TryBuildCalcDuctless AC Calculator (Single-Zone BTU Sizing)
Size your ductless AC instantly.
🕒 Last updated: September 11, 2026
Inputs
ℹ️IECC climate zone, shared across all zones — hotter zones need more cooling capacity per sqft.
ℹ️Shared across all zones — poor insulation increases every zone's load; good/newer insulation reduces it.
Zones / Rooms
Cost
System Type
Single-Zone
1 zone
Connected Indoor Capacity (Total)
6,000 BTU
Sum of 1 indoor head — NOT a required outdoor unit rating
Planning estimate only — not a Manual J load calculation.Real mini-split systems are sold as AHRI-certified indoor-head + outdoor-unit combinations — the outdoor unit's own nominal rating is NOT required to equal or exceed the connected indoor total above (many manufacturers allow connected indoor capacity to run higher, e.g. up to 130% for some Fujitsu systems). Match against an actual certified multi-zone system's own combination chart, not an assumed pairing of any two products that add up.
Zone 1
6,000 BTU Head200 sq ft — raw load 3,600 BTU/hr
Assumptions Used
Each zone/room's cooling load is estimated from its own square footage x a BTU/sqft figure for your shared IECC climate zone (hotter zones need more capacity per sqft), adjusted for that zone's ceiling height (+10% per ft above the 8 ft baseline), your shared insulation quality (poor +15%, good -12%), that zone's sun/window exposure (high +8%, low -5%), and that zone's occupancy beyond 2 people (+400 BTU/additional person). Each zone's raw load is rounded UP to the nearest nominal indoor head size (6,000 / 9,000 / 12,000 / 15,000 / 18,000 / 24,000 / 30,000 / 36,000 BTU) — these are commonly available across major manufacturers, though not every manufacturer sells every size (e.g. the 15,000 and 30,000 BTU tiers are offered by some brands but not all), so treat this as a realistic planning size, not a guarantee your preferred brand sells that exact number. The "Connected Indoor Capacity" shown is the SUM of all zone head sizes — this is NOT a requirement that the outdoor unit's own nominal rating must equal or exceed it. Real multi-zone systems commonly allow connected indoor capacity to run higher than the outdoor unit's nominal rating under a manufacturer-published combination ratio (e.g. up to 130% for some Fujitsu systems), so the actual outdoor unit is often smaller than this sum — match it against an actual AHRI-certified multi-zone combination chart from a manufacturer, not an assumption that outdoor capacity must meet or exceed the indoor total. A room whose raw load exceeds 36,000 BTU (the largest modeled head) is flagged rather than capped, since that room likely needs to be split into multiple zones. This is a square-footage planning estimate, not an ACCA Manual J load calculation— get a Manual J (and Manual S equipment selection) from a licensed HVAC contractor before purchasing. Cost estimation, if enabled, covers equipment only (priced per indoor head and per outdoor unit) — installation labor, line sets, and electrical work are not included, and is not estimated at all if any zone exceeds capacity, since the system spec itself isn't determined yet.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Mini-Split BTU Calculator.
Ductless AC sizing
A ductless (mini-split) AC unit is sized per room, not for a whole home at once — this calculator estimates the BTU load for your specific room, then rounds up to a standard nominal indoor head size.
This page defaults to a single zone — add more zones above if you're sizing a multi-room system sharing one outdoor unit.
Mini-Split BTU Calculator Formula: How Is It Determined?
Each zone is sized independently from its own inputs, then rolled up into a system-level total.
Step 1 — Per-Zone Base Load
Zone BTU (raw) = Cooling BTU/sqft(zone) x Zone Square Footage
x Height Factor x Insulation Factor x Sun Factor, + Occupancy Addend
Climate zone and insulation are shared across the whole installation; ceiling height, sun/window exposure, and occupancy are entered per zone, since those genuinely vary room to room — this is the core reason mini-split sizing is done per room instead of for the whole home at once.
Step 2 — Indoor Head Sizing
Nominal Head BTU = next available size ≥ Zone BTU (raw), up to 36,000 BTU
Indoor heads are commonly sold in these nominal sizes (6,000 / 9,000 / 12,000 / 15,000 / 18,000 / 24,000 / 30,000 / 36,000 BTU) across major manufacturers, though not every brand sells every size — each zone's raw load rounds UP to the next available size. A zone whose raw load exceeds 36,000 BTU is flagged as exceeding a single head's capacity, not capped to a number that would understate it.
Step 3 — System Rollup
Connected Indoor Capacity (Total) = sum of every zone's Nominal Head BTU
This total is NOT a requirement that the outdoor unit's own nominal rating must equal or exceed — many manufacturers allow connected indoor capacity to run higher than the outdoor unit's rating (a published combination ratio, e.g. up to 130% for some Fujitsu systems), so match this total against an actual manufacturer's AHRI-certified combination chart rather than assuming the outdoor unit must be this size or larger.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Climate / insulation | Zone 4 — Mixed (Mid-Atlantic, TN, OK, N. CA) / average | Sets the shared BTU/sqft multiplier for every zone |
| Zone count | 1 (single-zone) | Determines single-zone vs. multi-zone system rollup |
Step 1 — Base Load per Zone
Base load = climate zone's BTU/sqft x that zone's own square footage.
| Zone | Calculation | Base Load |
|---|---|---|
| Zone 1 | 18 BTU/sqft x 200 sq ft | 3,600 BTU/hr |
Step 2 — Adjustments per Zone
Insulation factor (average, x1) is shared across every zone. Height factor, sun factor, and occupancy addend are each zone's own.
| Zone | x Height | x Insulation | x Sun | + Occupancy | = Raw Load | Nominal Head |
|---|---|---|---|---|---|---|
| Zone 1 (200 sq ft, 8 ft ceiling) | x1 | x1 | x1 (average) | +0 (1 occupant) | 3,600 BTU/hr | 6,000 BTU |
Step 3 — System Rollup
| Connected indoor capacity (total) | 6,000 BTU |
Therefore, this 1-zone single-zone system has approximately 6,000 BTU of connected indoor capacity. Match this total against an actual AHRI-certified multi-zone system's own combination chart before purchasing — the outdoor unit's nominal rating is not required to equal or exceed it.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.