Thermopy Guide
How to size an air conditioner correctly
Oversized AC wastes money and comfort. Learn how to estimate cooling load before you buy.
Why sizing matters
An oversized unit short-cycles, leaving humidity and uneven temperatures. Undersized units run constantly and spike bills. Start with room area, ceiling height, sun exposure and insulation.
Quick sizing workflow
Measure the room, note window orientation, then use Thermopy AC Sizing to convert inputs into BTU/h or kW and a professional summary for installers.
How to measure your room
Measure length and width at the longest points, then ceiling height. For open-plan areas, treat each cooling zone separately. Use our printable measurement worksheet before entering data in the calculator.
Climate, humidity and design temperature
Cooling load depends on the gap between outdoor design temperature and your indoor setpoint. Humid regions need extra latent capacity — a square-foot chart alone will undersize dehumidification needs.
Insulation, glazing and solar gains
Poor wall or ceiling insulation and large south- or west-facing windows can dominate the load. Note glazing type (single/double/triple) and shading — blinds and trees materially reduce solar gains.
Worked example
A 20 m² bedroom (≈215 ft²) with average insulation, one double-glazed window, and two occupants often lands near 2.5–3.5 kW (8–12k BTU/h) in moderate climates — but kitchen adjacency, high ceilings, or poor sealing can push higher. Always run your exact dimensions in the calculator.
Oversizing vs undersizing
Oversized units cool quickly but may not run long enough to remove humidity. Undersized units struggle on peak days and wear out faster. Target the nearest standard size above calculated load with a modest safety factor — not double capacity 'just in case'.
When to get Manual J or pro assessment
Use Thermopy for single-room planning and contractor conversations. Request full Manual J (or equivalent) for new construction, multi-zone ducted systems, or when local code requires stamped load calculations.
Free downloadable resources
- Download (PDF)
AC Room Measurement Worksheet
Printable worksheet to capture room dimensions, windows, insulation, and climate inputs before using the AC sizing calculator.
- Download (PDF)
BTU, kW & Tons Quick-Reference Guide
Climate-qualified sizing ranges with dual metric/imperial units. Use alongside a room-level calculator — not square footage alone.
- Download (PDF)
HVAC Contractor Quote Comparison Checklist
Printable checklist to compare installer quotes: scope, efficiency, warranty, electrical, and commissioning.
- Download (CSV)
HVAC Contractor Quote Comparison (CSV template)
Spreadsheet-friendly CSV template to log and compare multiple HVAC contractor quotes.
Get exact numbers with our free calculator
Use your room data, climate and energy rates to generate actionable results in minutes.
Open calculatorFAQ
How many BTU per square meter?
Typical residential cooling needs range roughly 80–120 W/m² depending on climate, insulation and solar gain. Always validate with a room-level calculator.
Can I use one AC for multiple rooms?
Multi-split systems can serve several rooms, but each indoor unit still needs correct capacity based on that room load.
When should I recalculate?
Recalculate after major insulation upgrades, window replacements or if you change room usage (e.g. home office with extra heat load).
Is the free PDF summary enough for installers?
The free summary is great for planning. The paid AC report adds full inputs, window schedule, installer checklist, and methodology notes for contractor handoff.
Do I need different sizes for mini-split vs central AC?
The room load is similar, but ducted systems add distribution losses. Mini-splits are sized per zone; central systems need whole-home or zone-level Manual J.