AC Sizing & Running Costs Calculator

Air Conditioner Calculator

Calculate the exact cooling capacity required to cool your space. Accounts for dimensions, insulation quality, sunlight exposure, appliance wattage, occupant levels, and Installed vs Portable unit efficiency profiles.

Last reviewed 28 May 2026
Metric & Imperial
BTU, kW & Tons cooling Sizing
Installed vs Portable comparisons
Multi-room sizing multipliers
Rule of thumb: In standard residential rooms, allocate 20–25 BTU/hr per square foot of living space. Adjust upwards for sunny orientations (+10%), kitchens (+4,000 BTU/hr), or server environments (+2,000 BTU/hr).
Sizing Details
Enter dimensions and thermal gain descriptors
Unit System
Room Dimensions
m
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m
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m

Calculates volumetric thermal adjustments above standard 8ft (2.4m) ceilings.

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AC Sizing Prefs

Installed split systems are highly efficient (COP 3.6). Portable single-hose units draw more power (COP 2.6) and are capped at 12k BTU.

Aggregates total cooling requirements and electricity costs across multiple rooms.

Please enter a valid room count
Thermal Load Modifiers

Kitchens and server rooms generate high continuous heat loads requiring higher baseline sizing.

Adjusts base thermal gain depending on orientation and window solar radiation.

Calculated on standard heat gain. Adds 600 BTU/hr per person for sizes > 2 occupants.

Select matching hardware profiles or input a custom wattage for all devices in the room.

Running costs are calculated using dynamic electricity consumption models based on real seasonal heating parameters.

Cooling Guide

Understanding Air Conditioner Sizing

Undersizing an air conditioner results in continuous running without reaching comfortable temperatures. Oversizing leads to quick room cooling but fails to dehumidify the air, resulting in a cold yet clammy environment.

Why Efficiency (COP) Varies by System Type

The Coefficient of Performance (COP) measures cooling output relative to electrical input. Installed split systems represent the gold standard, achieving COPs of 3.6 or more (A++ class) because their heat-rejection condenser sits entirely outdoors. Portable single-hose units are mobile but inefficient (COP of 2.6) due to negative pressure: venting hot exhaust air out of a window hose pulls hot air from the rest of the building back into the cooled room through gaps, undermining the cooling cycle.

How Sizing Adjusts for Solar and Internal Gains

Calculating the cooling capacity is more complex than heating because solar radiation and occupants are massive heat contributors. Standard rules of thumb (20 BTU/sq ft) only cover empty, standard-glazed rooms. Adding occupants (+600 BTU per person), moving the unit into a highly glazed conservatory (+25%), or choosing a south-facing room (+10% solar gain) will heavily increase the load. If you are calculating winter heating requirements instead of summer cooling, you can use our standard Radiator BTU Calculator (designed using CIBSE guidelines) to map room heat loss and radiator size.

How UK Electricity Surcharges are Calculated

To calculate operational costs, we divide cooling requirements (kW) by EER/COP to yield the active electrical draw, then multiply by average UK electricity tariffs. A modern 3.5 kW split unit drawing ~0.97 kW costs only ~24p per active hour. Ensuring units have high efficiency and setting moderate target temperatures are key to maintaining low cooling bills during heatwaves.

Converting BTU to kW and Air Conditioning Tons

For trade professionals and engineers requiring precise thermodynamic unit conversions rather than residential rules of thumb, the explicit formulas are governed as follows:

  • BTU/hr to Kilowatts (kW): 1 Kilowatt of cooling capacity equates to exactly 3,412.142 British Thermal Units per hour. Formula: kW = BTU ÷ 3,412.142.
  • BTU/hr to Air Conditioning Tons: 1 Ton of refrigeration (TR) represents the rate of heat transfer required to melt 1 short ton of ice in 24 hours, equating to exactly 12,000 BTU/hr. Formula: Tons = BTU ÷ 12,000.
  • Kilowatts (kW) to Tons: One Ton of cooling capacity equates to 3.517 kW of heat extraction rate. Formula: Tons = kW ÷ 3.517.

These equations allow rapid conversions of heat exchange loads between imperial thermal metrics and metric-based mechanical engineering specifications.

Standard AC Capacity Bracket Guide
Quick references mapping floor footprint directly to typical equipment dimensions
Room Area Bracket Capacity Bracket Rating (Tons) System Suitability
Up to 10 m² (100 sq ft) 5,000 – 7,000 BTU/hr 0.5 Ton Small home office, box room, nursery. (Common in portable formats).
10 to 12 m² (130 sq ft) 8,000 BTU/hr 0.67 Ton Medium study, small bedroom, garden pods. (Common in portable formats).
12 to 15 m² (160 sq ft) 9,000 BTU/hr 0.75 Ton Small bedroom, single office, study. (Highly common in both portable & split formats).
15 to 18 m² (190 sq ft) 10,000 BTU/hr 0.83 Ton Standard double bedroom, larger workspace. (Very common portable format).
18 to 25 m² (270 sq ft) 12,000 BTU/hr 1.0 Ton Double bedroom, standard lounge, medium server room.
25 to 30 m² (320 sq ft) 14,000 BTU/hr 1.17 Tons Large lounge, master suites, commercial servers. (Upper limit for portable setups).
30 to 40 m² (430 sq ft) 18,000 BTU/hr 1.5 Tons Large open plan living area, double office garage conversion.
40 to 55 m² (600 sq ft) 24,000 BTU/hr 2.0 Tons Very large open offices, conservatories, commercial retail.

Common Questions

Air Conditioner FAQs

Generally, sizing requires 20 to 25 BTU/hr per square foot of floor area. However, standard rules of thumb do not account for ceiling height. If ceilings are over 8ft, the cooling load expands (+8% per foot). High occupant counts or continuous server equipment loads also require direct capacity increases to ensure the unit is correctly sized.
Split systems have a separate indoor cooling unit and outdoor condenser, resulting in whisper-quiet performance and high efficiency (typical COP of 3.6). Portable AC units contain the compressor inside the room and reject heat through a flexible hose. This hose gets hot, radiating heat back into the room, and negative air pressure draws warm external air inside, cutting their active efficiency down to a COP of 2.6.
It is far cheaper than heating. An efficient 12,000 BTU split system draws about 0.97 kW of electricity, costing approximately 24p per hour at standard 24.5p/kWh tariffs. Setting target temperatures to 22-24°C rather than freezing (16-18°C) allows compressors to run on lower speeds, slicing energy bills by up to 40%.
Single-hose portable ACs expel hot indoor air to the outside. Because this air is leaving the sealed room, it creates negative pressure. Warm outdoor air is immediately sucked into your home through door frames, windows, and floorboard gaps, requiring the AC to work twice as hard to keep the space cool.
Yes. Server rooms run continuous electronic equipment loads generating high heat. Sizing requires a base bracket load of 35 BTU/sq ft, and the total continuous power output of all servers, switches, racks, and accessories in Watts must be converted (multiplied by 3.41 BTU/hr) and added directly to the total capacity requirement.

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