Tools for HVAC contractors.

Heating and cooling loads

Manual J for the whole house, BTU numbers for one room or addition, design temperatures by ZIP, and the job-type pages for replacement, retrofit, new construction and permit work.

Pick the right one on the heating and cooling loads page →

• Load calculation

  Manual J Load Calculator

  Residential heating and cooling design loads, sensible and latent. Whole-house on the free page; room by room in the Pro plan.

  The free form takes a ZIP code and a short description of the house and returns the whole-house load with its breakdown. The room-by-room calculation, with window orientation and measured R-values, returns BTU/h per room and is part of the Pro plan.

  Based on ACCA Manual J 8th Edition

• Design conditions

  HVAC Design Temperatures by ZIP

  Cooling and heating design dry-bulb temperatures and the IECC climate zone for a US ZIP code.

  Maps ZIP → county → climate zone and returns approximate 1% / 0.4% cooling and 99% / 99.6% heating values for that zone - the same lookup the load calculators use.

  Based on IECC 2021 climate zones

• Load density

  BTU per Square Foot Calculator

  Cooling and heating BTU/h per square foot by climate zone, from the whole-house load engine.

  Puts Manual J engine results next to the 20 to 30 BTU per sq ft trade rule of thumb for each climate zone and marks the zones where the two land furthest apart.

  Based on ACCA Manual J 8th Edition

• Heating load

  Garage Heater Size Calculator

  Heat loss of an attached or detached garage held at a shop temperature, in BTU/h and kW.

  Counts walls, ceiling, the overhead door, windows, the slab edge and air leakage. Returns the electric heater kW and the gas heater input for the efficiency you enter.

  Based on U x A x dT heat loss, BuildSolver engine defaults

• Space load

  Sunroom BTU Calculator

  Heating and cooling BTU/h for a sunroom, with the solar gain through the wall glass and a glass roof.

  Takes glass area by side, glass type and roof type, and shows how much of the cooling load comes through the glass. The glazed side with the highest direction factor sets that factor for all of the wall glass. A simplified estimate for one space. Room-by-room Manual J for the whole house is part of the Pro and Team plans.

  Based on U x A x dT and solar gain, BuildSolver engine defaults

• Space load

  Basement BTU Calculator

  Heating and cooling BTU/h for a finished basement, with the below-grade walls and floor split out.

  Applies below-grade wall U-factors and floor F-factors by depth and insulation as a stated assumption, plus the above-grade wall, windows and air leakage.

  Based on Seattle Energy Code Table A104.1 below-grade defaults

• Space load

  Room Addition BTU Calculator

  Heating and cooling BTU/h for a new bedroom, family room or bonus room, separate from the house load.

  Compares the addition's load to the spare capacity you enter for the existing system, as a rough check. Room-by-room Manual J for the whole house with the addition is part of the Pro and Team plans.

  Based on U x A x dT and solar gain, BuildSolver engine defaults

• Space load

  Mini Split Sizing Calculator

  Heating and cooling BTU/h for one zone under a single ductless head.

  Stops at the load in BTU/h and leaves the head to you. Matching a unit to that load is Manual S, part of the Pro and Team plans, and a multi-zone split needs room-by-room Manual J.

  Based on U x A x dT and solar gain, BuildSolver engine defaults

• Replacement

  Replacement AC Sizing Calculator

  Whole-house load for a replacement from a description of the house, with no brand lock-in. Matching a condenser and coil to that load under Manual S is part of the Pro plan.

  For the changeout where the old unit's nameplate is the only documentation on site. Brand-agnostic: the load is the load, whichever manufacturer's line you quote from.

  Based on ACCA Manual J 8th Edition

• Retrofit

  Retrofit Sizing Without a Blueprint

  For houses that never had drawings, or whose drawings are three renovations out of date.

  Describe what you can see and measure on a walkthrough - floor area, window count and what the attic looks like. Type it in from your phone in the driveway.

  Based on ACCA Manual J 8th Edition

• New construction

  Manual J for New Construction

  Preliminary load numbers during design development, before drawings are stamped.

  Useful when the framing plan is still moving: re-run the load as glazing, orientation and insulation spec firm up, so the mechanical budget stops being a guess.

  Based on ACCA Manual J 8th Edition

• Permit

  When a Permit Needs a Stamped Manual J

  Where the line falls between sales-phase sizing and permit-grade submittal.

  What AHJs typically ask for, which jurisdictions require ACCA-approved software for the permit load (BuildSolver's reports are built for the sales phase), and when to hand the job to a PE.

  Based on IECC R403 / local AHJ practice

Equipment sizing

From a load to a nominal size: AC tons, furnace input, heat pump capacity at the design temperature, Manual S selection, and what a SEER2 rating costs to run, plus the cooling load of a garage in tons.

Pick the right one on the equipment sizing page →

• Equipment selector

  Manual S Equipment Selection

  Matches a Manual J load to a specific AHRI-certified system at your design conditions. Part of the Pro and Team plans.

  Checks total and sensible capacity at your design conditions with outdoor-temperature and altitude derate, plus the sensible/latent split that decides whether the unit dehumidifies.

  Based on ACCA Manual S

• Equipment selector

  How Manual S Equipment Selection Works

  The method itself, with four worked selections and the sizing limits for furnaces, boilers, air conditioners and heat pumps.

  Reading, not a form. It walks the selection a contractor does by hand, including jobs that clear the total capacity line and then fail somewhere else. Running it against a specific AHRI-rated system is the Pro and Team selector above.

  Based on ACCA Manual S Table 1 sizing limitations

• Equipment size

  Heat Pump Sizing Calculator

  How much of a heat pump's 47 degree rating survives at your design temperature, and what that means for the size you need.

  Takes a heating load and an outdoor design temperature and returns the capacity factor, the capacity needed at 47 degrees, and the next nominal size up, for a standard or a cold-climate unit. Runs in the browser with no daily limit.

  Based on AHRI 210/240 rating points, BuildSolver derate averages

• Heating load

  Furnace Size Calculator

  Whole-house heating load from a ZIP code and a short description of the house, with the furnace input bin.

  The form takes ten inputs (ZIP, floor area, stories, year built, air tightness, foundation, windows, occupants, attic ducts, optional site elevation). It returns the heating load, furnace input bin at the assumed AFUE and heat-pump capacity at 47 °F.

  Based on ACCA Manual J 8th Edition

• Equipment size

  What Size AC by Square Footage

  Cooling tons for 800 to 3,500 sq ft houses across 14 IECC climate zones, for an older and a newer house.

  Each area page puts the Manual J engine result next to the 400 to 600 sq ft per ton trade rule of thumb, so you can see how far apart they land for that size and zone. A form above the table runs your own ZIP.

  Based on ACCA Manual J 8th Edition

• Operating cost

  SEER2 Energy Cost Calculator

  Yearly cooling electricity cost of an existing and a proposed unit, with savings and payback.

  Starts from the EIA national residential rate. Enter the rate from the customer's bill and the full-load hours for the job.

  Based on DOE 10 CFR 430 Appendix M1

• Units

  BTU to Tons Converter

  BTU/h, tons of refrigeration, kW, watts and MBH, converted in one step.

  Uses the NIST factors, 1 ton = 12,000 BTU/h and 1 BTU/h = 0.2930711 W. Converts units only and never turns floor area into tons.

  Based on NIST SP 811 conversion factors

• Airflow

  Evaporative Cooler Size Calculator

  Swamp cooler supply temperature and the CFM for your sensible load.

  Saturation effectiveness from the data sheet, 1.08 corrected for elevation. Refuses when the cooler cannot get air below the room.

  Based on ASHRAE HVAC Systems and Equipment Ch. 41

• Heating load

  Boiler Size Calculator

  The Net AHRI rating, Heating Capacity window and input for a hot water boiler from the heat loss, or a check of a boiler against the load.

  Enter the design heat loss, the boiler type and the AFUE for the net rating to shop for, the Manual S capacity window and a ballpark input. Or enter a rating from a manufacturer's table for the percent of the load it covers and a verdict. Hot water boilers only; the heat loss comes from a Manual J.

  Based on ACCA Manual S (boilers 100-140%), Net AHRI rating with the 1.15 piping and pickup allowance

• Space load

  Garage AC Sizing Calculator

  Cooling load of an attached or detached garage, with sun through the glass, people and equipment.

  Counts walls, ceiling, the overhead door, windows, slab and air leakage, plus sun through the glass by direction, people and the watts of tools and lights. Stops at the load and leaves the unit to you.

  Based on Conduction, sun and infiltration heat gain, BuildSolver engine defaults

Ducts and airflow

Duct sizes, CFM, static pressure, blower speed changes and duct leakage limits.

Pick the right one on the ducts and airflow page →

• Duct sizing

  Duct Sizing Calculator

  Supply trunk and branch diameters from design CFM, longest run, available static pressure and duct material.

  Returns round sizes with rectangular equivalents, velocities and the design friction rate. Sales-phase sizing; room-by-room Manual D is part of the Pro plan.

  Based on ACCA Manual D (equal friction)

• Airflow

  HVAC CFM Calculator

  Design airflow from cooling tons, a sensible BTU/h load, or a furnace output and temperature rise.

  Uses Qs = 1.08 x CFM x dT at sea-level air and flags CFM per ton, supply dT or rise outside the usual range.

  Based on ASHRAE sensible heat equation

• Ventilation

  Duct Leakage Test Calculator

  CFM25 per 100 sq ft of floor area and the allowed total leakage for your code edition.

  Rough-in and postconstruction limits of the 2021 IECC, the 2024 table by ducted returns and system size. Leakage to outside gets its number but no pass or fail.

  Based on 2021 IECC R403.3.6, 2024 IECC Table R403.3.8

• Airflow

  Static Pressure Calculator

  Total external static pressure from supply and return readings, or the available static pressure left for the ducts.

  Adds the drop of a coil or filter the rating leaves out when your probe sits beyond it, then compares the pressure at the rating points with the blower's rated maximum. The design mode subtracts the filter, coil, grille and damper drops you enter and returns the friction rate over the total effective length of the longest supply and return runs.

  Based on Manufacturer blower data, ACCA Manual D method

• Airflow

  Fan Law Calculator

  New CFM, static pressure and brake horsepower for a blower speed change, and the speed for a target airflow.

  Applies the three fan laws to a known point on the same fan and duct system. The density mode converts static pressure and horsepower between standard air and the air at your altitude and temperature.

  Based on Fan laws (HVAC School), Aerovent FE-1600 density correction

• Duct sizing

  Rectangular to Round Duct Calculator

  The round duct equal in friction and airflow to a rectangle, or the rectangle for a round size with one side fixed.

  Inside dimensions in, equivalent round diameter and the next stock size out. An optional CFM adds the velocity in both shapes and the friction rate for galvanized duct. It converts a duct you already have; sizing from CFM is the duct sizing calculator.

  Based on ASHRAE Fundamentals Ch. 21, Huebscher equivalent diameter

• Duct sizing

  Duct Velocity and Friction Loss Calculator

  Friction rate and velocity of one round duct, the CFM a size carries at a friction rate, or the size for an airflow.

  Galvanized round or flex fully stretched, standard air, straight duct. Checks the velocity against the Manual D Table 3-1 limits for a supply or return trunk or branch. A whole trunk and branch layout from the system CFM is the duct sizing calculator.

  Based on ASHRAE Fundamentals Ch. 21 (Darcy, Colebrook), ACCA Manual D Table 3-1

Ventilation and exhaust

Whole-house ventilation under ASHRAE 62.2, bath fans, kitchen hoods and air changes per hour.

Pick the right one on the ventilation and exhaust page →

• Ventilation

  ASHRAE 62.2 Ventilation Calculator

  Whole-dwelling mechanical ventilation rate from floor area and bedroom count.

  Applies Qtot = 0.03 x floor area + 7.5 x (bedrooms + 1). The page text explains the IRC M1505.4.3 formula with its smaller 0.01 coefficient, which the calculator does not compute.

  Based on ANSI/ASHRAE Standard 62.2

• Local exhaust

  Bathroom Exhaust Fan Calculator

  Bath fan CFM by floor area or by fixture count, rounded up to a standard fan size.

  Scales the area method for ceilings above 8 ft and keeps the 50 CFM code minimum as its own check.

  Based on HVI guidance, IRC M1505.4.4

• Ventilation

  Air Changes per Hour Calculator

  ACH from CFM and room volume, CFM for a target ACH, and ACH50 from a blower door test.

  Checks ACH50 against the IECC limit for the climate zone and edition you pick. The natural infiltration estimate is shown as a rough figure.

  Based on IECC 2021 and 2024 air leakage limits

• Local exhaust

  Range Hood CFM Calculator

  Kitchen hood CFM by cooktop width and hood position, checked against the code minimum.

  Flags makeup air above 400 CFM when a fuel-burning appliance is inside the air barrier. The gas BTU rule of thumb is shown for reference only.

  Based on HVI guidance, IRC M1505.4.4 and M1503.6

Refrigerant and system checks

Superheat, subcooling, PT chart, line set charge, the temperature split across the coil and the temperature rise across a furnace.

Pick the right one on the refrigerant and system checks page →

• Refrigerant charge

  Superheat and Subcooling Calculator

  Superheat and subcooling from gauge readings for R-410A, R-454B, R-32 and R-22.

  Reads the dew point line for superheat and the bubble point line for subcooling, and gives a fixed-orifice target from indoor wet-bulb and outdoor dry-bulb.

  Based on CoolProp PT data, manufacturer charging charts

• Refrigerant charge

  Refrigerant PT Chart

  Saturation pressure from temperature and temperature from gauge pressure for R-454B, R-410A, R-32 and R-22.

  Shows the dew and bubble point lines and the glide on blends, with an optional elevation correction for gauges above sea level. Refuses readings outside -40 to 140°F instead of extrapolating.

  Based on CoolProp PT data, checked against Daikin, Honeywell and Chemours charts

• Diagnostics

  AC Temperature Split Calculator

  Expected cooling split from SHR and CFM per ton, or delivered BTU/h and SHR from your readings.

  Delivered mode uses return and supply dry-bulb and wet-bulb with the ASHRAE enthalpy equations. Cooling only, no charge diagnosis, no model ratings.

  Based on ASHRAE Fundamentals psychrometrics

• Refrigerant charge

  Line Set Charge Calculator

  Ounces to add past the factory line length, from the rate in your installation instructions.

  Reference liquid per foot from ACR tube ID and CoolProp density. No line sizing and no A2L charge limit.

  Based on Manufacturer's rate, Goodman TP-107M example

• Diagnostics

  Furnace Temperature Rise Calculator

  Rise from supply and return readings against the rating plate range, the CFM that rise implies, or the rise at a given CFM.

  Enter the return and supply temperatures and the rise range from the rating plate for a verdict, and the furnace output (or input and AFUE) for an airflow estimate. Or enter the output and CFM for the expected rise. Elevation is corrected with the ASHRAE standard atmosphere. Gas furnaces only; the output is not derated for altitude.

  Based on Furnace manufacturers' instructions, ASHRAE Fundamentals Ch. 1 (1.08)

Psychrometrics and moisture

Moist air properties, dew point, dehumidifier capacity and the condensate a coil produces.

• Psychrometrics

  Dew Point Calculator

  Dew point (or frost point) from dry-bulb temperature and relative humidity.

  Magnus-Tetens with Sonntag 1990 coefficients at sea-level pressure. For condensation checks on ducts, grilles and cold surfaces.

  Based on ASHRAE Fundamentals Ch. 1

• Psychrometrics

  Psychrometric Calculator

  Full moist-air state from dry-bulb plus relative humidity, wet-bulb or dew point.

  Returns wet-bulb, dew point, RH, humidity ratio, enthalpy, specific volume and density at sea-level pressure.

  Based on ASHRAE Fundamentals Ch. 1

• Diagnostics

  Condensate Drain Calculator

  Gallons per hour and per day off a cooling coil, with drain size and slope.

  Water rate from airflow and coil readings. 3/4 inch minimum for one unit, IMC Table 307.2.2 for shared drains up to 250 tons, 1/8 inch per foot slope.

  Based on 2021 IMC 307.2, IRC M1411.3

• Moisture

  Dehumidifier Size Calculator

  Pints of water a day to hold a basement, crawlspace or house at a target humidity.

  Adds infiltration moisture from outdoor and indoor humidity ratios to the moisture from occupants. The text explains the DOE test conditions behind a dehumidifier's rated pints.

  Based on ASHRAE psychrometrics, DOE 10 CFR 430 Appendix X1

Electrical

Breaker and wire for AC and heat pump circuits, running amps and watts, and voltage drop over a run.

Pick the right one on the electrical page →

• Electrical for HVAC

  AC Breaker Size Calculator

  Minimum circuit ampacity and the largest breaker for an AC, heat pump or mini split, plus heat strip circuits.

  Worked from the compressor and fan currents on the nameplate. The nameplate MCA and MOCP govern, and no values are given by tonnage or model. Heat strip mode is single phase, and a licensed electrician installs the circuit.

  Based on NEC 2023 Article 440 and Article 424

• Electrical for HVAC

  AC Wire Size Calculator

  Copper or aluminum conductor for the nameplate MCA, with the voltage drop over your run.

  Applies the 60°C (140°F) or 75°C (167°F) terminal rule, optional heat and conductor-count corrections, and the 3 percent voltage drop guideline from the Informational Note, which is not a requirement.

  Based on NEC 2023 Table 310.16, 110.14(C) and 310.15

• Electrical for HVAC

  AC Amps and Watts Calculator

  Running watts and amps of an AC or heat pump from its capacity and EER2, watts from a current reading, and starting kVA from the LRA.

  Works at the 95°F EER2 rating point, on single or three phase. No power factor is guessed, no generator size is given, and the breaker and wire come from the nameplate.

  Based on 10 CFR 430 Appendix M1 (EER2), DOE power equations

• Electrical

  Voltage Drop Calculator

  Volts and percent dropped over a run, the smallest wire for a target drop, and the longest run for a size.

  Single phase or balanced three phase, 120 to 480 V, copper or aluminum from 14 AWG to 4/0. Shows the Table 310.16 ampacity next to each size. The 3 percent figure is an Informational Note, not a requirement.

  Based on K method (Mike Holt), NEC 2023 Chapter 9 Table 8

• Electrical

  Baseboard Heater Breaker Size Calculator

  Breaker, wire and circuits for electric baseboard heaters, or how many heaters fit on a 15, 20, 25 or 30 amp circuit.

  Single phase at 120, 208 or 240 V, up to two heater sizes. Conductors at 125 percent of the load, the breaker at the next standard size, and 3,840 W on a 20 A circuit at 240 V. A 240 V heater on 208 V makes 75 percent of its watts. A circuit with more than one heater is held to 30 A.

  Based on NEC 2023 Article 424 (424.4), Table 240.6(A)

Fuel gas and water heating

Gas pipe sizing, combustion air for gas appliances, tankless water heater sizing, and the breaker and wire for an electric water heater.

• Fuel gas

  Gas Pipe Sizing Calculator

  Schedule 40 steel gas pipe for natural gas and propane by the longest length method.

  Reads the IFGC table row for the longest run with no interpolation and matches the code's Appendix A106.1 example line for line. CSST, copper, PE and 2 psi systems are not sized.

  Based on 2021 IFGC Tables 402.4(2) and 402.4(28)

• Fuel gas

  Combustion Air Calculator

  Room volume and combustion air openings for gas furnaces and water heaters.

  Standard 50 cu ft per 1,000 BTU/h and known infiltration rate methods, openings to other rooms, to the outdoors, combined and mechanical. Direct-vent appliances are left out of the sum.

  Based on 2021 IFGC Section 304

• Water heating

  Tankless Water Heater Size Calculator

  Peak gpm, temperature rise, BTU/h, gas input and electric kW for a tankless unit.

  Adds the flow of the fixtures that run together and heats it from the cold water you enter. It does not pick a model, and the gas input is what you take to the gas pipe calculator.

  Based on DOE Energy Saver sizing method, 10 CFR 430.32

• Electrical

  Water Heater Breaker Size Calculator

  Breaker limits and wire for an electric storage water heater, from its rating plate.

  Single phase at 120, 208 or 240 V, one heater on its own circuit. The breaker is at least 125 percent of the rating (422.13) and at most the plate rating or 150 percent rounded up (422.11(E)), 25 to 30 A for 4,500 W at 240 V. Checks the breaker on a circuit now. Tanks of 120 gallons or less; tankless units are not covered.

  Based on NEC Article 422 (422.13, 422.11)