How many BTUs do I need per square foot?

As a baseline rule of thumb, you need roughly 20 to 25 BTUs per square foot of living space with standard 8-foot ceilings. However, this must be adjusted upward for warm US climate zones, poor insulation, tall ceilings, kitchens, and sun-facing windows.

What happens if I install an oversized mini-split?

An oversized air conditioner will cool the room too quickly without running long enough to properly dehumidify the air, resulting in a cold, clammy, and humid indoor environment. It also leads to frequent short-cycling, which increases energy bills and wears out the compressor.

How many BTUs are in 1 ton of refrigeration/cooling?

There are exactly 12,000 BTUs in 1 ton of cooling capacity. For example, a 2-ton mini-split or heat pump provides 24,000 BTUs/hr.

Mini-Split HVAC Room Sizing & BTU Calculator (2026)

Room Dimensions

Length (Feet)

Width (Feet)

Ceiling Height

Total Area

400 sq. ft.

Insulation & Climate Factors

US Climate Zone

Insulation Quality

Sun Exposure

Internal Heat Loads

Occupants (# people) +600 BTU per person after 2

Is this a Kitchen?

Recommended Mini-Split Capacity 0.0 Tons

12,000

BTU / hr

Exact calculated thermal requirement: 10,840 BTU. Sized up to the nearest standard manufacturer split unit size.

9K 0.75 Ton

12K 1.0 Ton

18K 1.5 Ton

24K 2.0 Ton

30K 2.5 Ton

36K 3.0 Ton

Typical Electrical Circuit

115V or 208/230V

15A to 20A dedicated breaker

Square Footage Coverage

400 - 550 sq. ft.

Ideal conditioned envelope

Thermal Load Contribution Breakdown

Base Room Volume Load (Area × Height) 0 BTU

Climate Zone Multiplier Adjustment +0 BTU

Building Envelope & Insulation Variance +0 BTU

Solar Radiation / Sun Exposure +0 BTU

Internal Loads (Occupants & Kitchen Appliances) +0 BTU

Calculated Total Cooling / Heating Load 0 BTU/hr

How to Accurately Size a Ductless Mini-Split Air Conditioner

Selecting the proper capacity for a ductless mini-split or heat pump is essential for comfort and energy efficiency. While many HVAC rule-of-thumb charts suggest simply multiplying your square footage by 20, modern building science (ACCA Manual J) requires adjustments for thermal insulation, solar gain, and ceiling height:

Square Footage Baseline A room with standard 8-foot ceilings uses a baseline of ~25 BTUs per square foot for typical US conditions.

Ceiling Height Factor Cathedral, vaulted, or 10ft+ ceilings contain higher cubic air volume that must be conditioned, increasing BTU load proportionally.

Why Bigger Isn't Better An oversized unit cools too rapidly, shutting off before dehumidifying the air. This creates a cold, clammy room and short-cycles the compressor.

Cold Climate Heat Pumps In US Zones 5-7, ensure the outdoor condenser is rated for 100% heating capacity down to -5°F or -15°F (hyper-heat models).

Frequently Asked Questions

How many BTUs equal 1 Ton of cooling?

1 Ton of air conditioning capacity is equivalent to exactly 12,000 BTUs per hour. For example, a 1.5-ton mini-split produces 18,000 BTUs, and a 2-ton system produces 24,000 BTUs.

Can one outdoor unit run multiple indoor heads?

Yes. Multi-zone mini-split systems allow 2 to 5 indoor wall-mounted, floor-mounted, or ceiling-cassette units to connect to a single outdoor multi-port condenser. Each room's BTU requirement is sized individually, and the outdoor condenser is sized to match the sum of all zones.

Do mini-splits run on 115V or 230V electricity?

Most 9,000 BTU and select 12,000 BTU units are available in standard 115V configurations (useful for DIY plug-in kits). However, systems 18,000 BTU and larger nearly always require a dedicated 208/230V double-pole circuit breaker and an outdoor disconnect switch.