Free HVAC load calculator

Most free HVAC load calculators on the web are a multiplication. You give them square footage, they divide by 500 or 600, and they hand back a tonnage. That is the same rule of thumb that has been oversizing residential equipment for forty years — and oversizing is not a harmless error. A cooling system that is a ton too big short-cycles, never runs long enough to pull moisture out of the air, and leaves a house that is cold and clammy instead of comfortable. The homeowner then complains about humidity, and the fix costs more than the right-sized equipment would have.

This tool does something different. It runs a genuine whole-house block load: conduction through walls, ceilings, windows, and doors; solar gain through glazing; infiltration driven by the tightness implied by your insulation level; internal gains from occupants and appliances; and duct losses based on where the ductwork actually runs. Those pieces are summed into a sensible and latent cooling load and a heating load, then rounded to a nominal equipment size the way the standard expects.

Load is fundamentally driven by the temperature difference between the inside of the house and the outdoor design condition. Get that difference wrong and every downstream number is wrong by the same proportion, no matter how carefully you entered the rest.

The convention is the 99% winter dry-bulb and the 1% summer dry-bulb: the temperatures your location stays above and below for 99% and 1% of the hours in a year. They are not record highs and lows — sizing to a record cold snap is another route to oversized equipment. Most free calculators skip this entirely and offer a dropdown with four or five climate regions, which can put you 20°F or more away from your actual design condition. Minneapolis and St. Louis are both “cold,” and their heating design temperatures are about 40°F apart.

This calculator looks your ZIP up against a published design-temperature dataset and shows you exactly which location and which temperatures it used, along with how precisely your ZIP resolved — a specific weather station, a mapped reference city, or a regional approximation from the three-digit prefix. If it had to approximate, it says so rather than presenting a regional guess as a station-grade number.

Honesty about scope is the difference between a useful estimating aid and a number that gets someone in trouble. A block load from eight inputs answers the equipment-sizing question well enough to sanity-check a proposal, catch an obviously oversized quote, or get a budget number on the phone. It assumes typical values for a lot of things a full calculation would ask you about: window orientation and shading, foundation type and slab perimeter, occupancy, appliance load, ventilation, and the specific R-values of your assemblies.

What it will not do is tell you how much air each room needs. That is a room-by-room calculation, and it is the input to duct design — a block load cannot substitute for it, no matter how accurate the whole-house total is. It also will not produce Manual S equipment selection guidance, a heat-pump balance point, or a report you can hand to a building department. If you need any of those, the HVAC Load Calculator app does room-by-room loads, equipment sizing, and branded PDF reports, and it works with no signal in a crawlspace. If the job is Canadian, CSA F280-12 is a different rulebook and needs its own tool.

For more on that distinction, read block load vs room-by-room Manual J.

Once you know your load, the next question is usually where to buy. A local supply house is the right answer for most contractors; a homeowner, or a tech without a distributor relationship, can buy complete central systems online in the same standard sizes. Whatever you buy and wherever you buy it, size the equipment to your calculated load per Manual S — not the other way around. How our affiliate links work.

Complete central systems, heat pumps and ductless mini-splits in standard sizes are sold at HeatAndCool.com (affiliate link — we earn a commission on purchases, and your price is unchanged).

Ductless or window equipment in these sizes is also sold on Amazon (affiliate link — as an Amazon Associate we earn from qualifying purchases).

Going ductless, or running a window or portable unit? Sensibo makes smart controllers that add scheduling, remote control and energy tracking to any remote-controlled AC or heat pump (affiliate link — we earn a commission on purchases, and your price is unchanged).

Prefer to have it installed? Get quotes from local installers (a referral link — we may be paid for the lead, at no cost to you).

• Offline first. Crawlspaces, mechanical rooms, and new builds with no service. Every calculation runs on the phone. Nothing waits on a signal.
• Honest math. Results show what they assumed. A defaulted friction rate or an assumed equipment rating never hides in the fine print.
• Named sources. ASHRAE design temperatures, NBC Appendix C climatic design data, ACCA Manual J and Manual D methods, UL and ASHRAE charge limits. Methodology language only — never “approved” or “certified.” The data behind every number is cited in the app.
• Made for the truck. Big targets, few inputs, results you can screenshot and text to the office. Skimmable one-handed between calls.

• Free online ductulator Size round or rectangular duct from CFM, friction rate, and velocity limits.
• A2L P/T chart & superheat calculator R-454B, R-32, and R-410A superheat and subcooling with bubble/dew glide handled correctly.
• What size AC unit do I need? Calculated cooling load to nominal tonnage, size by size from 1.5 to 5 tons, with the Manual S window each size covers.
• AC tonnage per square foot chart The rule-of-thumb chart by climate band, an honest error range, and the one job a rule of thumb is good at.
• Matched systems and AHRI ratings Why the rating belongs to the combination and not the box, and how the A2L transition made it a hard stop.
• Block load vs room-by-room Manual J When a whole-house block load is enough, and when room-by-room earns its keep.
• CSA F280-12 vs Manual J Same physics, different rulebook — climate data, basements, ventilation, and AHJ expectations.
• Duct static pressure & friction rate Available static pressure, total effective length, and how field TESP readings drive Manual D.