Recommended BTU
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Cooling output needed to match your space.
Air Conditioner BTU Calculator for Accurate AC Size
Enter the dimensions of any room and this AC BTU calculator returns the right cooling capacity, tonnage, and comfort range in seconds. It works as an air conditioner size calculator, a room air conditioner size calculator, and a window AC BTU calculator for apartments or additions.
Designed for homeowners, contractors, and energy auditors who need a fast air conditioning size calculator with Manual J inspired checks. Also try our multi-room planner and BTU to tonnage converter.
Start Calculating BTUQuick AC Sizing Questions
What size AC do I need for my room?
Use our AC calculator below. Generally, you need 20-25 BTU per square foot, but our tool adjusts for ceiling height, insulation, and climate.
How do I calculate BTU for air conditioner?
Multiply room square footage by 25 BTU, then adjust for ceiling height, windows, and occupancy. Our calculator does this automatically.
What's the difference between BTU and tonnage?
1 ton = 12,000 BTU. Use our BTU tonnage converter to translate between equipment ratings.
Free AC BTU Calculator - Size Any Air Conditioner
Fill in the form once to run an air conditioner size calculator that adapts to ceiling height, solar exposure, and occupancy. Works for window AC, central air, and ductless systems.
Your AC sizing summary
Review the BTU total, comfort range, and AC unit size to select a central air system, ductless heat pump, or window AC that actually fits.
Comfort range (+/-10%)
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Stay in this window to prevent over or undersizing.
System tonnage
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Rounded to typical 0.5 ton increments.
Next steps
- Complete the form to reveal the checklist.
AC size chart by room square footage
Use this quick reference if you need a ballpark AC size calculator before running the detailed model. Values assume 8 ft ceilings, average insulation, and moderate sun.
| Room size (sq ft) | Suggested BTU | Typical AC unit size |
|---|---|---|
| 100 - 150 | 5,000 - 6,000 BTU | 0.5 ton window AC |
| 150 - 250 | 6,000 - 8,000 BTU | 0.75 ton ductless head |
| 250 - 400 | 8,000 - 11,000 BTU | 0.75 - 1 ton split AC |
| 400 - 600 | 11,000 - 14,000 BTU | 1 - 1.25 ton unit |
| 600 - 900 | 14,000 - 20,000 BTU | 1.5 - 1.75 ton central AC |
| 900 - 1,200 | 20,000 - 26,000 BTU | 2 - 2.25 ton central AC |
Need precision? The AC BTU calculator above adjusts these ranges for vaulted ceilings, poor insulation, and extreme climates, then the climate load adjuster lets you simulate new design temperatures.
How to calculate AC BTU the right way
Follow this five-minute process to size an air conditioner without guesswork using our AC calculator.
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1. Measure the footprint
Capture the length and width of the room in feet, then note ceiling height. Even a half-foot difference can shift cooling load by hundreds of BTU.
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2. Document the envelope
Check insulation quality, window orientation, and shading. Choose the insulation and climate options in the air conditioning calculator that match what you see.
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3. Count internal gains
Log the regular occupants plus any electronics, fitness gear, or lighting that runs for more than an hour at a time.
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4. Run the BTU calculator
Enter the data into our AC sizing calculator and review the output: recommended BTU, comfort range, and tonnage.
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5. Cross-check with tools
Use the multi-room planner for combined zones, then convert tonnage to BTU with the converter if you are comparing equipment with different ratings.
How the AC sizing calculator works
This air conditioning calculator blends the simplicity of a rule-of-thumb chart with the corrective factors HVAC pros rely on.
1. Sensible load baseline
We start with 25 BTU per square foot, scale for ceiling height, and verify that the AC unit size calculator result aligns with ACCA Manual J targets.
2. Envelope modifiers
Insulation, sun exposure, and glazing receive multipliers so your air conditioner room size calculator result fits both northern basements and sunrooms.
3. Internal gains
Occupants and plug loads add to the BTU total, giving you a realistic AC size calculator for home offices, gyms, or media rooms.
Deep-dive guides for every scenario
Move past basic BTU math with our step-by-step checklists, worksheets, and playbooks.
Manual J Simplified for Homeowners
Understand the industry-standard HVAC load calculation without getting lost in technical jargon.
Complete Climate Zone Guide
How ASHRAE climate zones affect AC sizing with detailed load adjustments for all US regions.
Heat Pump Sizing Checklist
Adapt your cooling calculation for heat-pump mode with Manual J adjustments for contractors.
Room-by-Room Load Worksheet
Print or download the form to log measurements, then consolidate each zone with the multi-room planner.
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Why this AC calculator stands apart
- Manual J alignment: We adopt ACCA's envelope factors so results can guide a professional energy audit.
- Multi-room planning: Combine zones with the dedicated multi-room planner for whole-house mini-split or central AC.
- BTU-to-tonnage conversion: Swap units on the fly with the converter when you compare equipment specs.
- Climate-load adjuster: Fine-tune humidity, altitude, and shading with the advanced tool.
- Free & transparent: No account, no credit card, no up-sell. Every tool is open and instant.
Frequently asked questions
How many BTU do I need per square foot?
You typically need 20-30 BTU per square foot, depending on insulation, sun exposure, and climate. Our BTU calculator applies those modifiers automatically so you don't guess.
Can I use a window AC BTU calculator for central air?
Yes, but you will need to sum every zone or use our multi-room planner. Central systems add duct loss, so consult an HVAC pro after you get the baseline.
What if my ceiling height is not standard?
Enter your actual ceiling height in the form. The calculator adjusts the volume, then scales the BTU result to match that extra air mass.
Should I round up or down when I see a range?
Round up if you have poor insulation, high sun, or many occupants. Round down if you have excellent insulation and minimal internal gains. The comfort range in the AC sizing calculator provides your safety margin.
Does this replace a Manual J load calculation?
No. This air conditioning calculator gives you an instant snapshot for budgeting and shopping, but a certified HVAC contractor should finalize duct sizing and equipment selection with a full Manual J.
How do I convert BTU to tons or kW?
One ton equals 12,000 BTU. Use our BTU-to-tonnage converter to flip between ratings, or divide BTU by 3,412 to get kilowatts.
Ready to size your AC with confidence?
Scroll back up to the AC calculator, enter your room details, and get your BTU recommendation in seconds.
Calculate Now