HVAC Load Calculator
Size the whole system in 60 seconds. Quote with confidence. Close more deals.
ZIP not found — pick your climate region:
- Excellent — post-2018 code or deep retrofit. R-19+ walls, R-38+ attic, spray foam.
- Better than average — 2010-era. R-15 walls, R-30 attic, double-pane low-E windows.
- Average — standard-code compliant. R-13 walls, R-30 attic, double-pane windows.
- Worse than average — pre-1990 unimproved. R-11 or less, single-pane windows, visibly drafty.
- Bad — no wall insulation. Pre-1950 construction; cold spots, daylight around outlets.
- A large amount — mostly south/west-facing with few trees or shade.
- A medium amount — mix of sun and shade across the day.
- Little or none — north-facing or heavily shaded by trees or neighbors.
- Standard — typical residential. Glass is ~15-20% of floor area.
- Many — large glass expanses, walls of windows, multiple skylights, sunroom-style spaces. More than 25% of floor area in glass.
- Sealed tight & double-paned — modern double-pane with weatherstripping. Newer construction or recent retrofit.
- Typical fit and seal — average 2000s-era double-pane, some minor drafts.
- Not well-sealed & single-paned — old single-pane, visible drafts, weatherstripping gone.
Count: TVs, desktop computers, servers, halogen lighting. Each adds ~600 BTU/h.
Skip: modern LED lighting, phone chargers, anything in standby. Baseline is 0; most homes don't need more than 2-3.
Yes if the space includes a kitchen — adds ~4,000 BTU for cooking + appliances. No for kitchen-less spaces (in-law suites, basement conversions without cooking) or when you're accounting for kitchen load elsewhere.
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Enter your email to get your whole-house load calculation — cooling BTU, heating BTU, and system tonnage.
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Your Load Calculation
Estimate only. For permit-ready Manual J, use ACCA-accredited software.
How to Use This Calculator
1. Enter a ZIP code. Auto-detects the climate region — a major factor in both cooling and heating load.
2. Add square footage and ceiling height. Total conditioned living space only. Exclude unfinished basements, garages, and unconditioned attics.
3. Rate insulation and window/door tightness. A 1980s retrofit with single-pane windows is "Bad" / "Not well-sealed." A 2020s tight envelope is "Excellent" / "Sealed tight."
4. Set sun, window count, and internal gains. South/west-facing homes with lots of glass take heavy cooling load. Each extra occupant adds ~600 BTU. Devices × 3.412 = BTU.
5. Click Calculate. Enter your email. We'll send the full results so you can reference them on-site or attach to a quote.
How the Calculation Works
We use a 20 BTU/sqft baseline with six adjustment factors (climate, insulation, sun, windows, tightness, ceiling height), then add internal gains for occupants, devices, and kitchen equipment.
base = sqft × 20 × (ceiling / 8)
modSum = climate + insulation + sun + windows + tightness
coolingBTU = base × (1 + modSum) + occupants + devices + kitchen
heatingBTU = coolingBTU × heatingFactor(region)
tonnage = round(coolingBTU / 12,000, 0.5)
Climate regions range from +10% (Hot) to -10% (Cold). Heating factor ranges from 0.70 (Hot) to 1.45 (Cold) — so a home in Minneapolis will have a heating load roughly 1.5× the cooling load, while a home in Phoenix will have heating at ~70% of cooling.
When You Need a Full Manual J
This tool is a pre-quote sanity check. Accurate enough to keep you from badly oversizing or undersizing a system in a sales conversation, and to back up a quote with a defensible number.
You need a full ACCA Manual J when:
- The job requires a permit (most new construction, many major replacements)
- The homeowner is pursuing ENERGY STAR certification or rebate programs
- A HERS rater or building department is involved
- Equipment warranties require a Manual J for the full capacity
For those cases, use ACCA-accredited Manual J software — free options exist alongside the paid industry standards.
Frequently Asked Questions
Is this a Manual J calculation?
How accurate is this compared to a full load calc?
Why does the ZIP matter so much?
Does this work for mini-splits?
Why is heating BTU different from cooling BTU?
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