People, design temps, ducts and moreOccupants, rooms, design temperatures, ducts, floor, doors, roof, equipment
Design temperatures fill in from the climate zone; type your own city's numbers to replace them. Leave outside wall length at 0 to use a square footprint.
Climate, design temperatures, insulation, glass, ceiling height, air tightness and ducts come from the Whole house tab.
See full breakdown
An HVAC load is the heat a home gains on a hot design day and loses on a cold one, measured in BTU per hour. It is driven by floor area and ceiling height, insulation R values, window glass and the direction it faces, air leaks, and your area's summer high and winter low. Divide the cooling load by 12,000 for tons and the heating load by 3,412 for kilowatts. A 2,000 sq ft, averagely insulated house in climate zone 4 comes to about 21,500 BTU/h of cooling (1.8 tons) and 33,300 BTU/h of heating. That is fine for planning, but permits and new equipment call for a full room by room Manual J.
How to calculate HVAC load
An HVAC load calculation adds up every path heat takes into the house in summer and out of it in winter. Cooling also counts sun through the glass, people and moisture. Heating leaves those out on purpose, which gives it a built in margin.
| Cooling load heat gain on a summer design day | Heating load heat loss on a winter design day | |
|---|---|---|
| Walls, attic, doors | U factor × area × the temperature difference, adjusted for sun on the wall or roof and the day's temperature swing | U factor × area × (68°F minus the winter design temperature) |
| Windows | U factor × area × temperature difference, plus sun through the glass: peak sun × SHGC × blinds × a factor for the direction it faces | U factor × area × temperature difference. Sun is ignored |
| Floor | Slab left out because it slightly lowers the load; a crawlspace floor is counted | Slab edge: 0.5 × perimeter × temperature difference. Crawlspace floor: U × area × temperature difference |
| Air leaks | 1.1 × leakage CFM × temperature difference, plus 4,840 × CFM × the outdoor to indoor moisture difference | 1.1 × leakage CFM × temperature difference |
| People and appliances | 464 + 0.7 × floor area + 75 per person, plus 68 + 0.07 × floor area + 41 per person of moisture, plus 3,412 per kW of extra equipment | Not counted |
| Ducts | Subtotal × a duct factor for where the ducts run: 0 inside the living space, up to 0.71 for cooling in a leaky, lightly wrapped attic | |
| Load | Sensible + latent = BTU/h, ÷ 12,000 = tons | Sum of the losses = BTU/h, ÷ 3,412 = kW |
Worked example: 2,000 sq ft, one story, zone 4, average insulation, clear double pane glass at 15% of the floor
The same house loses 33,256 BTU/h on Baltimore's 18°F design night (9.7 kW): walls, attic and doors 9,594, windows 7,650, slab edge 4,472, air leaks 7,714 and attic ducts 3,826. Windows are the biggest single cooling load here, and air leaks plus walls lead on the heating side.
How this HVAC load calculator works: it follows the residential load factor method published for quick whole house and room estimates, with design temperatures for a city in each climate zone, U factors for each insulation level and glass type, and leakage areas for each grade of air sealing. Change any input and every line of the breakdown updates.
When to use this HVAC load calculator
Run a quick HVAC load calculation any time someone needs a heating or cooling number before a full Manual J report is worth paying for.
- Sizing a new system: get the cooling and heating load first, then pick equipment that covers it without overshooting. An oversized cooling unit performs poorly, worst of all where summers are humid, so the load is the number to start from.
- Comparing properties: run two homes side by side and see which one needs more heating and cooling.
- Budgeting an install: the load sets the equipment size you get prices for.
- Contractor proposals: put a block load in the proposal so the homeowner sees where the tonnage came from. The breakdown shows windows, air leaks and ducts as separate lines, which makes upgrades easy to explain.
- Energy audits and upgrades: switch insulation, glass or air tightness and watch the load drop.
- Benchmarking a load: check a number from software or a bid against the same published method. If the two are far apart, ask what the other calculation assumed for windows, leakage and design temperatures.
- Problem rooms: use the One room tab on the bedroom that never cools.
- Additions and conversions: a finished attic, sunroom or garage conversion adds load that the existing unit may or may not cover. Run the new room on its own and compare it with the spare capacity you have.
How big is the heating and cooling load in each climate zone?
The same 2,000 sq ft, one story house run through the calculator in each zone. Cooling barely moves from Miami to Minneapolis; heating grows more than fivefold.
| Zone and city | Cooling, average insulation | Tons | Heating, average insulation | Cooling, good insulation | Heating, good insulation |
|---|---|---|---|---|---|
| 1, Miami | 22,473 | 1.87 | 10,040 | 20,611 | 8,956 |
| 2, Houston | 23,661 | 1.97 | 22,014 | 21,633 | 19,711 |
| 2 dry, Phoenix | 28,633 | 2.39 | 17,272 | 25,811 | 15,444 |
| 3, Atlanta | 21,595 | 1.80 | 27,565 | 19,780 | 24,720 |
| 4, Baltimore | 21,541 | 1.80 | 33,256 | 19,772 | 29,869 |
| 5, Chicago | 20,208 | 1.68 | 43,559 | 18,610 | 39,223 |
| 6, Minneapolis | 19,821 | 1.65 | 51,183 | 18,265 | 46,172 |
| 7, Duluth | 15,907 | 1.33 | 55,864 | 14,783 | 50,445 |
Both columns assume clear double pane glass at 15% of the floor area spread on all sides and half shaded, average air sealing, four people, a slab floor and well sealed attic ducts with an R8 wrap. Phoenix shows no moisture load because its design day air is drier than the house.
What design temperatures does a load calculation use?
Cooling uses the outdoor temperature a city tops in only 1% of the hours in a year, and heating uses the one it drops below in only 1%. Indoors, the usual targets are 75°F at no more than 50% to 65% humidity in summer and 68°F in winter.
| Zone and city | Summer design | Wet bulb | Daily swing | Winter design | Used here |
|---|---|---|---|---|---|
| 1, Miami | 90.8°F | 77.6°F | 11.8°F | 52.6°F | 91°F / 52°F |
| 2, Houston | 95.5°F | 76.5°F | 18.8°F | 34.1°F | 96°F / 34°F |
| 2 dry, Phoenix | 108.3°F | 69.3°F | 20.8°F | 41.8°F | 109°F / 41°F |
| 3, Atlanta | 91.6°F | 73.8°F | 16.7°F | 26.5°F | 92°F / 26°F |
| 4, Baltimore | 91.4°F | 74.0°F | 18.6°F | 18.4°F | 92°F / 18°F |
| 5, Chicago | 88.5°F | 72.9°F | 17.9°F | 4.4°F | 89°F / 4°F |
| 6, Minneapolis | 87.8°F | 72.0°F | 17.3°F | 5.8°F below zero | 88°F / 6 below |
| 7, Duluth | 81.1°F | 67.1°F | 18.8°F | 12.0°F below zero | 82°F / 12 below |
Summer values are rounded up and winter values rounded down before they go into the load, so each one leans to the safe side. Wet bulb sets how much moisture the outdoor air carries; the daily swing tells the method how much the house cools off overnight.
What affects HVAC load the most?
Starting from the worked example house (21,541 BTU/h cooling, 33,256 BTU/h heating), here is what one change at a time does to the load.
| Factor | What changes the load | Effect on the example house, BTU/h |
|---|---|---|
| Insulation | Walls and the attic are the largest surfaces, so their U factor sets the floor for both loads. Poor insulation hits heating hardest because the winter temperature gap is wider. | Poor: 32,778 cooling, 54,197 heating. Good: 19,772 and 29,869 |
| Window glass | Sun through clear glass is the biggest cooling line in the example house, so better glass moves cooling far more than heating. | Single pane, aluminum: 25,670 cooling. Double pane low E: 17,791 |
| Window direction | West glass catches the hot afternoon sun; north glass sees almost none. | All glass north: 16,310. All west: 27,157 cooling |
| Shading | Overhangs, porches, trees and the house next door block direct sun. Shaded glass still lets in diffuse light, so the load never drops to zero. | No shade: 26,157. Fully shaded: 16,926 cooling |
| Window area | More glass means more conduction and more sun. | 10% of the floor: 18,639. 25%: 27,344 cooling |
| Air tightness | Leaks add both heat and moisture in summer and cold air in winter. | Tight: 26,719 heating. Leaky: 41,974 |
| Duct location | Ducts in a hot attic pick up heat before the air reaches the rooms. Inside the living space, the loss is zero. | Inside: 17,361 cooling. Leaky attic, R4 wrap: 28,354 |
| Ceiling height | Taller walls add wall area and air volume. | 10 ft ceilings: 23,229 cooling, 36,991 heating |
| People | Each person adds body heat and moisture, plus cooking and showers. | About 116 BTU/h per extra person across the house; 430 per person in one room |
| Extra equipment | A second fridge, a server rack or a kiln running at peak hours adds straight to the sensible load. | 3,412 BTU/h per kW, before duct gains |
| Roof color | Changes how hot the attic gets above an insulated ceiling. | Light: 21,333. Dark: 21,687 cooling |
How much heat comes through each type of window?
Two numbers rate a window. The U factor sets how fast heat conducts through it, and the solar heat gain coefficient (SHGC) sets how much sunshine gets in. Low E glass cuts both.
| Glass | U factor | SHGC | Window cooling load | Window heat loss | House cooling / heating |
|---|---|---|---|---|---|
| Single pane, aluminum frame | 1.27 | 0.75 | 10,868 | 19,050 | 25,670 / 46,138 |
| Single pane, wood or vinyl frame | 0.89 | 0.64 | 8,794 | 13,350 | 23,036 / 39,697 |
| Double pane, clear | 0.51 | 0.57 | 7,617 | 7,650 | 21,541 / 33,256 |
| Double pane, low E | 0.39 | 0.31 | 4,664 | 5,850 | 17,791 / 31,222 |
| Triple pane, low E | 0.36 | 0.21 | 3,397 | 5,400 | 16,182 / 30,713 |
Values are for operable windows in wood or vinyl frames unless the frame is named; a rated label on the window beats any typical value. All four rows use the worked example's 300 sq ft of glass.
How much do air leaks add to the load?
Leakage is set by how carefully the house was sealed, and each step down in quality doubles the leakage area. Only a blower door test measures it; looking around the house does not give even a rough number.
| Air sealing | What it looks like | Leakage area | Winter air changes per hour | Cooling load | Heating load |
|---|---|---|---|---|---|
| Tight | Sealed under an air sealing specialist | 0.01 | 0.13 | 19,281 | 26,719 |
| Good | Carefully sealed by a knowledgeable builder | 0.02 | 0.26 | 20,035 | 28,897 |
| Average | Typical current production housing | 0.04 | 0.53 | 21,541 | 33,256 |
| Leaky | Typical house built before 1970 | 0.08 | 1.05 | 24,556 | 41,974 |
| Very leaky | Old house in original condition | 0.15 | 1.97 | 29,830 | 57,229 |
Leakage area is square inches of hole per square foot of outside wall and attic. Air changes per hour are for the worked example house on its winter design night, when wind and the indoor to outdoor temperature gap push the most air through.
When do you need a professional Manual J, and what does it cost?
The residential code and the energy code both call for loads worked out with ACCA Manual J and equipment picked with ACCA Manual S. A third party Manual J usually costs $79 to $800, and many contractors include one free with an installation quote.
| Who runs the HVAC load calculation | Typical price |
|---|---|
| Budget third party service | $79 to $100 |
| Mid range third party service | $150 to $300 |
| Complex or large homes | $300 to $800 |
| HVAC contractor, as a standalone job | $100 to $200 |
| Included with an installation quote | Free |
| Load software for a contractor | $45 per project, or $50 per user a month |
A pro Manual J is the right call for new construction, a permit for replacement equipment, a home with big glass or a big addition, and any time comfort complaints follow a new install. Send plans or measurements, window sizes and directions, insulation levels, the duct location and, if you have one, a blower door number.
How do HVAC contractors run a Manual J load calculation?
Manual J, 8th edition, is the procedure widely used across the US. Whatever the software, a room by room load comes down to these steps.
- Measure from the outsideTake floor area from the outer wall dimensions and wall height floor to floor. It is quicker, slightly conservative, and lines up with the plans.
- Pick design conditions1% summer and 99% winter temperatures for the location, 75°F and 68°F inside.
- Rate every surfaceAssign a U factor to each wall, ceiling, floor and door, and a U factor and SHGC to each window, from the NFRC label where there is one. Older single pane glass with a storm window counts as double pane.
- Set the air leakageUse a blower door number if the house has one, otherwise the construction quality class.
- Add people, appliances and ductsOccupants default to bedrooms plus one. Duct gains and losses go on last, and they can be large when the ducts sit in a hot attic.
- Split it room by roomEach room gets its own walls, glass and share of the air leakage by volume. That feeds Manual S for the equipment and Manual D for the ducts, and the One room tab above runs the same math for a single room.
What profit margin do HVAC contractors make?
Load calcs, proposals and installs all have to fit inside the margin. Gross margin is what is left after labor and equipment on the job; net margin is what survives trucks, office, insurance and the owner's pay.
| Margin | HVAC industry average | Well run HVAC shops |
|---|---|---|
| Gross margin, after job costs | 30% to 40% | 50% to 55% or more |
| Net margin, after overhead | Around 10% | 17% to 20% or more |
A net margin under about 10% leaves little cushion for a mild summer, a cash squeeze or a new competitor in town.
Your load calc is wasted if another shop books the job first.
A homeowner with a dead air conditioner keeps dialing until someone picks up, and your techs cannot answer from inside an attic with a tape measure in hand. ServiceAgent picks up every call around the clock, puts the site visit straight on your calendar and follows up on the proposal after the load is done, so the system you sized is the one your crew installs.
Book a free demoFrequently asked questions
What people ask most about heating and cooling loads, Manual J and what this calculator counts.
Book a free demoWhat is an HVAC load?
It is the rate at which a house gains heat on a hot design day or loses it on a cold one, in BTU per hour. The cooling load includes moisture as well as heat, which is why it splits into sensible and latent parts. The equipment has to match the load, not the floor area.
How do you calculate HVAC load?
Multiply each surface's U factor by its area and the indoor to outdoor temperature difference, add sun through the windows, air leakage, people, appliances and duct gains, and total it. Divide the cooling total by 12,000 for tons and the heating total by 3,412 for kilowatts.
Can I do an HVAC load calculation online?
Yes. This calculator runs a whole house or single room load with published factors, which is enough to plan, compare houses or sanity check a bid. For a permit or a new system, get a full Manual J from a contractor or load calculation service.
How accurate is this HVAC load calculator?
The method behind it gives cooling loads within about 10% of a full heat balance calculation for typical wood frame homes in warm and hot climates. Most of the error comes from the inputs: guessed insulation, window areas and air leakage move the answer far more than the math does.
Is this HVAC load calculator useful for contractors?
Yes. It gives a fast, consistent block load at the kitchen table, a check on one problem room, or a number for a proposal before the full Manual J is run. Every assumption, from design temperatures to duct location, is an input you can change, and the breakdown prints on one page.
How much does a Manual J load calculation cost?
Usually $79 to $800 from a third party service, depending on the size and complexity of the home, or $100 to $200 from an HVAC contractor as a standalone job. Many contractors include it free when they quote the installation.
Can I use it for a commercial building?
No. It is built for houses, where the envelope and air leaks drive the load. Offices, shops and restaurants have much higher internal gains and need a nonresidential load method.
What happens if the system is oversized or undersized?
An oversized air conditioner performs poorly and is worst at removing moisture where summers are humid. Being a little under is less harmful than it sounds: a 20% shortfall usually means the house drifts only a degree or two above the setpoint on the hottest afternoons.
Does the load include ductwork or installation costs?
It includes the heat ducts gain or lose based on where they run, but not what they cost. Size the ducts with the duct calculator, and pick the equipment size with the HVAC sizing calculator.
Why is my heating load bigger than my cooling load?
The winter temperature gap is usually much wider than the summer one, 50°F against 17°F in the zone 4 example. Heating also leaves out sun and internal gains, so the number carries a built in safety margin.
How many tons do I need for my load?
Divide the cooling load by 12,000 to get tons; the calculator also shows the next half ton step above it, since most units come in half ton sizes. Choosing the actual unit is a Manual S job that checks the equipment's real output at your design conditions.
Leads growth and product marketing for ServiceAgent at SaaS Labs, where he has worked in product marketing since 2020. Writes ServiceAgent's coverage of AI call answering, CRM and home services operations.
NATE certified Master HVAC Technician with 14 years in residential and light commercial HVAC. Former Senior Service Tech at Trane Austin, now reviews HVAC content for ServiceAgent.
Published by ServiceAgent, the AI front office for home service teams. Loads are planning estimates built from typical construction values. Equipment for a permit or a new install should be sized from a full room by room ACCA Manual J and selected with Manual S for the actual house.