HVAC Calculators

Tools for HVAC contractors.

Every BuildSolver tool runs an ACCA, ASHRAE or IECC procedure in deterministic code. The assistant collects your inputs, passes them to that code and explains each line of the result. Results list the standard and the assumptions behind each number, so you or a plan reviewer can check the math line by line.

The on-page calculators and whole-house loads in the assistant are free without signup, within daily limits. Room-by-room Manual J, Manual S equipment selection and full Manual D are part of the Pro plan.

Maintained by BuildSolver · Updated

The calculations

Two ACCA procedures run as deterministic code. The assistant collects the inputs in conversation, runs the formula and shows the table behind every row.

  • 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

  • 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

Free on-page calculators

Single-purpose forms that run the same deterministic tools as the assistant and return the number on the page. No signup; the on-page calculators share a free daily and monthly limit.

  • 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

  • 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)

  • 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 that zone's worst-case 1% / 0.4% cooling and 99% / 99.6% heating values - the same lookup the load calculators use.

    Based on IECC 2021 climate zones

  • 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

Field and sizing formulas

The CFM, superheat, ventilation, bath fan and SEER2 calculators run a published formula in your browser, with no daily limit. The AC size and BTU per square foot tables come from the Manual J engine and are computed when the site is built. On those pages the load form counts toward the shared daily and monthly limit.

  • Equipment size

    What Size AC by Square Footage

    Cooling tons for 800 to 3,500 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

By job type

The same Manual J procedure, walked through for the four situations contractors actually quote. Each page has a worked project example and a side-by-side with the tool you would otherwise use for that job.

  • 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

Manual J by state

Ten state pages carrying the inputs that change with location: 99% heating and 0.4% cooling design temperatures for the top metros (ASHRAE 169-2021), which IECC edition the state has adopted, and what the licensing board expects of whoever signs the job. These pages are reference data; the calculator looks up design conditions from the project location, not from the state page.

Working somewhere else? The main Manual J calculator takes a US ZIP code and looks up the same ASHRAE design conditions.

Background reading

Longer write-ups on the procedures behind the tools - what each calculation decides, and where the rules of thumb go wrong.

More in the engineering blog.

Before you quote off these numbers

BuildSolver follows ACCA Manual J procedures but is not ACCA-approved software for permit submission. It is built for the sales phase: preliminary sizing and client quotes. For a submittal that an AHJ will stamp, use certified software or a licensed engineer - the permit page covers where that line falls.