HVAC Load Calculator

What this calculator does

Every shortcut in HVAC sizing fails the same way: it replaces the building with its floor area. A shaded, tight, well-insulated 300 sq ft room and a glassy, leaky one of the same size can differ threefold in load, and the difference is not noise, it is the whole subject. This estimator therefore does the simplified physics properly: conduction through each surface at stated U-values and the local design temperature difference, solar gain through the glass at its SHGC and your shading, infiltration from an air-change rate, and internal gains from people (sensible and latent, at the standard seated rates) and electrical watts. Latent load is reported on its own line because humid-climate failures are usually moisture failures: a system can hit the temperature and still leave the room clammy, which is an oversizing symptom the results section discusses.

How to use it

• Design temperatures are local facts. The 1 percent summer and 99 percent winter design values for your town are published (ASHRAE tables, and every local contractor knows them); 95 and 20 F are common but not universal, and the fields take your real numbers.
• Measure windows in total, honestly. Glass is the dominant cooling input in most rooms: its conducted gain is modest, its solar gain is not, and the shading percentage is where honesty pays, since a shaded west window and a naked one are different appliances.
• Indoor conditions are fixed and stated. Cooling is computed against 75 F indoors and heating against 70 F, the standard design points; a household that lives at 72 in summer genuinely needs more capacity, and a professional calculation would move that input.
• Read the result as a range, not a size. The output gives total load and the sensible-latent split; equipment is then chosen by a professional at the next nominal size with the split in mind, not by rounding this number up at a sales desk.

The method

The solar factor of 140 BTU per hour per sq ft is the stated peak-sun assumption for glass at typical US latitudes before SHGC and shading; orientation moves it considerably, which is one of the named limitations. Heating takes no credit for sun or internal gains, the conservative professional convention, since the coldest design hour is a dark one. Capacity in tons is the total cooling load over 12,000 BTU per hour per ton.

Worked examples

A 20 by 15 ft room, 9 ft ceiling. Walls net of 40 sq ft of glass: 590 sq ft at U 0.08 and a 20-degree difference (95 out, 75 in) conduct 944 BTU/hr; the ceiling adds 300; the glass conducts 400 and admits 2,240 in sun at SHGC 0.40 unshaded. Infiltration at 0.5 air changes adds 486 sensible and, at the moderate humidity setting, about 536 latent. Four people add 1,000 sensible and 800 latent; 500 watts of equipment adds 1,706; attic-duct losses at 10 percent add 213. Totals: 7,289 BTU/hr sensible, 1,336 latent, 8,625 in all, about 0.72 tons. Heating the same room at a 50-degree difference (70 in, 20 out) needs 5,325 BTU/hr, envelope and infiltration only.

What moves the answer. Shade that glass fully and the solar line’s 2,240 nearly vanishes, taking the total under 6,400: the single largest lever in the room was a blind. Swap to single glazing and winter’s number jumps by 800 on conduction alone. The estimator’s value is exactly this: it shows which input owns the load, which no per-area rule can even describe.

Reading your results

Read the sensible-latent split before the total. In humid climates a latent share above roughly a quarter of the total warns that short-cycling equipment will under-dehumidify, which argues for the smaller end of the capacity range and longer runtimes, the opposite of the oversize instinct. Read the heating figure as a floor for furnace or heat-pump sizing at the design temperature, remembering that heat pumps lose capacity as it gets colder and their balance point is a separate professional calculation. And read any result that lands implausibly far from a contractor’s Manual J as a prompt to compare assumptions line by line; the breakdown exists so the argument can be about inputs, not totals.

Limitations

• This is a preliminary estimate, not Manual J. The professional method models orientation per window, construction assemblies layer by layer, duct geometry, blower heat and local design data. For equipment purchase, permits and any guarantee of comfort, that calculation is the requirement, and this page says so on its results as well as here.
• One averaged solar factor stands in for orientation. East glass peaks in the morning, west in the afternoon, north glass (in the northern hemisphere) barely at all; the 140 factor is a planning value, and glass-heavy designs deserve the per-orientation treatment.
• Multi-storey and attached buildings differ. Shared walls and floors between conditioned units carry little load; the envelope fields here describe one exposed space, and a whole house is properly calculated room by room and summed.
• Oversizing is a defect, not a margin. Equipment well above the load short-cycles: worse humidity control, more wear, more noise, higher cost. The capacity conversation should start at the calculated load and justify every step up, rather than starting big and calling it safety.

Frequently asked questions

Why not just use BTU per square foot?

Because the worked example is the refutation: one 300 sq ft room whose total swings from about 6,400 to 8,625 BTU/hr on shading alone, and would swing far wider on insulation or glass area, all at the same square footage. Rules like 20 or 25 BTU per sq ft are averages of other people’s buildings wearing your building’s clothes; they persist because they are easy, and they mislead in both directions, oversizing tight modern rooms (with the humidity problems that follow) and undersizing glassy leaky ones.

What is the difference between sensible and latent load?

Sensible load is heat you can feel on a thermometer; latent load is moisture the system must condense out of the air, which costs capacity without changing temperature. People, cooking, showers and humid infiltrating air all add latent load. Air conditioners split their capacity between the two in a ratio fixed by their design and runtime, so a load with a large latent share needs equipment and runtimes chosen with that split in mind, which is why the results never merge the two into one silent number.

Why does oversizing an air conditioner cause humidity problems?

Because dehumidification needs runtime: moisture condenses on a cold coil steadily while the unit runs, and an oversized unit reaches the temperature setpoint quickly and stops, over and over, wringing little water in its short runs. The room then sits cool and clammy, occupants lower the setpoint to compensate, and the bills rise for worse comfort. Correct sizing, with long gentle cycles near the design load, is a moisture strategy as much as a temperature one.

When is this estimate enough, and when do I need Manual J?

This estimate is enough for planning: comparing insulation or window upgrades, sanity-checking a quote’s ballpark, sizing expectations before a contractor visit. Manual J (or your region’s equivalent professional method) is needed for buying equipment, replacing a system, permitting new construction and any situation where the installer will stand behind the result. The honest sequence is this page first for understanding, the professional calculation second for the signature.

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Method: simplified steady-state heat balance with stated U-values, design temperature differences, a stated peak solar factor, standard air constants (1.08 sensible, 0.68 latent) and standard per-person gains; preliminary only, not a substitute for a professional load calculation. Last reviewed: 15 October 2026.