Free estimate — verify against local code before building
AC Size / BTU Calculator
Estimate a preliminary BTU and AC tonnage range for one room, with every adjustment shown.
What this calculator includes
This room-level planning estimator starts with floor area and ceiling height, then applies visible adjustments for insulation, solar exposure, broad climate, occupants, and major heat-producing appliances. It returns a range rather than a single equipment size because windows, air leakage, assemblies, ducts, humidity, and design weather are not captured. Use Manual J and equipment selection by a qualified HVAC professional before purchasing.
How to use this ac size & btu calculator
- 01
Measure one conditioned room
Enter inside length, width, and average ceiling height. Calculate communicating spaces together only when air moves freely between them.
- 02
Choose broad conditions
Describe insulation, sun, and climate conservatively without treating the labels as a building audit.
- 03
Add internal gains
Count regular occupants and major heat-producing appliances in the room.
- 04
Use the range for planning
Compare the preliminary range with a Manual J load and manufacturer performance data before selecting equipment.
Worked example
Example: 20 x 15 ft sunny room
A 300 sq ft room starts at the 7,000 BTU/h ENERGY STAR area-table value. With an 8 ft ceiling, average insulation, sunny exposure, moderate climate, two occupants, and no major appliance, the expected planning value is 7,700 BTU/h and the displayed range is about 6,545 to 8,855 BTU/h.
Practical buying and overage guidance
Do not round the result directly to the biggest available unit. Compare a professional design load with equipment capacity at local design conditions, dehumidification needs, sound, electrical requirements, and installation constraints.
Continue the project
Frame & Finish a Room
Plan wall framing, insulation, drywall, paint, flooring, baseboard, trim, materials, supplies, and contractor pricing in one workflow.
Open the project workflow →Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula review completed July 13, 2026. What this review covers
- ENERGY STAR - Air conditioner sizing guidance (opens in a new tab)
Room-area cooling-capacity table, sun/shade adjustments, occupant allowance, and oversizing guidance.
- ACCA Manual J overview (opens in a new tab)
Industry residential load-calculation method required for equipment sizing decisions.
Frequently asked questions
How many BTUs do I need per square foot?
A single BTU-per-square-foot number is too crude for equipment selection. This tool starts with the ENERGY STAR room-area capacity table, then shows broad height, insulation, sun, climate, occupant, and appliance adjustments.
How many BTUs are in one ton of AC?
One nominal ton of cooling equals 12,000 BTU per hour. Nominal tonnage does not guarantee delivered capacity under every indoor and outdoor condition.
Is a larger AC safer?
No. Oversized equipment can short-cycle, control humidity poorly, create temperature swings, and waste energy. Capacity should match a calculated design load and the selected equipment's performance data.
Can this replace a Manual J calculation?
No. It omits envelope assemblies, windows, infiltration, ducts, ventilation, latent load, and local design weather. Use it only for early planning.
Should a kitchen be adjusted?
Yes, major cooking appliances add heat. The appliance input adds a visible planning allowance, but a professional load calculation should model actual use and ventilation.