How many BTU do I need per square metre?

A common rule of thumb is approximately 337 BTU per hour per square metre of floor area for a standard room with average ceiling height (2.4 m) and moderate sun exposure. This is equivalent to roughly 100 watts per square metre. For a very sunny or poorly insulated room, add 10 to 20 percent. For a heavily shaded or well-insulated room, subtract 10 percent. Kitchens generate significant extra heat and need an additional 4,000 BTU/h on top of the base figure.

What is the difference between BTU and kW for air conditioners?

BTU (British Thermal Units) per hour and kilowatts (kW) are both measures of cooling capacity. To convert BTU/h to kW, divide by 3,412. To convert kW to BTU/h, multiply by 3,412. In New Zealand and Australia, air conditioners are typically rated in kW, while in the United States and on many online retailers the BTU rating is used. A 2.5 kW unit has a cooling capacity of approximately 8,530 BTU/h, which is suitable for a room of about 15 to 20 square metres.

Can I use this calculator for a heat pump in New Zealand?

Yes. The sizing method is the same for heat pumps used in cooling mode. Most modern heat pumps sold in New Zealand are reverse-cycle, meaning they provide both heating and cooling. The BTU or kW figure from this calculator is the cooling capacity you need. When buying, check the unit's stated cooling kW matches or exceeds the recommended figure. For heating in cooler climates, you may want to size up slightly as heating capacity in very cold conditions can be lower than the rated cooling capacity.

Air Conditioner BTU Calculator

This calculator works out the air conditioner or heat pump cooling capacity you need for a room, in both BTU per hour and kW, so you buy a unit that is neither undersized, which runs constantly without cooling properly, nor oversized, which cycles on and off and leaves the air clammy. You enter the room's length and width, its ceiling height, how much sun it gets, the number of regular occupants, the room type (bedroom, living room, kitchen, home office or dining room), and your climate zone based on where in New Zealand you live. It starts from a base rate of 337 BTU/h per square metre of floor area, then adjusts for ceiling height, sunlight, extra people beyond two, the heat load of the room type, and your local climate. It returns your floor area, the recommended BTU/h and kW cooling capacity, and the nearest standard unit size to buy, along with a full breakdown of each adjustment step and a table comparing standard unit sizes to typical room sizes. Use it before buying a split-system heat pump, portable unit or window unit, and choose a unit that meets or slightly exceeds the recommended figure. This is a general sizing guide; actual requirements depend on insulation, window area and building construction, so for commercial or unusual buildings it is worth checking with a qualified HVAC engineer.

Calculate.co.nz is proud to be partnered with Premium Homes, a recognised leader in eco-friendly, sustainable, and energy-efficient homebuilding. With a dedicated team and award-winning experience, they create homes that prioritise health, comfort, and long-term performance. Their founders, Andrew and Kelly, set out to raise the standard of residential construction in New Zealand by combining practical building expertise with a clear commitment to doing things better for homeowners.

Updated June 2026 Based on ASHRAE and industry-standard BTU sizing guidelines.

1. Room Dimensions

Room length (metres)

Room width (metres)

Ceiling height (metres)

Sunlight exposure

2. Occupancy and Use

Number of regular occupants

Room type

Climate zone

Recommended Cooling Capacity

Floor Area

Square metres

Recommended BTU/h
-
Adjusted for all factors

Recommended kW
-
Cooling capacity

Nearest Standard Size

Typical unit to buy

Calculation Breakdown

Floor area-

Base capacity (337 BTU/m²)-

Ceiling height factor-

Sunlight adjustment-

Extra occupant adjustment-

Room type adjustment-

Climate zone factor-

Total BTU/h required-

Standard Unit Sizes

Unit Size (BTU/h)	kW	Suitable for (m²)

Recommendation: Enter your room details above.

How Air Conditioner BTU Sizing Works

The cooling capacity of an air conditioner is measured in BTU per hour (BTU/h) or kilowatts (kW). Choosing the right size matters: an undersized unit runs constantly and fails to cool the room, while an oversized unit cycles on and off too quickly, leaving the air humid and uncomfortable. The standard sizing method starts with a base rate of approximately 337 BTU/h per square metre of floor area (roughly 100 W/m²), then adjusts for ceiling height, sun exposure, occupancy, room use, and climate.

The Sizing Formula

The recommended BTU/h is calculated as follows:

Base BTU/h = floor area (m²) x 337
Ceiling height factor: multiply by 1.0 for standard height (up to 2.4 m), 1.15 for slightly high ceilings (2.4 to 3.0 m), or 1.30 for high ceilings (over 3.0 m)
Sunlight adjustment: subtract 10% for heavily shaded rooms, add 10% for sunny or west-facing rooms, add 20% for very sunny or poorly insulated rooms
Extra occupants: add 600 BTU/h for each person beyond two (each person generates approximately 600 BTU/h of heat)
Room type: add 4,000 BTU/h for kitchens, 2,000 BTU/h for offices with multiple computers, or 1,000 BTU/h for busy dining rooms
Climate zone: multiply the adjusted total by 0.90 for cooler New Zealand locations (Christchurch, Dunedin), 1.0 for temperate (Wellington, Hamilton), 1.10 for warm (Auckland, Bay of Plenty), or 1.20 for the hottest areas (Northland, Nelson/Marlborough)

The result is your total recommended BTU/h. Divide by 3,412 to convert to kW. Choose the nearest standard unit that meets or slightly exceeds this figure.

BTU to kW Conversion

BTU/h	kW (approx)	Typical room size (m²)	Common use
5,000	1.5 kW	Up to 15 m²	Small bedroom
7,000	2.0 kW	15 to 20 m²	Bedroom, small office
9,000	2.6 kW	20 to 25 m²	Master bedroom, dining room
12,000	3.5 kW	25 to 35 m²	Living room, open-plan bedroom
18,000	5.3 kW	35 to 50 m²	Large living room
24,000	7.0 kW	50 to 70 m²	Open-plan living/dining
36,000	10.5 kW	70 to 100 m²	Large open plan

Worked Example

A 5 m x 4 m living room (20 m²) with standard ceiling height (2.4 m), normal sun exposure, 2 occupants, temperate climate (Wellington):

Base BTU/h = 20 x 337 = 6,740
Ceiling height factor = 1.0 (no change)
Sunlight adjustment = 0% (no change)
Extra occupant adjustment = 0 BTU/h (2 people is the baseline)
Room type adjustment = 0 BTU/h (living room)
Climate factor = 1.0 (temperate)
Total = 6,740 BTU/h = 1.97 kW
Nearest standard size: 7,000 BTU/h (2.0 kW)

This matches the calculator's default output.

Tips for Choosing the Right Unit

Always choose the nearest standard size that meets or exceeds the calculated BTU/h. Going slightly over is better than being undersized.
Split-system heat pumps (the most common type in NZ homes) are the most efficient option. A unit with a high Energy Efficiency Ratio (EER) or Coefficient of Performance (COP) will cost less to run.
Portable and window units lose some efficiency due to air leaks around installation. If using one of these, consider adding 10 to 15% to the calculated BTU/h.
If the room has very high ceilings, poor insulation, or large single-glazed windows facing north or west, err on the larger side.
For open-plan spaces connecting a kitchen and living area, add the areas together and include the kitchen adjustment.

Related Calculators

Science and Engineering Calculators
Appliance Running Cost Calculator: estimate the electricity cost of running an air conditioner or heat pump.
Electricity Cost Calculator: work out your total home electricity costs.
Solar Panel Count by kW Calculator: offset your air conditioning running costs with solar.
Cost of Owning a Pool Calculator: factor in pool heat pump running costs.

Sources and method: ASHRAE Handbook of Fundamentals (standard BTU/m² sizing basis); industry-standard 337 BTU/h per m² base rate (equivalent to approximately 100 W/m²); 600 BTU/h per additional person beyond two occupants; 4,000 BTU/h kitchen adjustment. BTU to kW conversion factor: 1 kW = 3,412 BTU/h.

This calculator provides a general sizing guide only. Actual requirements depend on insulation quality, window area, local climate, and building construction. For commercial spaces or buildings with unusual features, consult a qualified HVAC engineer or refrigeration mechanic.