BTU Calculator Widget

Embed a room air conditioner sizing calculator next to window, portable or mini-split listings. Shoppers enter the room's floor area, how sunny it is, how many people use it and whether it is a kitchen, and get the BTU/h needed, the same figure in tons and kW, and the next standard capacity up.

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<iframe src="https://a2z.tools/embed/w/btu-calculator" title="BTU Calculator by A2Z Tools" width="100%" height="640" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

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How it works

The starting capacity comes straight from the ENERGY STAR room air conditioner sizing chart, which lists 14 floor-area bands from 100 to 2,500 sq ft: a 300-350 sq ft room needs 8,000 BTU/h, 450-550 sq ft needs 12,000 BTU/h and 1,500-2,000 sq ft needs 30,000 BTU/h. Square metres are converted at 10.764 sq ft per m². The widget then applies the chart's published adjustments in order - reduce by 10% for a heavily shaded room or add 10% for a very sunny one, add 600 BTU/h for each person beyond two who regularly uses the room, and add 4,000 BTU/h in a kitchen. The result is shown in BTU/h, in tons (12,000 BTU/h each) and in kW (3,412.142 BTU/h each), and the nearest common size is the next chart capacity at or above it. Rooms under 100 sq ft use the 5,000 BTU/h minimum; areas above 2,500 sq ft are refused because they need a proper load calculation.

Calculation method

  • Base capacity = ENERGY STAR chart band for the floor area (e.g. 400 up to 450 sq ft = 10,000 BTU/h)
  • Heavily shaded: x 0.90; very sunny: x 1.10
  • Occupants: + 600 BTU/h x (people - 2) when more than two people regularly use the room
  • Kitchen: + 4,000 BTU/h
  • Tons = BTU/h / 12,000; kW = BTU/h / 3,412.142; m² x 10.7639 = sq ft
  • Nearest common size = smallest chart capacity at or above the adjusted figure

Worked examples

Living room

Inputs: 400 sq ft, average sun, 2 people, not a kitchen

Result: 10,000 BTU/h; 0.83 tons; 2.93 kW; nearest size 10,000 BTU/h

The 400 up to 450 sq ft band of the ENERGY STAR chart.

Sunny open-plan kitchen-diner

Inputs: 500 sq ft, very sunny, 4 people, kitchen

Result: 18,400 BTU/h (12,000 + 1,200 sun + 1,200 people + 4,000 kitchen); nearest size 21,000 BTU/h

All four ENERGY STAR adjustments at once.

Bedroom in metres

Inputs: 14 m² (150.7 sq ft), heavily shaded, 2 people

Result: 5,400 BTU/h; nearest size 6,000 BTU/h

The 150 up to 250 sq ft band gives 6,000, less 10% for shade.

Limitations

  • A room air conditioner rule of thumb, not an ACCA Manual J load calculation; insulation, glazing, ceiling height and climate are not entered.
  • Limited to the ENERGY STAR chart range of 100 to 2,500 sq ft (rooms under 100 sq ft get the 5,000 BTU/h minimum).
  • Assumes a normal ceiling height of about 8 ft; tall or vaulted rooms need more capacity.

Where publishers use it

  • Window and portable air conditioner product pages on an appliance retailer's site
  • Home-improvement blogs answering 'what size AC for my bedroom'
  • HVAC installers' websites pre-qualifying mini-split enquiries
  • Landlord and student-housing guides choosing units for individual rooms
  • Energy-saving articles warning against oversized air conditioners

Questions

What size air conditioner do I need for a 400 sq ft room?

The ENERGY STAR chart gives 10,000 BTU/h for 400 up to 450 sq ft. If the room is very sunny that becomes 11,000 BTU/h, and the next common size up is 12,000 BTU/h.

How much do people and sun change the size?

ENERGY STAR adds 600 BTU/h for every regular occupant beyond two, and moves capacity 10% down for a heavily shaded room or 10% up for a very sunny one. A 500 sq ft sunny room used by four people needs 12,000 x 1.1 + 1,200 = 14,400 BTU/h.

Why does a kitchen need 4,000 BTU/h more?

Cooking appliances release a lot of heat into the room. ENERGY STAR's sizing guidance adds a flat 4,000 BTU/h when the unit cools a kitchen.

Is bigger always better?

No. An oversized room air conditioner cools the air quickly and switches off before it has removed much moisture, leaving the room cold and clammy, and it cycles more often. Choose the next size up from the estimate, not the largest you can afford.

How do I enter a room measured in metres?

Switch to Metric and enter square metres; 25 m² is 269 sq ft, which falls in the 250-300 sq ft band of 7,000 BTU/h, or about 2.05 kW of cooling.

What about a portable air conditioner?

Portable units with a single exhaust hose pull replacement air into the room from outside or neighbouring spaces, so they deliver less effective cooling than their rating suggests. Many shoppers choose the next size up for a portable, especially in a west-facing room under a hot attic.

Can I use it for a whole house?

Not reliably. The chart stops at 2,500 sq ft and is meant for single rooms. Whole-house central systems should be sized with an ACCA Manual J calculation that accounts for insulation, windows, orientation and climate.

Sources

  1. Room Air Conditioners - Find the correct cooling capacity for your room size - ENERGY STAR (US EPA and DOE) . The 14-band area-to-capacity chart (100 up to 150 sq ft = 5,000 BTU/h ... 2,000 up to 2,500 sq ft = 34,000 BTU/h) and the four adjustments: heavily shaded -10%, very sunny +10%, +600 BTU per person beyond two, kitchen +4,000 BTU. Checked 2026-10-01.
  2. NIST Guide to the SI, Appendix B.8 - National Institute of Standards and Technology . Ton of refrigeration = 12,000 Btu_IT/h; Btu_IT/h = 0.2930711 W, used for the tons and kW cards.

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A2Z Tools BTU Calculator
https://a2z.tools/embed/btu-calculator

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