What Size Air Conditioner Do I Need? Air Conditioner Buying Guide
Selecting the right air conditioner size is a blend of science and practicality—especially in the UK, where summers are warming but remain unpredictable. An improperly sized unit can waste energy, inflate bills, or leave your home clammy. This guide answers “how do I calculate what size air conditioner I need?” while addressing BTUs, room variables, and system types. Whether you’re cooling a compact flat or a family home, we’ll ensure your investment delivers year-round comfort.
Why Does Air Conditioner Size Matter?
Air conditioner sizing isn’t just about cooling power—it’s about precision. A unit that’s too small or too large creates inefficiencies:
Undersized AC: Struggles to cool the room, runs nonstop, and inflates energy bills.
Oversized AC: Cools the room too quickly, causing “short cycling” (rapid on/off cycles). This fails to remove humidity effectively, leaving the air clammy and increasing wear on the system.
In the UK’s temperate climate, where summer temperatures average 18–25°C, an oversized unit is a common mistake. Proper sizing ensures your AC maintains consistent temperatures without wasting energy.
How Do You Calculate the Right AC Size for Your Space?
Step 1: Measure Your Room
Calculate the room’s square footage (length x width in metres). For instance:
A 5m x 4m living room = 20m².
Step 2: Apply the BTU Formula
The general rule is 1m² ≈ 600 BTUs (British Thermal Units). Multiply your room’s area by 600:
20m² x 600 = 12,000 BTUs.
Step 3: Adjust for Key Factors
Modify the BTU calculation based on:
Sunlight: Add 10% for south-facing rooms.
Insulation: Poor insulation? Add 15–20%.
Occupancy: Include 600 BTUs per person (or heat source like a computer).
Example: A 15m² home office with large south-facing windows and two occupants:
Base BTUs: 15m² x 600 = 9,000
Sunlight adjustment: 9,000 + 10% = 9,900
Occupancy adjustment: 9,900 + 1,200 = 11,100 BTUs.
Key Question: “How do I determine what size air conditioner I need?”
Use this formula or leverage tools like BOXT’s calculator for precision.
What’s the Role of BTUs in Sizing an Air Conditioner?
BTUs (British Thermal Units) measure an AC’s cooling capacity per hour. The higher the BTU, the larger the space it can cool. Here’s a room size breakdown:
Room Size (m²) | Recommended BTUs | Best AC Type |
9–14 m² | 5,000–7,000 BTU | Portable or window unit |
14–23 m² | 7,000–10,000 BTU | Portable or split-system |
23–28 m² | 10,000–12,000 BTU | Split-system |
28+ m² | 12,000+ BTU | Multi-split or ducted AC |
Is a higher number of BTUs always better? No. An oversized AC cools the room rapidly but fails to dehumidify properly, creating a damp environment. Conversely, undersized units overwork themselves. Always match BTUs to your calculated needs.
What Other Factors Affect the AC Size You Need?
Geographic Location: While the UK is generally mild, southern regions like London may require 5–10% more BTUs than Scotland.
Ceiling Height: Rooms taller than 2.4m need extra capacity (add 10% per 0.3m above 2.4m).
Window Quality: Single-pane windows lose cool air faster—consider upgrading to double glazing or adding 10% BTUs.
Appliance Heat: Kitchens or rooms with ovens, TVs, or gaming PCs require 400–600 BTUs per device.
Kitchens or media rooms need adjustments:
Oven: +600 BTUs
TV/Gaming PC: +400 BTUs
Fridge: +200 BTUs
Case Study: A 25m² attic bedroom with sloped ceilings (3m tall), two occupants, and a north-facing window:
Base BTUs: 25m² x 600 = 15,000
Ceiling adjustment: 15,000 + 20% = 18,000
Occupancy: 18,000 + 1,200 = 19,200 BTUs.
Solution: A 20,000 BTU split-system AC or a portable portable air cooler unit for efficiency.
What Kind of Air Conditioner Is Best for the Size of Your Room?
Portable Air Conditioners
Best for: Small to medium rooms (up to 25m²), renters, or temporary cooling.
BTU Range: 8,000–14,000 BTUs.
Pros: No installation needed; easy to move.
Cons: Louder than split systems; requires venting via a window.
Explore top-rated portable air cooler UK models for flexibility. A 20,000 BTU split-system AC or a portable unit like the WAVE 3 with dual-hose design for efficiency.
The EcoFlow WAVE 3 Portable Air Conditioner excels in 15–25m² rooms, offering quiet operation (52 dB) and easy installation.
EcoFlow WAVE 3 Portable Air Conditioner
Split-System ACs
Best for: Medium to large rooms (15–60m²); long-term use.
BTU Range: 9,000–36,000 BTUs.
Pros: Quiet, energy-efficient, and sleek design.
Cons: Requires professional installation.
Window Units
Best for: Single rooms (10–40m²) with compatible windows.
BTU Range: 5,000–12,000 BTUs.
Pros: Affordable and easy to install.
Cons: Rarely suitable for UK sash windows; blocks natural light.
Ducted Central AC
Best for: Whole-house cooling (60m²+).
BTU Range: 24,000–60,000 BTUs.
Pros: Uniform cooling; integrates with heating systems.
Cons: High upfront cost; requires ductwork.
Conclusion
Choosing the right AC size is a science, not a guess. Start with accurate room measurements, adjust for sunlight, insulation, and usage, and match BTUs to your needs. For UK homes, portable generator and split-system units from brands like EcoFlow often strike the best balance between efficiency and practicality. Still unsure? Consult experts like D-Air Conditioning for personalised advice
FAQs
What happens if your AC unit is too big or too small?
An improperly sized AC leads to inefficiencies and higher costs:
Too Small: An undersized unit runs continuously to meet cooling demands, increasing energy bills by 20–30%. Prolonged operation strains components, shortening the system’s lifespan. For example, a 7,000 BTU AC in a 20m² room may never reach the desired temperature during UK heatwaves, leaving you uncomfortable.
Too Big: An oversized unit cools the room rapidly but fails to run long enough to dehumidify properly. This creates damp, clammy air and wastes energy through frequent on/off cycles (short cycling). For instance, a 14,000 BTU unit in a 10m² bedroom may cool the space in minutes but leave humidity levels unchecked, promoting mold growth. To avoid these problems, always calculate the correct air conditioner size before purchase to ensure optimal performance and efficiency.
Is 8000 BTU enough for a bedroom?
Yes, an 8,000 BTU AC typically suffices for a standard 14–18m² UK double bedroom. However, add 10–20% BTUs if the room is south-facing, has high ceilings, or houses multiple occupants.
South-facing windows: Add 10% BTUs (8,800 BTU) to offset solar heat gain.
High ceilings (over 2.4m): Increase capacity by 10–15% (up to 9,200 BTU).
Multiple occupants/appliances: Add 600 BTUs per person or device (e.g., a gaming PC). For example, a 16m² bedroom with two occupants and a TV would need 8,000 + 1,200 = 9,200 BTUs. Portable models offer flexibility for such scenarios.
How do I determine what size portable air conditioner I need?
If you’re asking, “what size do I need portable air conditioner?”, follow these steps:
Measure your room: Calculate square metres (length x width). A 4m x 5m room = 20m².
Calculate base BTUs: Multiply by 600 (20m² x 600 = 12,000 BTUs).
Adjust for variables:
Sunlight: +10% for south-facing rooms.
Insulation: +15% for poorly insulated spaces.
Occupancy: +600 BTUs per person.
Example:
A 15m² sunlit lounge with three people needs:
15 x 600 = 9,000 BTUs (base) + 900 (sun) + 1,800 (occupants) = 11,700 BTUs.
Most portable air-cooler UK models, like the WAVE 3 (10,000–12,000 BTU), fit 12–25m² spaces. For precise answers to “what size do I need portable air conditioner?”, use BOXT’s calculator.
Can I use one AC unit for multiple rooms?
Portable or window units are designed for single-room cooling. For multi-room coverage:
Split-system ACs: Opt for multi-zone systems with indoor units in each room. A 24,000 BTU system can cool 2–3 rooms (e.g., a 15m² bedroom and 20m² living area).
Ducted central AC: Ideal for whole-house cooling but requires ductwork and a £5,000+ investment.
Workarounds: Use fans to circulate cool air from a portable AC, but this reduces efficiency by 15–20%. For open-plan spaces, a 12,000 BTU portable unit can cover up to 35m². Always prioritize BTU calculations for the largest connected area.