU to HP Calculator
Use this BTU to HP calculator to convert any BTU per hour rating into mechanical horsepower, with a 12,000 BTU/hr worked example and the exact factor.
Water Heater Size Calculator
Use this water heater size calculator to estimate a storage tank size from household size and usage, with a 3-person example and a First Hour Rating note.
Tankless Water Heater Sizing Calculator
Use this tankless water heater sizing calculator to find the BTU/hr a unit needs from your flow rate, incoming water temperature, and target output.
Glycol Calculator
Use this glycol calculator to work out concentrate and water volumes for hydronic freeze protection, with a 50 gallon, 30% example you can adjust.
Pipe Thermal Expansion Calculator
Use this pipe thermal expansion calculator to see how far copper, steel, PVC, CPVC, or PEX grows over a run, from its length and temperature change.
Sump Pump Sizing Calculator
Use this sump pump sizing calculator to estimate pump cycles per hour from basin size, drawdown, pump capacity, and inflow rate, with a worked example.
Pipe Weight Calculator
Use this pipe weight calculator to get the weight per foot and total weight of steel or stainless pipe by nominal size, schedule, and length.
Dehumidifier Sizing Calculator
Use this dehumidifier sizing calculator to find the pints-per-day capacity a room needs from its square footage and dampness, rounded to a standard unit.
Baseboard Heater Length Calculator
Use this baseboard heater length calculator to find the feet of heater a room needs, its wattage, and the circuit amps it draws from a BTU/hr load.
Water Pressure to Head Height Calculator
Use this water pressure to head height calculator to convert PSI to feet of head, or back, for well and irrigation planning, with a 50 PSI example.
Rolling Offset Calculator
Use this rolling offset calculator to get the travel and run for a compound pipe offset from its vertical and horizontal legs, with a worked 45° example.
Water Softener Sizing Calculator
Use this water softener sizing calculator to find the grain capacity your household needs from occupant count and water hardness, with a worked example.