AC Calculators That Know Your Climate
Most online AC calculators use a formula designed for mild UK and US climates. Apply it in Dubai, Delhi, or Bangkok and you get an AC that is half the size you actually need. These calculators use a climate-zone baseline that scales from 25 BTU per sq ft for cool northern regions to 100 BTU per sq ft for very hot climates above 40 degrees, so the result is correct wherever you are.
Quick Room Calculator
Click a room size to calculate required size
Results below are for a hot climate (33–40°C peak). Use the full BTU Calculator for your specific climate, floor level and window direction.
Custom Room SizeWhy does climate matter?
The same 12x12 room needs 1 ton in South Asia, 1.5 ton in the Gulf, and only 0.75 ton in the UK, because the AC fights a larger temperature gap in hotter climates. The quick buttons above show hot-climate sizing. Use the full calculator for any other climate.
ASHRAE-Based Formulas
Sizing methodology from ASHRAE fundamentals
5 Climate Zones
From Cool under 20°C to Very Hot above 40°C
Feet and Metres
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How These Calculators Work
Calculations are adjusted to your specific local conditions and environment.
Select your climate zone
Five climate bands from Very Hot (above 40°C, Gulf states, desert regions) to Cool (below 20°C, UK, northern Europe, Canada). Each band uses a different BTU per square foot baseline.
Enter your room details
Length, width, ceiling height, floor level, window direction, insulation, and any special loads such as a kitchen or computer equipment.
Get an accurate size
The result shows the required BTU per hour, the recommended tonnage, and a plain-English explanation of the key factors driving the number.
Free HVAC Calculators
Industry-standard tools designed by HVAC professionals for accurate results you can trust.
Trusted by HVAC professionals worldwide
Our calculators use industry-standard formulas from ASHRAE and Manual J guidelines, ensuring you get accurate results every time.
Instant Results
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Industry Standard
Formulas based on ASHRAE and Manual J guidelines
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Multi-Unit Support
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Built on Verifiable Methodology
CalcArcond was built by Shahzad Arsi, an HVAC researcher and content specialist, after noticing that every freely available AC calculator online gave dangerous undersizing recommendations for hot-climate countries. The sizing formulas on this site are derived from ASHRAE Fundamentals, the Manual J residential load calculation method published by ACCA, and peer-reviewed heat transfer references. Where the calculator makes a simplifying assumption, the tool page explains it. Where precision matters, such as server rooms, kitchens, or high-ceiling rooms, specialised tools are provided rather than stretching a general formula beyond its valid range.
Shahzad Arsi
Founder and HVAC Researcher, CalcArcond
About CalcArcondBackground in building science and HVAC systems, focused on making professional-grade sizing tools accessible to homeowners and engineers worldwide.
AC Running Costs by Country
Calculate electricity costs using local tariffs. Each country page uses actual local electricity rates.
Rates shown are approximate domestic averages. Your actual rate may vary by state, provider, and consumption tier. Enter your exact rate in any cost calculator for a precise result.
From the Blog
HVAC tips, efficiency guides, and sizing logic to help you maintain the perfect indoor climate.
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Read articleFrequently Asked Questions
Why does my climate zone change the AC size so much?
The BTU requirement scales directly with the temperature gap the AC must overcome. In the UK peaking at 20°C and cooling to 24°C indoors, the gap is 4 degrees. In Dubai peaking at 46°C with the same indoor target, the gap is 22 degrees, which is more than five times larger. The same room therefore needs five times more cooling capacity per square foot in Dubai than in the UK.
What is the correct BTU for a 12x12 room?
It depends on your climate. Cool climate such as the UK: about 3,600 BTU, 0.75 ton. Hot climate such as South Asia and South-East Asia: about 10,800 BTU, 1 ton. Very hot climate such as the Gulf states: about 14,400 BTU, 1.5 ton. The figure of 3,600 BTU is only correct for cool-climate rooms.
Are these calculators free to use?
All 23 calculators are completely free. No account, no registration, and no paywall at any point. The site is funded by display advertising.
Do the tools work in metric units?
Yes. Every sizing tool has a feet and metres toggle. Enter your room dimensions in whichever unit you measure in and the result is identical either way.
How accurate are the results?
The results are accurate for the inputs you provide and the assumptions documented on each tool page. For a standard room with known dimensions and a clear climate, the error is typically within one standard tonnage class. For complex rooms, such as irregular shapes, very large spaces, or commercial installations, the tool pages recommend consulting a licensed HVAC engineer.