How to use this calculator
- Enter the room area and height. Use the room or zone served by one indoor head, not the entire house unless it is one open zone.
- Set cooling and heating design deltas. Use outdoor design temperature minus indoor cooling setpoint, and indoor heating setpoint minus outdoor winter design temperature.
- Adjust for exposure and envelope. Pick sun exposure and insulation/leakage factors that match the room.
- Add internal gains. Enter typical occupants and equipment watts for the cooling side of the screen.
- Check the suggested size. Use the next common nominal size as a screening target, then verify manufacturer capacity and installation limits.
How it works
This mini-split sizing calculator starts with an area-based room screen and then adjusts it for volume, design weather and simple room modifiers:
q_cool = A x LF_c x h/8 ft x dT_c/20 F x exposure x envelope + internal gains
q_heat = A x LF_h x h/8 ft x dT_h/70 F x envelope
The required capacity is the larger of cooling and heating after your sizing margin. The suggested nominal size is the next common ductless BTU/h size at or above that requirement. The airflow screen then uses:
CFM = nominal tons x target CFM/ton
Use this result as a front-end check before a real load calculation. For air-side comparison, carry the CFM into the HVAC airflow and BTU load calculator. For ducted or mixed systems, use the duct size calculator, duct friction loss calculator, HVAC static pressure calculator and grille size calculator. For the refrigerant-side installation screen, use the refrigerant line size calculator and line-set charge calculator.
Worked example
Verified against the live calculator
A 350 ft2 room with an 8 ft ceiling, average exposure, average envelope, 20 F cooling design difference, 70 F heating design difference, two occupants, 150 W of equipment heat and a 10% margin screens near 9,900 Btu/h cooling and 12,250 Btu/h heating. The heating side controls, so the next common nominal size is 15,000 Btu/h, about 1.25 tons.
Reference data
Common nominal sizes are convenient for screening only; actual capacity depends on the exact outdoor/indoor combination and test condition.
| Nominal size (Btu/h) | Tons | Typical use |
|---|---|---|
| 6,000 | 0.50 | small room or tight zone |
| 9,000 | 0.75 | small room or tight zone |
| 12,000 | 1 | small room or tight zone |
| 15,000 | 1.25 | larger room or open zone |
| 18,000 | 1.50 | larger room or open zone |
| 24,000 | 2 | larger room or open zone |
| 30,000 | 2.50 | large zone or multi-room screen |
| 36,000 | 3 | large zone or multi-room screen |
| 42,000 | 3.50 | large zone or multi-room screen |
| 48,000 | 4 | large zone or multi-room screen |
Source: Nominal screening sizes only. Final equipment selection must use manufacturer capacity tables, local design weather and room-by-room load calculations.
Frequently asked questions
How many BTU do I need for a mini split?
For an early screen, estimate the room cooling and heating load from area, ceiling height, design temperature difference, sun exposure, insulation, occupants and equipment gains. Then select the next common ductless nominal size that covers the larger load after a modest margin.
Is this a Manual J calculator?
No. This is a room-by-room screening calculator. A final mini-split selection should use ACCA Manual J / Manual S or the applicable local method, with real window areas, infiltration, latent load, room adjacency, manufacturer capacity tables and local design weather.
Should I size a heat pump by cooling or heating?
Use the larger design requirement, then verify the actual equipment capacity at your outdoor design temperature. Cold-climate mini splits may have strong low-temperature performance, but the rated nominal BTU/h is not enough by itself.
Why does the calculator show CFM?
The CFM result is a sanity check based on nominal tons and an entered CFM/ton target. Ductless indoor heads have their own fan tables, throw patterns and noise ratings, so use manufacturer data for final airflow and placement.
Can I use one large multi-zone number?
Do the screen room by room. Multi-zone systems also need connected-capacity, branch length, diversity, minimum turndown and simultaneous-load checks. Do not simply add nameplate sizes without checking the manufacturer combination rules.
Method & assumptions
- Area-based cooling load factor is normalized to a 20 F indoor-outdoor design difference and an 8 ft ceiling.
- Area-based heating load factor is normalized to a 70 F indoor-outdoor design difference and an 8 ft ceiling.
- Sun exposure modifies cooling load only; envelope quality modifies both cooling and heating load.
- Occupants beyond the first add a rough cooling allowance; equipment watts are added to cooling load as heat gain.
- Final mini-split sizing still needs Manual J / Manual S or equivalent, latent-load checks, manufacturer low-temperature heating capacity, indoor-head airflow/noise, line length, added charge and local-code review.