Free estimate — verify against local code before building
AC Size / BTU Calculator
Follow the ENERGY STAR room-AC sizing table exactly, or explore a separately labeled planning scenario.
What this calculator includes
The exact mode follows the ENERGY STAR room-air-conditioner table for rooms from 100 through 2,500 square feet, including its published shade, sun, additional-occupant, and kitchen adjustments. A separate CountStruction planning scenario can apply clearly labeled ceiling-height, insulation, and climate assumptions and show a range, but those scenario factors are not part of the ENERGY STAR method. Use Manual J and equipment selection by a qualified HVAC professional for central, ducted, or permanent systems.
How to use this ac size & btu calculator
- 01
Choose the sizing method
Use the exact ENERGY STAR mode for a room air conditioner. Use the separately labeled planning scenario only to explore additional assumptions.
- 02
Measure one conditioned room
Enter inside length and width. The ENERGY STAR table covers rooms from 100 through 2,500 square feet and is based on an 8-foot ceiling.
- 03
Apply the published adjustments
Select heavy shade, mixed exposure, or very sunny; enter regular occupants; and identify whether the unit will serve a kitchen.
- 04
Verify before purchase
Match the result to an available room-AC capacity and installation requirements. Use Manual J and manufacturer performance data for system equipment.
Worked example
Example: 20 x 15 ft sunny room
A 300 sq ft room starts at the 7,000 BTU/h ENERGY STAR table value. Very sunny exposure increases that by 10% to 7,700 BTU/h. Two regular occupants add nothing, and a non-kitchen room adds no kitchen allowance, so exact mode recommends 7,700 BTU/h.
Practical buying and overage guidance
For a room AC, compare the exact-mode capacity with available ENERGY STAR certified models, airflow direction, sound, electrical requirements, window or wall fit, and installation instructions. For central or ducted equipment, use a professional design load and manufacturer performance data.
Scope and safety note
Planning estimator only. It is not Manual J, Manual S, an HVAC design, or purchasing advice. Have a qualified HVAC professional calculate the load and select the equipment.
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 and test check recorded July 14, 2026. What this review covers
Formula version 2026-07-14.1
No named qualified reviewer is recorded for this calculator. Treat the result as planning support and obtain the professional or code review identified in its safety note.
- ENERGY STAR - Air conditioner sizing guidance (opens in a new tab)
100–2,500 sq ft room-area table; shade/sun, additional-occupant, and kitchen adjustments; 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?
ENERGY STAR uses room-area bands rather than one BTU-per-square-foot multiplier. Exact mode looks up the published capacity, then applies only the published shade/sun, additional-occupant, and kitchen 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. The ENERGY STAR table is for room air conditioners. It does not model envelope assemblies, windows, infiltration, ducts, ventilation, latent load, or local design weather for system selection.
Should a kitchen be adjusted?
Yes. ENERGY STAR says to increase room-air-conditioner capacity by 4,000 BTU/h when the unit is used in a kitchen. The exact mode applies that adjustment once.