The AC Calculator determines the BTU requirement and recommends AC capacity based on room size.
Considers area, ceiling height, number of occupants, floor level, and window orientation. Includes monthly AC electricity cost estimates for inverter and non-inverter models.
Calculator information
📋 How to use this calculator
- Measure the length and width of the room to get the area in square feet (or square meters).
- Enter ceiling height (standard 8-9 ft / 2.4-2.7 m); higher ceilings need more BTUs.
- Specify the typical number of occupants and the number of heat-generating electronics in the room.
- Select the floor position (top, middle, or ground) and window orientation relative to the sun.
- Review the recommended BTU/hour rating and tonnage (1 ton = 12,000 BTU/h).
- Tip: Add a 10-20% margin for rooms with heavy direct sun or uninsulated roofs.
🧮 AC BTU Sizing Estimate
BTU/h ~= Area (sq ft) x 25 + (Occupants - 1) x 600 + Heat_devices x 1,200
- Area = length x width (sq ft); use the metric form ~600 BTU/m2 for SI inputs
- Base factor: 20-25 BTU/sq ft for typical rooms; up to 30+ for kitchens or sun-exposed rooms
- 1 ton = 12,000 BTU/h = ~3,517 W cooling capacity
- 1 BTU/h = 0.293 W
- Power draw ~= Cooling_capacity / EER (modern inverter units: EER ~10-14 or SEER 14-22)
This is a quick estimate. For precision, HVAC contractors run an ASHRAE Manual J load calculation that accounts for insulation, glazing, infiltration, and climate zone.
💡 Worked example: Bedroom 13 ft x 10 ft, 9 ft ceiling, 2 occupants, west-facing
Given:- Area = 130 sq ft
- Ceiling = 9 ft (factor +10%)
- Occupants = 2
- 1 LED TV (heat-generating device)
- Orientation = west (factor +15%)
Steps:- Base load = 130 x 25 = 3,250 BTU/h.
- Second occupant = +600 BTU/h.
- TV = +1,200 BTU/h.
- Subtotal = 5,050 BTU/h; ceiling factor +10% = 5,555; orientation factor +15% = 6,388 BTU/h.
- Round to a common size: a 6,000 BTU window unit covers it; choose 8,000 BTU if the room runs hot.
Result: Roughly 6,400 BTU/h needed. A 6,000 BTU unit is the minimum; an 8,000 BTU unit gives faster cooldown in hot conditions.
❓ Frequently asked questions
What is a BTU and what does AC tonnage mean?
BTU/h (British Thermal Units per hour) is the standard unit for cooling capacity. AC tonnage is industry shorthand: 1 ton of cooling = 12,000 BTU/h (~3,517 W of cooling). Tonnage refers to cooling output, not electricity consumption. A 3-ton central AC delivers 36,000 BTU/h of cooling.
How much electricity does a 12,000 BTU (1-ton) AC use?
A typical 12,000 BTU window unit draws around 1,000-1,200 W (EER ~10-12), so running it 1 hour uses about 1.0-1.2 kWh. Inverter mini-splits with SEER 18-22 can draw 30-50% less under partial load. At the US average residential rate of about $0.16/kWh, running it 8 hours/day costs roughly $40-50 per month.
Inverter vs single-stage AC - which saves more?
Inverter (variable-speed) units save more on long daily run times (>6 hours) because the compressor modulates instead of cycling on/off. Single-stage units are cheaper upfront and work fine for short runs. Look for ENERGY STAR certified models and the highest SEER2 rating you can afford - the 2023 federal minimum is SEER2 14-15 depending on region, and SEER2 18+ models qualify for federal tax credits under the Inflation Reduction Act.
Can a small room use an oversized AC?
It can, but it's inefficient. An oversized AC short-cycles (rapid on/off), which dehumidifies poorly and wastes energy on each startup surge. Right-size to the heat load, or use an inverter unit that throttles down at low load.
Which factors increase BTU requirements?
BTU needs rise with: west-facing rooms hit by afternoon sun, ceilings above 9 ft, uninsulated attics or roofs, heavy heat-generating electronics (gaming PCs, ovens), high occupant density (meeting rooms), and large glass walls. Insulation, attic radiant barriers, blackout curtains, and reflective window film can cut needs by 10-20%.
📚 Sources & references
Last updated: May 11, 2026