Mini-Split Sizing Calculator — Free Online Calculator
A ductless mini-split is sized per room or zone, not for the whole house at once, so the size you need depends on that space's area, insulation, sun, and ceiling height. This tool turns those into a BTU figure and the nearest standard mini-split head size, then explains how to add up zones for a multi-zone condenser.
Enter your room or zone details
Recommended mini-split size
Calculated load
10,000 BTU/hr
Equivalent to
1.0 ton
What it covers
A 12,000 BTU (1-ton) single-zone head suits a room this size.
See the breakdown
This is a planning estimate. Confirm with a full ACCA Manual J load calculation. For multi-zone, size each room then pick a condenser rated at or above the total.
Mini-split size by room size
Average construction (~25 BTU/sq ft), 8 ft ceilings, moderate climate. Use a bigger head for poor insulation, sun, tall ceilings, or kitchens.
| Room size | Recommended BTU | Tons | Typical use |
|---|---|---|---|
| 150–250 sq ft | 6,000 BTU | 0.5 | Small bedroom, office |
| 250–350 sq ft | 9,000 BTU | 0.75 | Bedroom, small living room |
| 350–450 sq ft | 12,000 BTU | 1.0 | Living room, large bedroom |
| 450–700 sq ft | 18,000 BTU | 1.5 | Open living/dining |
| 700–1,000 sq ft | 24,000 BTU | 2.0 | Studio, small apartment |
| 1,000–1,300 sq ft | 30,000 BTU | 2.5 | Large open floor |
| 1,300–1,600 sq ft | 36,000 BTU | 3.0 | Whole small home (often multi-zone) |
Rule of thumb: a 12,000 BTU (1-ton) head covers roughly 450–550 sq ft in good conditions.
Sources & standards: ENERGY STAR — Ductless Heating & Cooling · ACCA Manual J (load calculation standard) · U.S. DOE Energy Saver — Ductless, Mini-Split Heat Pumps
The formula, explained in plain English
Sizing a ductless mini-split starts with the heat a single room gains, then rounds up to the nearest head a manufacturer actually sells. Here is exactly what this calculator does.
Why per-room?
Mini-splits condition individual zones — each head is sized to its own room, unlike one central system that serves the whole house from a single unit.
20–30 BTU/sq ft
Well-insulated rooms land near 20 BTU/sq ft, average ones near 25, and drafty older spaces closer to 30. Pick the band that matches the construction.
Don't oversize
An oversized head short-cycles and won't dehumidify — it's the #1 ductless sizing mistake. Aim for the right size, not the biggest one.
Multi-zone
Size each room, then choose a condenser rated at or above the total. Manufacturers allow some diversity, so you rarely sum exactly.
Worked examples
Three real-world scenarios showing how the same method scales from a single bedroom to a whole home.
Small bedroom — 250 sq ft, well-insulated
Well-insulated (20 BTU/sq ft) · 8 ft ceiling · moderate climate · average sun · no kitchen.
→ nearest head = 6,000 BTU (0.5 ton)
Result: about 5,000 BTU of load — a 6k or 9k single-zone head is the right fit for a small bedroom.
Sunny open living + kitchen — 600 sq ft
Average (25 BTU/sq ft) · 8 ft ceiling · warm climate (×1.1) · very sunny (×1.1) · kitchen included.
× 1.1 (warm) × 1.1 (sunny) = 18,150 BTU
+ 4,000 (kitchen) = 22,150 BTU → 24,000 BTU (2-ton) head
Result: about 22,150 BTU — a 24,000 BTU (2-ton) head, or split the open space into two zones.
Multi-zone whole home — 300 + 300 + 500 sq ft rooms
Three rooms, each sized on its own, then combined for the condenser.
→ choose a ~36,000 BTU (3-ton) multi-zone condenser
Result: a ~36,000 BTU (3-ton) multi-zone condenser. Thanks to load diversity you don't always size the condenser to the exact sum.
Frequently asked questions
Common questions about mini-split sizing and ductless head capacity.
What size mini-split do I need?
Size per room: about 20 BTU/sq ft for a well-insulated space, 25 average, and 30 for drafty, sunny, or kitchen areas. A 400 sq ft average room needs ≈10,000 BTU, so a 12,000 BTU (1-ton) head. Enter your room above for an exact figure.
How many square feet does a 12,000 BTU mini-split cool?
Roughly 450–550 sq ft in good conditions — less with poor insulation, big windows, sun, or tall ceilings, and a bit more in a tight, shaded room.
What size mini-split for a 1,000 sq ft space?
About 24,000 BTU (2 tons) for an average open 1,000 sq ft area — but if it's several rooms, a multi-zone system with a head per room is usually better than one big head.
How do I size a multi-zone mini-split?
Size each room individually, then pick an outdoor condenser rated at or above the combined indoor load. Because rooms rarely peak at once (load diversity), manufacturers let connected head capacity modestly exceed the condenser rating.
Can a mini-split be too big?
Yes — oversizing is the most common ductless mistake. An oversized head short-cycles, leaving the room cold and clammy because it never runs long enough to remove humidity, and it wears the compressor. Right-sizing matters more than going big.
Do mini-splits heat too?
Most are heat pumps that both heat and cool. In cold climates check the heating load and pick a cold-climate model; see our Heat Pump Sizing Calculator for the heating-side math.
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