What Size Mini Split Do I Need? BTU Sizing Guide
A mini split sizing guide with a BTU-per-square-foot chart, room-by-room table, the 1-ton rule, climate and sun adjustments, and the Manual J caveat.
The short answer: size a mini split at roughly 20 BTU per square foot, so a 300 sq ft room needs about 6,000 BTU and a 600 sq ft room about 12,000 BTU. One ton equals 12,000 BTU. That rule gets you to the right model class; a Manual J load calculation fine-tunes it for your insulation, windows and climate. This guide gives the full chart and the adjustments that matter.
How many BTU do I need per square foot?
Start at about 20 BTU per square foot of conditioned space. The U.S. Department of Energy and ENERGY STAR use room square footage as the primary sizing input, and 20 BTU/sq ft is the long-standing rule of thumb for cooling in a typical home.
That number is a starting point, not a final answer. Ceiling height above 8 ft, poor insulation, large west-facing windows and a hot climate all push the load up. Good insulation, shade and a mild climate pull it down. Use the rule to pick a size class, then adjust.
Mini split BTU chart by room size
This chart adapts the ENERGY STAR room air conditioner sizing table to mini split head sizes. Match your room’s square footage to the BTU column, then pick the nearest real model size (6k, 9k, 12k, 18k, 24k).
| Room size (sq ft) | BTU needed | Typical head size |
|---|---|---|
| 100-150 | 5,000-6,000 | 6,000 (6k) |
| 150-250 | 6,000-7,000 | 6,000-9,000 |
| 250-350 | 7,000-8,000 | 9,000 (9k) |
| 350-450 | 9,000-10,000 | 9,000-12,000 |
| 450-550 | 10,000-12,000 | 12,000 (12k) |
| 550-700 | 12,000-14,000 | 12,000-18,000 |
| 700-1,000 | 18,000 | 18,000 (18k) |
| 1,000-1,200 | 21,000 | 18,000-24,000 |
| 1,200-1,400 | 23,000-24,000 | 24,000 (24k) |
| 1,400-1,500 | 24,000 | 24,000 (2 ton) |
Mini splits are sold in fixed sizes, so you round to the nearest model. A 400 sq ft bedroom at 9,000 BTU takes a 9k head; a 600 sq ft open living room at 12,000 BTU takes a 12k. The smallest mini splits start at 6,000 BTU for rooms under 250 sq ft.
What adjustments change the BTU number?
Four adjustments move the base number up or down. ENERGY STAR publishes the first three as part of its room AC guidance; the kitchen figure is a standard HVAC allowance.
- Strong sun: add 10% for a room that gets heavy afternoon sun.
- Heavy shade: subtract 10% for a well-shaded room.
- Occupancy: add 600 BTU for each person beyond two who regularly occupies the room.
- Kitchen: add 4,000 BTU if the head serves a kitchen.
Example: a 450 sq ft sunroom with full west sun starts at about 10,000 BTU, plus 10% for sun, which lands near 11,000 BTU, so a 12k head fits. The adjustments are why two rooms of the same size can need different units.
How does tonnage relate to BTU?
One ton of cooling equals 12,000 BTU per hour. The term comes from the cooling once produced by melting a ton of ice in a day, and it is still how HVAC pros talk about capacity.
| Tonnage | BTU per hour | Rough coverage (sq ft) |
|---|---|---|
| 0.5 ton | 6,000 | 100-250 |
| 0.75 ton | 9,000 | 250-400 |
| 1 ton | 12,000 | 400-600 |
| 1.5 ton | 18,000 | 700-1,000 |
| 2 ton | 24,000 | 1,000-1,400 |
So a 2 ton mini split is 24,000 BTU and covers roughly 1,000 to 1,400 sq ft in a moderate climate. A whole floor of separate rooms at that size usually needs a multi-zone system rather than one large head, because conditioned air will not pass a closed door.
How do I size a mini split in square meters?
Outside the U.S., size by square meters using about 100 to 150 watts per square meter, or roughly 400 to 600 BTU per square meter for a standard room. A 25 square meter bedroom needs about 2.5 to 3.5 kW, which is close to 9,000 to 12,000 BTU.
The conversion is 1 kW equals about 3,412 BTU per hour, so a 3.5 kW unit is a 12k head. Hot climates and large glazed areas sit at the top of the watts-per-square-meter range. Our sizing calculator works in both square feet and the BTU equivalent and matches the result to real DIY models.
When do I need a professional load calculation?
For a single room in a well-built house, the rule and chart are close enough to buy with confidence. For a whole-house project, a multi-zone system or an older or leaky building, ask an HVAC contractor for a Manual J load calculation.
Manual J is the Air Conditioning Contractors of America standard. It accounts for insulation R-values, window type and orientation, air leakage, ceiling height, local design temperatures and internal heat gains. It is the method that tells you whether to round a borderline room up or down.
Oversizing is the common mistake. A head that is too large cools fast, then shuts off before it removes humidity, so the room feels cold and damp. Inverter compressors are most efficient at part load, so a right-sized unit that runs long and slow beats an oversized one that short-cycles. When a room falls between two sizes, size for the real load and lean smaller, not larger.
How does sizing differ for heating?
If heat is the main job, size on the heating load and check capacity at your coldest design temperature, not just the nameplate rating. A mini split’s heating output drops as it gets colder: ENERGY STAR lists MRCOOL’s DIY 5th Gen 12k at 8,400 BTU per hour at 5 F, about 70% of its 12,000 BTU rating.
Pick a model whose published low-temperature capacity still covers the room at your design temperature, and keep a backup heat source for the coldest days. Our sizing calculator checks each model’s rated and 5 F capacity so a heating-led choice does not come up short. The right-sized DIY models are in our best DIY mini split guide, and installed pricing by size is in the installation cost guide.
Where do these numbers come from?
- ENERGY STAR room air conditioner sizing guidance and ductless heating and cooling pages, read October 2026: square-footage-to-BTU table and the sun, shade, occupancy and kitchen adjustments.
- U.S. Department of Energy, energy.gov sizing guidance: the 20 BTU per square foot rule of thumb and the role of a Manual J load calculation.
- ACCA Manual J, Residential Load Calculation: the standard method HVAC contractors use.
- ENERGY STAR Certified Mini-Split Heat Pumps dataset and the MRCOOL DIY 5th Gen catalog (V 09.16.26): model sizes and heating capacity at 5 F.
Frequently asked questions
Quick answers to what readers ask us most
How many BTU do I need per square foot?
About 20 BTU per square foot of living space as a starting point. A 500 sq ft room lands near 10,000 BTU. Raise it 10% for strong sun, lots of people or a kitchen, and lower it 10% for a well-shaded room. A Manual J load calculation is the accurate method.
What size mini split do I need for a 1,000 sq ft space?
Roughly 18,000 to 24,000 BTU, which is 1.5 to 2 tons. A single 1,000 sq ft open area can run on one 18,000 BTU head, but 1,000 sq ft split across rooms with doors usually needs a multi-zone system with a head in each room.
How many square feet does a 2 ton mini split cover?
A 2 ton mini split is 24,000 BTU and covers roughly 1,000 to 1,400 sq ft in a moderate climate, less in a hot or sunny space. Tonnage is a cooling measure: 1 ton equals 12,000 BTU per hour.
What happens if a mini split is too big?
An oversized mini split short-cycles. It cools the air fast, shuts off before it pulls moisture out, and leaves the room cold and clammy. It also wastes the inverter compressor's part-load efficiency. Sizing down to the real load is usually better than rounding up.
Can I size a mini split myself?
You can get a close estimate with the 20 BTU per square foot rule and the room chart below, then refine it with our calculator. For a final purchase, a Manual J calculation from an HVAC contractor accounts for insulation, windows, ceiling height and local design temperatures.