HVAC Load Calculator

Find out how many BTUs of heating and cooling your home or small commercial space actually needs, and what equipment tonnage to look for, with an HVAC load calculator that goes further than a flat square-footage rule of thumb. Enter your ZIP code to auto-detect your climate zone, describe your building's envelope and openings, and get separate heating and cooling numbers — plus a line-by-line breakdown of exactly what drove your result, so you can see why the number is what it is, not just what it is.

Choose Residential or Light Commercial, enter your ZIP code and building details, then calculate. Cooling load accounts for occupants, sunlight, and equipment heat since those all add warmth in summer; heating load leaves those out, since proper heating design sizes for the coldest morning with nobody home and nothing running. Formula: Cooling = Envelope + Windows + Doors + Occupants + Room/Appliance Loads · Heating = Envelope + Windows + Doors.

Building Type

Light Commercial uses higher occupancy and equipment-load assumptions than Residential. Large or complex commercial buildings need a licensed engineer's Manual N/ASHRAE assessment — see the FAQ below.

Location & Climate Zone

Enter your ZIP code to auto-detect your climate zone, then verify it — this is a broad regional approximation, not a precise map. See the FAQ if your ZIP doesn't resolve.

ZIP Code

Climate Zone — verify / adjust

Building Basics

Square Footage — conditioned area

Ceiling Height — feet

Envelope

Not sure what a field means? Tap the ⓘ next to its label.

Insulation Quality

Sun Exposure

Air-Tightness

Windows & Doors

Windows — count

Window Type

Exterior Doors — count

Occupancy & Heat Sources

Occupants

Equipment Heat — watts, optional

Kitchen Present?

Sunroom / Glassed Room?

Your Results

Cooling Load

—

BTU/h

Recommended Size

—

tons (cooling)

Heating Load

—

BTU/h

Climate Zone

—

used in this result

Formula: —

Cooling Load Breakdown

Results are for reference and estimation only. See full disclaimer below.

How to Use the HVAC Load Calculator

Choose Residential or Light Commercial

Pick the scope that matches your building — the calculator adjusts occupancy assumptions accordingly.

Enter Your ZIP Code and Building Basics

Enter your ZIP code to auto-detect your climate zone, then add square footage and ceiling height.

Describe the Envelope and Openings

Select insulation quality, sun exposure, and air-tightness, then enter window count and type, exterior doors, and occupants. Residential users can flag a kitchen or sunroom.

Review Your Results

Click Calculate to see cooling load, heating load, and recommended equipment tonnage, plus a line-by-line breakdown of what drove the number. Use Print to capture the results as they appear on screen.

Frequently Asked Questions

Is this the same as a Manual J calculation?

No. This tool uses a simplified adjusted-square-footage method, similar in spirit to the quick estimates HVAC contractors run on-site, and is designed to land within roughly 10-15% of a full Manual J result for standard homes. A full ACCA Manual J calculation accounts for far more construction detail and is required for new-construction permits — use this tool for planning and quote-checking, not for permit documentation.

Why does the calculator show separate heating and cooling numbers?

Heating and cooling loads are driven by different physics. Cooling load includes heat from occupants, sunlight through windows, and equipment, because all of that makes a space warmer in summer. Heating load leaves those out, because proper heating design sizes for the worst case — an empty house on a cold morning with no solar gain and no equipment running — so those internal and solar gains aren't counted on as free heat.

What if my ZIP code doesn't match a climate zone automatically?

The ZIP lookup uses a simplified state-level climate zone approximation, not a county-by-county map, so it can occasionally miss or land on a neighboring zone in climate-diverse states. The detected zone always appears in an editable dropdown — check it against your local climate and adjust manually if it looks off.

Can I use this for a commercial building?

The Light Commercial scope works for small, simple, enclosed spaces like offices and retail units, using higher occupancy and equipment-load assumptions than residential. Larger, more complex, or multi-zone commercial buildings need a full ACCA Manual N or ASHRAE assessment from a licensed mechanical engineer, which almost every commercial permit office requires — this tool is planning-level only for that scope.

How accurate is this compared to a professional load calculation?

Treat the result as a solid planning-level estimate, not a final answer. It's useful for budgeting, sanity-checking a contractor's quote, or catching an obviously oversized or undersized recommendation before you commit — but construction details, duct condition, and site-specific factors this tool can't see mean the final equipment selection should come from a licensed HVAC professional's on-site assessment.