About the HVAC Sizing (BTU) Calculator
Ask what size air conditioner a house needs and you will usually be told 20 BTU per square foot, or a ton for every 500–600 square feet. That rule predates modern insulation and low-E glass, and it oversizes most houses built or renovated in the last thirty years — which is why so many systems short-cycle and leave rooms clammy.
This calculator does the job properly: a room-envelope heat-loss and heat-gain estimate, surface by surface. Walls, windows by orientation, the attic, the floor or slab, doors, air leakage and the people and appliances inside. The component methods follow ACCA Manual J 8th edition as implemented in NREL’s open-source OpenStudio-HPXML, and the engine reproduces the published component loads of the ACCA example houses. The design temperatures are not looked up from the nearest airport: they are derived from ten years of hourly weather at your address, the same percentile method ASHRAE uses.
Said plainly: this is a Manual J-style estimate, not an ACCA Manual J report. Manual J is a licensed, room-by-room procedure, and a contractor should run one before you buy equipment. What this gives you is a number grounded in physics to hold theirs against — and a clear answer when someone quotes a 5-ton unit for a tight 2,000 square-foot house.
Enter design conditions by hand instead
Describe each room’s exposed wall, glass and whether it sits under the attic or over the foundation. Loads use the same per-square-foot multipliers as the whole-house result; air leakage is shared by exposed wall area and internal gains by floor area, as Manual J does. CFM is each room’s share of the blower airflow at a 20 °F supply temperature difference — the starting point for duct sizing (Manual D).
Each upgrade applied on its own to the house above, then all of them together. Fix the envelope before you buy the equipment and you may buy a smaller system.
Capacity retention defaults: 62.6% at 17 °F for single/two-stage (RESNET HERS Addendum 82) and 69% for variable-speed (NEEP database), as used by NREL OpenStudio-HPXML. Below 17 °F the capacity line is extended linearly — cold-climate models hold up better, so enter your unit’s listed numbers if you have them. The heat-loss line ignores internal gains, which is the conservative convention.
Design conditions, every input, the component breakdown and the equipment sizes on one page — to hand a contractor and ask for their Manual J next to it.
How to Use the HVAC Sizing Calculator
Start with the address: the tool reads ten years of hourly temperatures and humidity for that spot and works out the design conditions a load calculation needs. Then describe the house — floor area, stories, foundation, insulation, windows and how leaky it is. The defaults are a typical 2,000 square-foot two-story house with R-13 walls, an R-38 attic and double-pane low-E glass; change anything you know to be different. The result is the heating and cooling load in BTU per hour, the equipment size that fits it, and a breakdown showing which part of the house is driving the number.
Why Square Feet Alone Gives the Wrong Answer
A house gains and loses heat through its surfaces and its leaks, not through its floor. Two 2,000 square-foot houses can differ by a factor of three: one with an R-49 attic, sealed ductwork and north-facing glass in Seattle, the other with an uninsulated attic, single-pane windows facing west and a drafty envelope in Dallas. The 20 BTU per square foot rule gives both of them the same 3.3-ton air conditioner. For the first house that is roughly double what it needs; the second might be marginal. The comparison card shows how far the rule misses for the house you entered.
Design Temperatures: the 99% and 1% Days
A system is not sized for the hottest hour ever recorded; it is sized for a design condition that is exceeded only rarely. For heating that is the 99% dry-bulb — the temperature the air is colder than for just 1% of the hours in a year, about 88 hours. For cooling it is the 1% dry-bulb. Sizing for the absolute extreme would produce a unit that runs short, inefficient cycles on the other 8,670 hours. On the few hours beyond design the house drifts a degree or two, which is the intended trade.
Humidity matters too. The summer design moisture — how much wetter the outdoor air is than the 75 °F, 50% indoor target — sets the latent load: the water the air conditioner has to condense out of the air that leaks in. In Houston that can be a quarter of the cooling load; in Denver it is close to zero.
What Each Part of the Load Means
- Windows usually dominate the cooling load, and west-facing glass most of all: the late-afternoon sun lands on it when the outdoor air is already at its hottest. Solar-control low-E glass or exterior shading can cut that line in half.
- Infiltration is outdoor air leaking in. In cold climates it is often the largest heating component, which is why air sealing is the cheapest load reduction there is.
- Ceiling loads come from the attic, which reaches 120–130 °F on a design afternoon under dark shingles. Insulation and a radiant barrier attack this directly.
- Slab and floor losses are heating-only; an uninsulated slab edge leaks heat into the ground around the perimeter all winter.
- Internal gains — people and appliances — add to cooling and are ignored for heating, so the heating number is conservative on purpose.
Choosing the Equipment Size
Air conditioners and heat pumps come in half-ton steps (one ton = 12,000 BTU per hour). ACCA Manual S allows a single-stage unit between 90% and 115% of the total cooling load; the calculator picks the standard size inside that band that is closest to the load. Remember that a nominal “3-ton” rating is measured at 95 °F outdoors and 80 °F indoors; the capacity at your conditions comes from the manufacturer’s expanded performance data, which is part of a proper Manual S selection. Furnaces are sized on output: a 60,000 BTU input furnace at 96% AFUE delivers about 57,600 BTU per hour of heat to the house.
Before You Sign a Quote
Ask the contractor for their Manual J and compare it line by line with the breakdown here. If their total is far higher, ask which inputs differ: design temperatures, window areas, infiltration and duct losses are where padding usually hides. If they have not done a load calculation at all and are replacing like for like, the old unit’s size is the least reliable number in the house — it was very likely oversized to begin with, and the house has probably been tightened and re-insulated since.
Related tools: compare running costs with the Gas vs Electric Cost Calculator, plan the replacement with the Home Systems Replacement Budget, size backup power with the Generator Wattage Calculator, and see what panels could offset the bill with the Solar Savings Calculator. Browse every Home & Real Estate tool for more.
Frequently Asked Questions
How many BTUs do I need per square foot?
There is no single right number, which is the point of this calculator. The common rule of 20 BTU per square foot (about 600 square feet per ton) comes from room air-conditioner charts and older, leakier houses. A well-insulated modern home in a mild climate can need under 12 BTU per square foot of cooling; an older house with single-pane west-facing glass in Phoenix can exceed 30. The load depends on the design temperature at your address, the insulation, the glass and its orientation, and air leakage — not on floor area alone.
Is this a Manual J calculation?
It is a Manual J-style estimate, not an ACCA Manual J report. The component methods (conduction by U-value and design temperature difference, CLTD for walls, the vented-attic design temperature, window peak solar factors and cooling load factors, Table 5A infiltration) follow Manual J 8th edition as implemented in NREL's open-source OpenStudio-HPXML, and they reproduce the published component loads of the ACCA example houses. But a whole-house input form simplifies the house to a rectangle, uses the average-load procedure for glass, and cannot see overhangs, duct layout or your actual air leakage. A contractor should run a room-by-room Manual J before you buy equipment — and you can hold their number up against this one.
Why is an oversized air conditioner a problem?
An oversized unit cools the air to the thermostat setting before it has run long enough to pull moisture out, so the house ends up cool and clammy. It also short-cycles — starting and stopping frequently — which wastes energy on every start, wears the compressor and contactor, and makes rooms far from the thermostat swing in temperature. ACCA Manual S limits single-stage cooling equipment to 90–115% of the calculated total cooling load for exactly this reason.
Where do the design temperatures come from?
ASHRAE publishes design conditions only for weather stations, usually airports. This tool derives them the way ASHRAE does — as annual percentiles of hourly dry-bulb temperature — from ten years (2015–2024) of hourly reanalysis data at your address, via the Open-Meteo archive. The 99% heating temperature is the one the air is colder than for only 1% of the year's hours (about 88 hours); the 1% cooling temperature is exceeded only 1% of the time. Checked against fourteen ASHRAE stations, the derived values average about 1 °F warmer for heating and within 0.3 °F for cooling, consistent with a warmer recent decade.
What is a heat pump balance point?
A heat pump loses capacity as it gets colder outside, while the house loses heat faster. The balance point is the outdoor temperature where the two lines cross: above it the heat pump carries the house alone, below it you need backup heat (resistance strips or a furnace). The Pro balance-point panel draws both lines from your heating load and the heat pump's rated capacity at 47 °F and 17 °F, and tells you how much backup you need on the design day.
