For estimation and sales-phase use. A simplified heating and cooling estimate for one zone, not a Manual J load calculation. It does not select an indoor unit.

Maintained by the BuildSolver engineering team · Updated September 21, 2026

Calculator · Mini split zone

Mini split sizing calculator

A ductless head holds one zone, so sizing starts with the load of that zone. Enter the space one indoor unit has to cover, how it is built, the glass on each side and your design temperatures. The calculator adds up heat loss and heat gain line by line and reports heating and cooling in BTU/h. It stops at the load. Picking the unit that covers it at your design conditions is a job for the manufacturer's performance data.

Enter the inside size and wall height of the space one indoor unit has to hold, and pick which walls face outside.
Choose the wall and ceiling insulation, enter the glass on each side with its type, add any solid exterior door and pick what sits under the floor.
Enter the climate zone, the winter and summer design temperatures for your location, the indoor settings, the shading on the glass and how many people use the zone.
The result gives heating and cooling in BTU/h with every line behind them, plus the larger of the two. Take those numbers to the manufacturer's performance data to pick a unit.

One zone, heating and cooling at design. Walls against heated rooms are left out. The load of the space, never an equipment size. The form runs in your browser, so there is no daily limit on it.

Zone length, ft

Inside dimension of the space one indoor unit has to hold.

Zone width, ft

Inside dimension. For an open plan, measure the whole open area, not one end of it.

Wall height, ft

Floor to ceiling. Sets the wall area and the air volume. For a cathedral ceiling use the average height.

Exterior walls

Long means the longer of length and width. Walls against heated rooms are left out of the load. Enter glass only on the sides that face outside, one side per exterior wall.

Exterior wall insulation

Ceiling

Attic floor or cathedral ceiling insulation over the zone. With a heated room above, the ceiling line is zero.

Windows facing north, sq ft

Glass and frame together. Count a sliding glass door as glass. Put 0 on a side with no glass.

Windows facing east, sq ft

Windows facing south, sq ft

Windows facing west, sq ft

Window type

Solid exterior door area, sq ft

Solid doors in exterior walls. A 3 x 6.8 ft door is about 20 sq ft. Glass patio doors go in the window fields instead.

Under the floor

This calculator has no separate garage floor. A zone over an unheated space is counted like a floor over a vented crawlspace, with outdoor air underneath, so the heating line runs on the high side for an insulated floor.

Air leakage

Shading factor on the glass

1 is bare glass. For blinds or screens the engine points at 0.5, a house number with no published table behind it, so treat it as a starting point and not a rating. The lowest value the calculator takes is 0.2.

IECC climate zone

Sets the sun factor and the wall and ceiling cooling factors. It sits next to the design temperatures on the HVAC design temperatures page.

Winter outdoor design temperature, °F

The 99% heating design temperature for your location, from /hvac-design-temperatures.

Indoor heating temperature, °F

Summer outdoor design temperature, °F

The 1% cooling design dry-bulb for your location, from /hvac-design-temperatures.

Indoor cooling temperature, °F

People in the zone

People in the space at the same time. They count on the cooling side only.

Example result for the default inputs

20 x 15 ft zone, 8 ft walls, at a corner, one long and one short wall outside, 2 exterior walls, zone 4A, 15°F winter and 92°F summer design. One zone, heating is the larger of the two.

Heating load

8,439 BTU/h

Cooling load

5,454 BTU/h

Larger of the two loads

8,439 BTU/h

Calculation details
Floor area of the zone	300 sq ft
Exterior wall length	35 ft
Windows and doors in exterior walls	80 sq ft
Heating temperature difference	55°F
Heating, walls net of openings	846 BTU/h
Heating, windows	1,056 BTU/h
Heating, exterior door	440 BTU/h
Heating, ceiling	434 BTU/h
Heating, floor over the crawlspace	4,950 BTU/h
Heating, air leakage at 0.30 ACH	713 BTU/h
Cooling temperature difference	17°F
Cooling, walls net of openings	275 BTU/h
Cooling, window conduction	326 BTU/h
Cooling, sun through windows, W factor 1.00 on all glass, shading factor 1.00	2,040 BTU/h
Cooling, exterior door	136 BTU/h
Cooling, ceiling	154 BTU/h
Cooling, floor over the crawlspace	1,530 BTU/h
Cooling, air leakage at 0.18 ACH	132 BTU/h
Cooling, 2 people, sensible	460 BTU/h
Cooling, people, latent	400 BTU/h
Sensible part of the cooling load	5,054 BTU/h
Heating load per sq ft of floor	28.1 BTU/h
Cooling load per sq ft of floor	18.2 BTU/h

For estimation purposes only. Consult a licensed engineer for final designs.

Run the whole-house load in chat

Free without signup, 3 questions a day, whole house only

How the zone load is calculated

The zone is added up surface by surface. On the heating side each wall, window, door, ceiling and floor loses heat in proportion to its area, its U-value and the gap to the winter design temperature, and air leakage adds its share. Cooling walks the same surfaces at the summer gap, then adds sun through the glass and the people in the space. The coefficients come from the BuildSolver load engine. Its code cites ACCA Manual J 8th Edition Speed Sheet tables, and it marks the handful of values it recut against Wrightsoft reports, which are named under standards further down. The same caveat applies to every coefficient in this section, so read them as this page's working numbers rather than as quoted ACCA values, and watch the few that are flagged below as having nothing published behind them. The two totals come out as they are, with no rounding to a nominal capacity.

Temperature differences: heating dT = indoor - winter design, cooling dT = summer design - indoorA difference at or below zero zeroes the surface and air leakage lines on that side. The calculator never reports a negative load.
Exterior walls: Q = (exterior wall length x wall height - window and door area) x (1 / R) x dT x wall factorExterior wall length follows the walls you pick. A long side against heated rooms drops one long side, a corner drops one long and one short side. Nominal R stands in for the whole wall, studs and all, and R-4 for an empty stud bay is a guess this page makes on its own. Cooling puts the wall on the engine's CLTD band, which the code traces to the Manual J Speed Sheet CLTD table recut against the Wrightsoft per-room subtotals, 1.15 in zones 1 and 2, 1.05 in zones 3 to 5 and 1.0 in zones 6 to 8. Heating uses a straight 1.
Windows and doors: Q = area x U x dTWindow U by type is 1.04 single pane, 0.49 double clear, 0.32 double low-e and 0.22 triple. A solid door uses U 0.40. Enter the glass area with its frame, the way it measures on the wall.
Sun through glass: Q = total glass area x SHGC x zone sun factor x direction factor x shading factorSHGC is 0.86 single, 0.62 double clear, 0.40 double low-e and 0.35 triple. The zone sun factor follows the climate zone you pick, 55 to 100 BTU/h per sq ft from the cool marine zones up to the hot ones. The direction factor is 0.4 north, 0.7 south and 1.0 east or west, and the highest factor among the sides you gave glass is applied to all of the glass, as the room-by-room solver does it. Your shading factor multiplies the result. Cooling only.
Ceiling: Q = floor area x (1 / R) x dT x ceiling factorThe ceiling factor stands for sun on the roof and rides the same CLTD band as the walls, 1.4 in zones 1 and 2, 1.15 in zones 3 to 5 and 1.0 in zones 6 to 8. Heating uses 1. With a heated room above, the line is zero.
Floor: slab heating Q = F2 x exterior wall length x dT, slab cooling Q = 0.04 x floor area x dT, crawlspace Q = 0.30 x floor area x dTF2 is 1.0 with no edge insulation, 0.7 with R-5 and 0.5 with R-10. A floor over an unheated garage, porch or storage space borrows the vented crawlspace numbers with outdoor air below, a substitution this page makes because the engine carries no garage floor of its own. Over a heated room the floor adds nothing.
Air leakage: CFM = ACH x zone volume / 60, Q = 1.08 x CFM x dTAir changes per hour are 0.20 tight, 0.30 average and 0.50 leaky for heating, and 0.10, 0.18 and 0.30 for cooling, anchored in the engine to the measured whole-house rates in those same Wrightsoft reports. The sensible factor 1.08 is a sea-level number and picks up no altitude correction here.
People: sensible Q = people x 230, latent Q = people x 200230 BTU/h sensible and 200 BTU/h latent per person, counted on the cooling side only. The cooling load is the sensible total plus the people latent line.
Larger of the two loads: larger = max(heating, cooling)One head carries the zone through both seasons, so the bigger number is the one it has to cover. What a given head actually delivers at those temperatures comes from its own performance table.

Worked examples for one zone, four ways

The first row is the default on the form. Each row after it changes one thing about the same 20 x 15 ft zone in climate zone 4A, 15°F winter and 92°F summer design: what is under the floor, which way the glass faces, or the insulation, air sealing and shading.

Heating and cooling load for four versions of one mini split zone
Zone	Heating	Cooling
20 x 15 zone at a corner over a vented crawlspace, R-13 walls, R-38 ceiling, 60 sq ft low-e glass facing south and west	8,439 BTU/h	5,454 BTU/h
Same zone on a second floor, over heated rooms	3,489 BTU/h	3,924 BTU/h
Same zone with all 60 sq ft of glass facing south	8,439 BTU/h	4,842 BTU/h
Same zone with R-21 walls, R-49 ceiling, tight air sealing and blinds on the glass	7,782 BTU/h	4,236 BTU/h

Two lines carry this zone. The vented crawlspace under the floor is 59% of the heating load, and sun through all of the glass on the zone, south and west together, is 37% of the cooling load. West is the strongest direction the zone has glass on, so the engine prices every square foot of that glass at the west rate. Put the same zone on a second floor over heated rooms and heating falls 59%, which is why two rooms of 300 sq ft in the same house can need very different units. Turning all of the glass south instead of west takes 11% off cooling and leaves heating where it was. The last row spends money on R-21 walls, an R-49 ceiling, air sealing and blinds. Cooling drops 22% while heating drops only 8%, because the crawlspace floor and the glass conduction are untouched by any of it.

Standards and data sources

BuildSolver load engine coefficients - window U and SHGC, door U, slab F2 and floor U, air change rates, zone sun and wall and ceiling cooling factors, direction factors and people gains. The engine code names ACCA Manual J 8th edition and ASHRAE Fundamentals tables as their origin, and it records where values were recut against Wrightsoft load reports for two homes, a 2018 build in zone 3B and a 2006 Honolulu build in zone 1A, plus one more older home behind the air change rates. Those are the reports behind the recut, and they are a handful of houses rather than a published study. This page uses the numbers as its own working values, not as quoted ACCA figures.
Building America Solution Center, Ductless (Mini-Split) Heat Pumps - the guide from Pacific Northwest National Laboratory for the U.S. Department of Energy states that "Correctly sizing the outdoor unit and each indoor unit (head) to the space loads is imperative for efficient and comfortable operation" and that "Oversized or incorrectly located indoor units can result in short cycling, which wastes energy and does not provide proper temperature or humidity control." It also says to "Calculate heating and cooling loads for each zone and for the entire home using ACCA Manual J." This page estimates one zone and is not Manual J.
ACCA Manual S Residential Equipment Selection - ACCA describes Manual S as a guide "for selecting and sizing residential heating, cooling, dehumidification, and humidification equipment" that uses "heating and cooling loads, the manufacturer's performance data, and your company's design conditions to select equipment." Picking the indoor unit is that procedure, so it is not on this page.
Outdoor design temperatures and climate zone - entered by you. The 99% heating and 1% cooling design temperatures and the IECC climate zone for your location are on the BuildSolver HVAC design temperatures page.

Limitations of this calculator

This is a simplified heating and cooling estimate for one zone. It is not a Manual J load calculation and not the room-by-room Manual J of a house.
It does not pick an indoor unit and does not name a nominal capacity. Capacity at your design temperatures comes from the manufacturer's performance data. Equipment selection is ACCA Manual S, on Pro and Team.
One zone at a time. A multi-zone condenser serving several heads has its own derating and diversity rules, and this page does not touch them. Run each zone separately and take the numbers to your equipment data.
Moisture in the outside air leaking into the zone is not counted. The only latent line is 200 BTU/h per person, a BuildSolver engine default, so there is no sensible heat ratio to check the equipment against. In a humid climate the real latent load is higher and moisture removal has to be checked against the equipment data.
The highest direction factor among the sides with glass is applied to all of the glass. A zone with most of its glass facing north and one small west window is counted as if all of the glass faced west.
Duct losses and gains are left out. That suits a ductless head in the room, and it also means a ducted mini split with a short duct run in an attic needs more than the number here.
No altitude correction. Run the standard atmosphere the engine carries, the equation its code cites to the ASHRAE Handbook of Fundamentals Chapter 1, and air at 6,000 ft comes out about 20 percent low on density, so the air leakage line here runs about 20 percent high at that elevation. The ZIP-based whole-house tools do apply that correction, this single-zone page does not.
Below-grade walls are outside this page. A walkout or a finished basement belongs on the basement BTU calculator, which has the below-grade wall and floor factors.
Nominal R-values are used as the whole wall or ceiling, with no allowance for framing or gaps, and no credit for thermal mass, lights or plug loads.

Common mini split sizing mistakes

Floor area alone will not size the head. The same 300 sq ft lands far apart depending on the glass, what is under the floor and how many walls face outside. Move the worked example off its vented crawlspace onto heated rooms below and the heating load drops by more than half, with nothing else touched.
Treating the zone load as the unit capacity. A heat pump puts out less at 5°F than at 47°F, and its cooling output shifts with outdoor temperature too. Take the load to the manufacturer's performance tables, at your design conditions, before naming a unit.
Room loads do not add up into a multi-zone condenser. Heads on a shared condenser do not all run at peak at the same time, and a multi-zone outdoor unit derates by how many heads are calling. Summing zone loads and buying that number in a condenser is how systems end up oversized.
A head in every room. Building America Solution Center says a head in every room usually does not make sense, because the smallest head on the market still puts out more than a bedroom needs. For the rooms that go without one it points at small air transfer fans moving air from a room that has a head, with a return path such as a vent or an undercut door, or at a ducted air handler with short duct runs to several nearby rooms.
Ignoring humidity because the sensible number looks fine. This page counts one latent line, the people. A ductless head that is oversized for the sensible load runs at low speed and pulls very little water out of the air, which is how a cool room still feels clammy in August.
Only part of an open plan on the tape measure. A single head over a kitchen, dining and living area has to hold the whole open volume. Measure what the air actually reaches, not the room the unit is bolted to.

Common questions about mini split sizing

What size mini split do I need for one room?

Start from the load of that room in BTU/h, not from its floor area. In the default example on this page, a 20 x 15 ft zone at a corner of an older house, over a vented crawlspace, with R-13 walls, an R-38 ceiling and 60 sq ft of low-e glass, needs about 8,400 BTU/h of heat at 15°F outside and about 5,500 BTU/h of cooling at 92°F outside. The unit that covers those numbers at your design temperatures comes from the manufacturer's performance data.

Does this page tell me which indoor unit to buy?

No. It reports the load of the zone in BTU/h and stops there. A nameplate rating is measured at one test condition, and real output moves with outdoor temperature, so the choice belongs to the manufacturer's performance tables read at your design conditions. Matching equipment to a load is ACCA Manual S, which is in the BuildSolver Pro and Team plans.

Can I use this for a multi-zone system?

Run it once per zone to get each zone's load. Do not add the zone loads up and buy that number in a condenser. Heads on a shared outdoor unit rarely peak together, and a multi-zone condenser derates by how many heads are calling, both of which are handled in the equipment data and in Manual S rather than here.

Is this a Manual J calculation?

No. It is a simplified estimate for one space, built from U times area times temperature difference plus air leakage, sun through glass and people. ACCA describes Manual J 8th Edition as the national ANSI-recognized standard for producing HVAC equipment sizing loads for homes. The room-by-room Manual J of a whole house is a BuildSolver Pro and Team feature.

Why does the cooling number jump when glass faces west?

Sun through glass carries a direction factor of 1.0 for east or west, 0.7 for south and 0.4 for north, and the highest factor among the glazed sides is applied to all of the glass. Moving the same 60 sq ft of glass from west to south drops the cooling load in the worked example by 11%, with heating unchanged.

Where do the shading factor and the climate zone come from?

The shading factor is yours to set. 1 is bare glass, and 0.5 for blinds or screens is a BuildSolver house number rather than a published coefficient, so move it if you know the glass better than that. The IECC climate zone and the 99% winter and 1% summer design temperatures for your location are listed on the HVAC design temperatures page.

Does the calculator handle humidity?

Only the people. Moisture in the air leaking into the zone is not counted, so the page reports no sensible heat ratio and gives no dehumidification number. In a humid climate check moisture removal against the equipment data, and if the space needs standalone dehumidification, the dehumidifier size calculator covers that.

How many calculations a day do I get?

There is no limit on this page. The form runs in your browser, so nothing is sent to a server and nothing counts against a quota. The chat and the pages with a server form share a daily allowance. This one has none.

From the zone load to an indoor unit

A ductless head answers the number on this page with capacity that moves with outdoor temperature, so the last step happens in the manufacturer's performance tables, read at your winter and summer design temperatures rather than at the nameplate rating. ACCA Manual S is the written procedure for that step, and a multi-zone outdoor unit adds its own derating by how many heads are calling.

BuildSolver runs Manual S equipment selection in the Pro and Team plans, together with room-by-room Manual J for a whole house. Run the zone in the form above, then open the manufacturer's performance table at your winter design temperature and read the capacity there. If you do not have the design temperature yet, pull it for your ZIP code on the HVAC design temperatures page first.

Used on the same job

What the free chat can and cannot do here

The chat has no single-zone tool, so the zone number comes from this page. Describe the house in chat and the assistant runs the free whole-house Manual J heating and cooling load, the design conditions for your ZIP code and the psychrometrics, and that same whole-house load is free on the Manual J calculator page. Room-by-room Manual J and Manual S equipment selection are Pro and Team features. Take each zone total into chat for the whole-house Manual J and your ZIP design temperature.

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