Mini Split Sizing Calculator
A mini split sizing calculator multiplies room area by about 30 BTU per sq ft, then rounds up. A 500 sq ft room needs 15,000 BTU/hr, so an 18,000 BTU unit.
A mini split sizing calculator estimates the heat a room needs to lose, then picks the next standard unit above it. The load is room area × BTU per square foot. At the default of 30 BTU per sq ft, a 500 sq ft room has a load of 15,000 BTU/hr, and the next unit size up is 18,000 BTU.
The formula in plain words
Start with how many square feet the unit has to serve. Multiply by the factor, which is the BTU of heat load for each square foot. That gives the load in BTU per hour.
The factor is a rule of thumb. It folds in sun, insulation, ceiling height, and climate into one number. The default of 30 fits a moderate room with average insulation, and you can change it.
Mini-splits are sold in steps: 9,000, 12,000, 18,000, 24,000, 36,000, and 48,000 BTU. The calculator picks the first step that is at or above your load. It also reports tons, where 12,000 BTU/hr is 1 ton. Above 48,000 BTU/hr no single head is big enough, so the calculator divides the load by 48,000, rounds up, and shows the result as “N × 48,000 BTU”.
Worked examples
Example 1, the default: 500 sq ft at 30 BTU per sq ft.
- Load: 500 × 30 = 15,000 BTU/hr
- Next size up: 18,000 BTU
- Load in tons: 15,000 ÷ 12,000 = 1.25 tons
Example 2, a sunny room with a higher factor: 400 sq ft at 35 BTU per sq ft.
- Load: 400 × 35 = 14,000 BTU/hr
- Next size up: 18,000 BTU
- Load in tons: 1.17 tons
At 30 the same room would be 12,000 BTU/hr and fit a 12,000 BTU unit exactly. The factor can move the choice by a full size.
Example 3, a load above the largest head: 2,000 sq ft at 30 BTU per sq ft.
- Load: 2,000 × 30 = 60,000 BTU/hr
- Units: 60,000 ÷ 48,000 = 1.25, rounded up to 2
- Result shown: 2 × 48,000 BTU
- Load in tons: 60,000 ÷ 12,000 = 5 tons
Unit size by room area at 30 BTU per sq ft
| Room area | Load | Unit size |
|---|---|---|
| 200 sq ft | 6,000 BTU/hr | 9,000 BTU |
| 300 sq ft | 9,000 BTU/hr | 9,000 BTU |
| 500 sq ft | 15,000 BTU/hr | 18,000 BTU |
| 750 sq ft | 22,500 BTU/hr | 24,000 BTU |
| 1,000 sq ft | 30,000 BTU/hr | 36,000 BTU |
| 2,000 sq ft | 60,000 BTU/hr | 2 × 48,000 BTU |
Notice the jump at 1,000 sq ft. A 30,000 BTU load has no 30,000 BTU unit to match, so it moves up to 36,000, which is 20% more than needed. Two smaller heads can fit better.
How to measure
Measure the floor area of the room or open space the unit will serve. If a doorway or wide opening joins two rooms, include both, because air moves between them. Closed rooms with doors need their own head or a plan for how the air will reach them.
Ceilings taller than 8 ft add to the load, so raise the factor a little for a loft or vaulted room.
Common mistakes
- Sizing for the whole house with one head. A single indoor unit cools the space around it, and far rooms get little.
- Choosing the next size up “to be safe.” An oversized unit shuts off too soon and removes less moisture from the air.
- Using the cooling number for a cold climate. Heating output from a heat pump drops as the outdoor temperature falls, so check the maker’s heating capacity at your low temperature.
- Forgetting windows. A wall of south-facing glass can add more load than the factor assumes.
- Reading “2 × 48,000 BTU” as one big unit. Above 48,000 BTU/hr the calculator shows how many 48,000 BTU units the load needs, so plan for separate heads or systems and let an installer pick real combinations.
Reading the equipment label
Each indoor and outdoor unit lists its cooling capacity and its heating capacity in BTU/hr, and the two numbers are not the same. Inverter-driven units can run at a fraction of full output, so they handle a part-load day more gracefully than an on/off system. That does not make a large oversize a good idea. Match the nominal size to the load and check the heating rating at the lowest outdoor temperature in your area.
Multi-room layouts
One outdoor unit can feed several indoor heads. Size each room on its own, then check that the outdoor unit can support the total at the same time. A 500 sq ft living room at 15,000 BTU/hr and a 200 sq ft bedroom at 6,000 BTU/hr together call for about 21,000 BTU/hr of capacity, and the installer will choose combinations that exist.
Related calculators
For a whole house, try the furnace size calculator or the heat loss calculator. The BTU calculator helps with unit conversions. For what the unit costs to buy, see the mini split cost calculator, and for what it costs to run, use the electricity cost calculator.
Frequently asked questions
How do I size a mini-split for a room?
Multiply the room's floor area by a BTU-per-square-foot factor, then round up to the next unit size. At 30 BTU per sq ft, a 500 sq ft room needs 15,000 BTU/hr, and the next size up is 18,000 BTU. Open doorways to other rooms count toward the area being conditioned.
What is a BTU and how does it relate to tons?
A BTU is a unit of heat, and a BTU per hour is a rate of heat removal or addition. One ton of cooling is 12,000 BTU/hr, so an 18,000 BTU unit is 1.5 tons. The calculator shows the raw load in tons too, where 15,000 BTU/hr is 1.25 tons.
Why round up to a nominal unit size?
Mini-splits are built in set sizes, and the calculator rounds up to 9,000, 12,000, 18,000, 24,000, 36,000, or 48,000 BTU. A 1,000 sq ft space at 30 BTU per sq ft is 30,000 BTU/hr, which lands on the 36,000 size. Many installers would use two smaller heads for a space that large. Above 48,000 BTU/hr the calculator shows the unit count instead, such as 2 × 48,000 BTU for a 60,000 BTU/hr load.
Does the BTU-per-square-foot factor change?
Yes, and you can edit it. The default is 30 for a moderate climate and average insulation. A sunny room, a vaulted ceiling, or poor insulation may call for a higher number, and a well-insulated bedroom may call for less. Moving from 30 to 35 changes a 400 sq ft room from 12,000 to 14,000 BTU/hr.
Can I trust this for a final equipment choice?
Treat it as a starting point. A contractor's load calculation counts windows, insulation, ceiling height, and climate. An oversized unit cycles on and off and handles humidity poorly, and an undersized one runs flat out. Have an HVAC professional confirm the size before you buy.
Estimates only. Verify quantities with your supplier before purchasing.