Calculators

HVAC equipment sizing calculators

Picking a unit is a second step after the load. A furnace is sized by input and AFUE, a heat pump by the capacity left at the outdoor design temperature rather than its 47 degree rating, and an AC by tons with a check on the sensible and latent split.

These calculators take a load you already have, or estimate one from a ZIP code, and return the nominal size. Manual S, the ACCA procedure that checks a specific system against the load, is explained on its own page and runs against AHRI-rated equipment in the Pro plan.

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Which one you need

Every tool in this section

  • 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

Other calculators

Every calculator here lists the standard it works from and what it leaves out. The numbers are for sales-phase sizing and field checks, not a permit submittal. Methodology lists the standard behind each one.