HVAC Load Calculator
Cooling load in BTU and tons.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Cooling load
- 48,918 BTU/h
- Tons of cooling
- 4.08
- Kilowatts
- 14.34
- Recommended unit
- 4.5 ton
Quick answer
A quick cooling load estimate is floor area × load factor + 600 BTU/h per person + about 1,000 BTU/h per sun-facing window + equipment watts × 3.412. Divide by 12,000 for tons. A 1,600 ft² home at 22 BTU/ft² with 6 people, 5 sunny windows and 1,500 W of equipment needs about 48,900 BTU/h, or 4.08 tons.
What the HVAC Load calculator does
This calculator gives a screening estimate of the cooling load for a home or small commercial space, in BTU per hour, tons of refrigeration and kilowatts, and suggests the nearest half-ton equipment size above it.
It is useful early on — comparing quotes, checking whether an installer's proposed system is in the right range, or budgeting for a new addition. It adds the main heat sources: the building fabric (via the area-based load factor), people, sun through windows and heat from appliances, computers and lighting.
It is not a substitute for an ACCA Manual J calculation, which accounts for insulation levels, window types and orientation, infiltration, duct losses and local design temperatures. Oversized systems short-cycle and remove humidity poorly, so treat this as a sanity check. For a single room, the air conditioner size calculator is quicker, and the BTU calculator covers heating as well.
The formula
BTU/h = A × F + 600 × people + 1,000 × windows + W × 3.412 Tons = BTU/h ÷ 12,000
| Symbol | Meaning | Unit |
|---|---|---|
| A | Conditioned floor area | ft² |
| F | Base load factor for climate and insulation | BTU/h per ft² |
| people | Typical occupants during the hottest part of the day | count |
| windows | Windows facing the afternoon sun (south or west in the northern hemisphere) | count |
| W | Heat-producing equipment: computers, TVs, cooking, lighting | W |
| 3.412 | BTU/h per watt | BTU/h/W |
| 12,000 | BTU/h in one ton of refrigeration | BTU/h/ton |
The area-based factor follows the rule-of-thumb approach used in ENERGY STAR room air conditioner sizing guidance (about 20 BTU/h per ft²), with additions for occupants and internal gains. 600 BTU/h per person is the common sizing allowance for an occupant. Proper residential sizing uses ACCA Manual J for the load and Manual S to select equipment; commercial buildings use ASHRAE methods.
Worked examples
Typical family home
A 1,600 ft² house in a warm climate (22 BTU/ft²) with 6 occupants, 5 west-facing windows and 1,500 W of electronics and cooking.
Inputs
- Floor area1600 ft²
- Base load factor22 BTU/ft²
- Occupants6
- Sun-facing windows5
- Equipment load1500 W
Result
- Cooling load48,918 BTU/h
- Tons of cooling4.08
- Kilowatts14.34
- Recommended unit4.5 ton
About 4.1 tons. The calculator rounds to 4.5 tons, but a Manual J with good insulation figures often comes in lower, so a 4-ton unit may be the better fit — ask the installer to show their load calculation.
Home office or garage conversion
A 500 ft² converted garage with poor insulation (25 BTU/ft²), 2 people, 2 sunny windows and 300 W of computers.
Inputs
- Floor area500 ft²
- Base load factor25 BTU/ft²
- Occupants2
- Sun-facing windows2
- Equipment load300 W
Result
- Cooling load16,724 BTU/h
- Tons of cooling1.39
- Kilowatts4.9
- Recommended unit1.5 ton
About 16,700 BTU/h, which suits a 1.5-ton ductless mini-split. Insulating the garage door and walls first could noticeably lower the load and the equipment cost.
Large, well-insulated newer home
A 2,400 ft² newer home in a moderate climate (18 BTU/ft²), 4 occupants, 6 sunny windows and 1,000 W of equipment.
Inputs
- Floor area2400 ft²
- Base load factor18 BTU/ft²
- Occupants4
- Sun-facing windows6
- Equipment load1000 W
Result
- Cooling load55,012 BTU/h
- Tons of cooling4.58
- Kilowatts16.12
- Recommended unit5 ton
About 4.6 tons, rounded up to 5 tons. In larger homes, splitting the load into two zones or systems can give better comfort than one large unit.
Choosing a load factor for your building
The base factor carries most of the result, so pick it carefully. These ranges are rough guides for residential cooling; local design temperature, humidity and construction all shift them.
| Situation | Load factor | Examples |
|---|---|---|
| Mild climate, well insulated, shaded | 15–18 | Coastal or northern areas, new construction |
| Average climate and insulation | 18–22 | Most suburban homes |
| Hot climate or average insulation, lots of sun | 22–28 | Southern US, top-floor apartments |
| Very hot or poorly insulated | 28–35 | Desert climates, uninsulated additions, sunrooms |
Why bigger is not better for air conditioners
An oversized system cools the air quickly and shuts off before it has run long enough to pull moisture out, leaving rooms cold but clammy. Frequent short cycles also wear compressors and waste energy. Manual S guidance generally limits cooling equipment to modestly above the calculated load. If your estimate falls between sizes — say 4.1 tons — the smaller of the two nearby sizes is often the better match once a detailed load calculation has been done. Variable-speed and inverter units handle a range of loads and reduce the penalty of slight oversizing.
From load to equipment and ducts
- Tons to kW: one ton is 12,000 BTU/h, about 3.517 kW of cooling. Equipment kW in the result is cooling output, not electrical input.
- Airflow: central systems typically move around 350–400 CFM per ton; the BTU to CFM calculator converts load into airflow.
- Ducts: undersized ducts limit how much of the unit's capacity reaches rooms — check them with the duct size calculator.
- Heating: heating loads are driven by winter temperatures and are calculated separately.
How to use the HVAC Load Calculator
- 1Enter the conditioned floor area in square feet (multiply m² by 10.764).
- 2Pick a base load factor from the table to match your climate and insulation.
- 3Enter the usual number of occupants and the windows that get direct afternoon sun.
- 4Enter equipment heat in watts (computers, TVs, cooking, lighting) and read BTU/h, tons and the suggested unit size.
Frequently asked questions
How many BTU do I need per square foot?
About 20 BTU/h per ft² is the common rule of thumb for cooling, ranging from roughly 15 in mild, well-insulated homes to 30 or more in hot climates or poorly insulated spaces.
What size AC do I need for a 2,000 sq ft house?
At 20 BTU/ft² the base load is 40,000 BTU/h, roughly 3.3 tons before people, windows and equipment. Most homes this size land between 3 and 4 tons, but a Manual J calculation should decide.
How many BTU is one ton of air conditioning?
One ton equals 12,000 BTU/h, the cooling from melting one short ton of ice over 24 hours. A 3-ton unit delivers 36,000 BTU/h.
Is this the same as a Manual J calculation?
No. Manual J models each wall, window, roof, infiltration and duct losses using local design temperatures. This calculator is a screening estimate to check that a proposal is in the right range.
How much heat does a person add to a room?
Sizing guidelines allow about 600 BTU/h per occupant, covering both sensible heat and moisture from breathing. Activity raises it — a gym needs a much higher allowance per person.
References
- Manual J: Residential Load Calculation — Air Conditioning Contractors of America (ACCA)
- Manual S: Residential Equipment Selection — Air Conditioning Contractors of America (ACCA)
- ASHRAE Handbook – Fundamentals — ASHRAE
- Room air conditioner sizing guidance — ENERGY STAR (US EPA)
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
- Report an error