Interactive tool

Mini Split Sizing Calculator: What Size Do You Need?

Room, sun, insulation and climate in. The BTU size you need, the real DIY models that match it, and every step of the math out. Built on the ENERGY STAR sizing chart and manufacturer data checked October 6, 2026.

The room

Room size

Square feet Length x width

Area (sq ft)

Ceiling height

Room type

Sun on the room

People who regularly use the room

Insulation and windows

Climate and setup

Climate zone (IECC)

Nearest listed city is close enough. The US Department of Energy publishes a county map of the zones.

Use

Heat and cool Cooling only

Line-set route, indoor to outdoor unit (ft)

Follow the real path: through the wall, along it and down to the outdoor unit. Leave blank to skip the check.

Power at the outdoor unit

115-120 V only 208-230 V possible

Either way the unit needs its own circuit, run by a licensed electrician.

The short answer With DIY mini splits starting at 9,000 BTU, the ENERGY STAR chart works out to 9,000 BTU for rooms up to 400 sq ft, 12,000 BTU for 400 to 550 sq ft, 18,000 BTU for 550 to 1,000 sq ft, 24,000 BTU for 1,000 to 1,500 sq ft and 36,000 BTU for 1,500 to 2,500 sq ft. Sun, people, a kitchen, tall ceilings, poor insulation and cold-climate heating push the size up.

Mini split sizing chart by square feet

The cooling capacity ENERGY STAR recommends for each room size, before any adjustment, and the DIY mini split size that covers it. One ton is 12,000 BTU per hour.

Room area (sq ft)	ENERGY STAR capacity (BTU/h)	DIY mini split size
100 to 150	5,000	9,000 BTU (0.75 ton)
150 to 250	6,000	9,000 BTU (0.75 ton)
250 to 300	7,000	9,000 BTU (0.75 ton)
300 to 350	8,000	9,000 BTU (0.75 ton)
350 to 400	9,000	9,000 BTU (0.75 ton)
400 to 450	10,000	12,000 BTU (1 ton)
450 to 550	12,000	12,000 BTU (1 ton)
550 to 700	14,000	18,000 BTU (1.5 ton)
700 to 1,000	18,000	18,000 BTU (1.5 ton)
1,000 to 1,200	21,000	24,000 BTU (2 ton)
1,200 to 1,400	23,000	24,000 BTU (2 ton)
1,400 to 1,500	24,000	24,000 BTU (2 ton)
1,500 to 2,000	30,000	36,000 BTU (3 ton)
2,000 to 2,500	34,000	36,000 BTU (3 ton)

Source: ENERGY STAR room air conditioner sizing chart, checked October 6, 2026. Rooms above 2,500 sq ft are past the chart and past what one indoor head can serve.

Which DIY mini splits does the calculator match?

The table covers the pre-charged, quick-connect single-zone systems whose specs we have verified from primary sources: three MRCOOL lines. Other pre-charged systems will be added once their specs are verified the same way. Capacities are rated figures; heat at 17°F and 5°F comes from the ENERGY STAR listing and is blank where the model is not listed.

Line and size	Outdoor / indoor model	Volts	SEER2	HSPF2	Heat at 17°F / 5°F	Line set in box / max run	Max breaker
MRCOOL DIY 5th Gen 9k	DIY-09-HP-C-115D25-O DIY-09-HP-WMAH-115D25-O	115	23.6	11.5	7,500 / 7,300	24.6 ft / 75 ft	20 A
MRCOOL DIY 5th Gen 12k	DIY-12-HP-C-115D25-O DIY-12-HP-WMAH-115D25-O	115	23.5	10.2	8,500 / 8,400	24.6 ft / 75 ft	20 A
MRCOOL DIY 5th Gen 18k	DIY-18-HP-C-230D25-O DIY-18-HP-WMAH-230D25-O	208-230	22.5	11.5	13,400 / 12,800	24.6 ft / 75 ft	20 A
MRCOOL DIY 5th Gen 24k	DIY-24-HP-C-230D25-O DIY-24-HP-WMAH-230D25-O	208-230	22.7	9.5	17,900 / 16,100	24.6 ft / 75 ft	25 A
MRCOOL DIY 5th Gen 36k	DIY-36-HP-C-230D25-O DIY-36-HP-WMAH-230D25-O	208-230	18.0	8.5	n/l / n/l	24.6 ft / 75 ft	35 A
MRCOOL DIY Easy Pro 9k	EZPRO-09-HP-C-11511DE-O EZPRO-09-HP-WMAH-115DE-O	115	20.2	9.2	7,000 / 6,200	11.5 ft / 75 ft	20 A
MRCOOL DIY Easy Pro 12k	EZPRO-12-HP-C-11511DE-O EZPRO-12-HP-WMAH-115DE-O	115	19.5	9.3	n/l / n/l	11.5 ft / 75 ft	20 A
MRCOOL DIY Easy Pro 18k	EZPRO-18-HP-C-23011DE-O EZPRO-18-HP-WMAH-230DE-O	208-230	20.0	8.5	n/l / n/l	11.5 ft / 75 ft	20 A
MRCOOL DIY Easy Pro 24k	EZPRO-24-HP-C-23011DE-O EZPRO-24-HP-WMAH-230DE-O	208-230	18.9	8.7	n/l / n/l	11.5 ft / 75 ft	20 A
MRCOOL DIY Select 9k	DIYS-09-HP-C-115A11-O DIYS-09-HP-WMAH-115A13-O	115	17.5	8.8	n/l / n/l	11.5 ft / not stated	20 A
MRCOOL DIY Select 12k	DIYS-12-HP-C-115A11-O DIYS-12-HP-WMAH-115A13-O	115	17.5	9.0	n/l / n/l	11.5 ft / not stated	20 A
MRCOOL DIY Select 18k	DIYS-18-HP-C-230A11-O DIYS-18-HP-WMAH-230A13-O	208-230	17.3	8.5	n/l / n/l	11.5 ft / not stated	20 A
MRCOOL DIY Select 24k	DIYS-24-HP-C-230A11-O DIYS-24-HP-WMAH-230A13-O	208-230	16.5	8.3	n/l / n/l	11.5 ft / not stated	20 A

n/l: not listed by ENERGY STAR. Sources, checked October 6, 2026: the ENERGY STAR certified mini-split heat pump data, AHRI certificates of product ratings for the models ENERGY STAR does not list, and MRCOOL's spec brochures and installation manuals (DIY 5th Gen, DIY Easy Pro, DIY Select). DIY Select figures come only from MRCOOL's brochure, which labels its specs preliminary, because we found no ENERGY STAR listing or MRCOOL-hosted AHRI certificate for that line. All three lines use R-454B refrigerant. Prices are left out on purpose because they change weekly; see the installation cost guide for dated prices.

How does the calculator work?

Every step is shown under "Show the math" in your result. ENERGY STAR steps come from its room sizing guidance; the steps marked "rule of thumb" are our editorial assumptions, listed here so you can judge them.

Chart capacity (ENERGY STAR). The room's area picks a row in the chart above. Rooms under 100 sq ft use the first row.
Sun (ENERGY STAR). Heavily shaded: minus 10 percent. Very sunny: plus 10 percent. A sunroom always counts as very sunny.
Ceiling height (rule of thumb). The chart assumes 8 ft ceilings, so capacity scales by height divided by 8. A 10 ft ceiling adds 25 percent.
Insulation and windows (rule of thumb). Poor: times 1.2. Good: times 0.9.
Room type (rule of thumb). A garage or workshop is times 1.2 unless its insulation is good, for the uninsulated door and slab. A sunroom is times 1.15 for its glass.
People and kitchens (ENERGY STAR). Plus 600 BTU for each regular occupant beyond two, plus 4,000 BTU in a kitchen.
Heating screen (rule of thumb). Area times 0.25 BTU/h per sq ft per °F, times the gap between 70°F inside and a typical winter design temperature for the zone: zone 1 45°F, zone 2 32°F, zone 3 22°F, zone 4 12°F, zone 5 0°F, zone 6 -10°F, zone 7 -20°F. The insulation, ceiling and room-type factors apply here too. The 0.25 figure is our working average for a room with two outside walls, windows, a ceiling and normal air leakage.
Size. The smallest of 9,000, 12,000, 18,000, 24,000 and 36,000 BTU that covers the cooling load. If you heat, each model's published heating capacity is checked against the heating load at 47°F in zones 1 and 2, at 17°F in zones 3 and 4, and at 5°F in zones 5 to 7. When no model at the cooling size carries the heat, the size steps up to the smallest model that does, and the smaller size is offered with backup heat.
Checks. Voltage (115 V DIY units stop at 12,000 BTU), line-set length against what comes in the box and the manual's maximum, and climate notes.

What this calculator cannot tell you

It sizes one room. It does not know your wall construction, window area, air leakage or local design temperature, which is what an ACCA Manual J load calculation measures. Use the result to shortlist and budget; for a whole house, a new build, or heating a cold-climate home with no backup, pay for a Manual J. It also does not cover installation: the dedicated circuit and disconnect are an electrician's job, and the installation manual and local code decide mounting, clearances and drainage. For which system to buy, see the DIY mini split comparison.

Mini split sizing questions

What size mini split do I need for my room?

On the ENERGY STAR sizing chart, and with DIY units starting at 9,000 BTU, a room up to 400 sq ft takes a 9,000 BTU unit, 400 to 550 sq ft a 12,000 BTU unit, 550 to 1,000 sq ft an 18,000 BTU unit, 1,000 to 1,500 sq ft a 24,000 BTU unit and 1,500 to 2,500 sq ft a 36,000 BTU unit. Sun, extra people, a kitchen, high ceilings, poor insulation and heating in a cold climate can each push that up a size.

How many square feet does a 12,000 BTU mini split cover?

The ENERGY STAR chart puts 12,000 BTU at 450 to 550 sq ft for cooling, and MRCOOL rates its 12,000 BTU DIY units at up to 550 sq ft under ideal conditions. A sunny room, a kitchen, a ceiling above 8 ft or poor insulation lowers that figure.

Is it bad to oversize a mini split?

Somewhat. An oversized unit reaches the set temperature before it has pulled much moisture out of the air, so the room can feel clammy, and it cycles on and off more in mild weather. Inverter mini splits slow down to soften this, which is why one size up is tolerable and two sizes up is not.

Should I size a mini split for heating or for cooling?

Size it for cooling, then check heating against the capacity the model actually publishes at a cold temperature (ENERGY STAR lists capacity at 17°F and 5°F). In climate zones 5 to 7 heating often decides the size, or calls for a backup heat source.

Can a 115 V mini split heat a room in winter?

Yes, within limits. The MRCOOL DIY 5th Gen 12,000 BTU unit runs on 115 V and is listed by ENERGY STAR at 8,400 BTU/h of heat at 5°F, enough for a small, well-insulated room. Every 115 V DIY unit stops at 12,000 BTU; bigger units need a 208-230 V circuit.

Is this calculator a Manual J load calculation?

No. It is a room-level screening tool built on the ENERGY STAR chart plus labelled rules of thumb. For a whole house, a new build, or a system that must carry all the heat in a cold climate, ask an HVAC contractor for an ACCA Manual J load calculation.