{ "slug": "mini-split-sizing", "title": "Mini Split Heat Pump Sizing Calculator", "heading": "Ductless Mini Split Sizing Calculator", "category": "other", "url": "https://www.revenuelab.fyi/toolbox/mini-split-sizing", "summary": "BTU capacity needed for a room or zone based on size and use.", "description": "Ductless mini splits are commonly oversized by installers and homeowners alike, which hurts both efficiency and humidity control since an oversized unit short-cycles instead of running steady, dehumidifying cycles. This calculator uses the standard rule-of-thumb Btu-per-square-foot method adjusted for ceiling height, insulation quality, sun exposure, and occupancy/heat-generating equipment, which is close enough for a single-zone sizing decision without a full Manual J. It also flags when a room's load is small enough that even the smallest common mini split size (9,000 Btu) will be oversized, which is a very common real-world outcome for small, well-insulated rooms.", "formula": "Base Btu = sq ft × 20 (avg climate); adjust for ceiling height, insulation, sun exposure, and occupants × 400 Btu each.", "dateModified": "2026-09-30", "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=mini-split-sizing", "inputs": [ { "id": "sqft", "label": "Room/zone area", "kind": "number", "hint": null, "default": 350, "unit": "sq ft", "min": 50, "max": null }, { "id": "ceilingHeight", "label": "Ceiling height", "kind": "number", "hint": null, "default": 8, "unit": "ft", "min": 7, "max": 20 }, { "id": "insulationQuality", "label": "Insulation quality", "kind": "select", "hint": null, "default": "average", "options": [ { "value": "poor", "label": "Poor (older, uninsulated)" }, { "value": "average", "label": "Average" }, { "value": "good", "label": "Good (well insulated, new construction)" } ] }, { "id": "sunExposure", "label": "Sun exposure", "kind": "select", "hint": null, "default": "moderate", "options": [ { "value": "shaded", "label": "Shaded / north-facing" }, { "value": "moderate", "label": "Moderate" }, { "value": "high", "label": "High / south or west-facing, lots of glass" } ] }, { "id": "occupants", "label": "Typical occupants", "kind": "number", "hint": null, "default": 2, "unit": null, "min": 0, "max": null } ], "outputs": [ { "id": "recommendedSize", "label": "Recommended unit size", "format": "number", "hint": null, "primary": true }, { "id": "btu", "label": "Calculated load", "format": "number", "hint": null, "primary": false }, { "id": "oversized", "label": "Smallest unit still oversized? (1=yes)", "format": "number", "hint": null, "primary": false } ], "worked_example": { "inputs": [ "Room/zone area: 350 sq ft", "Ceiling height: 8 ft", "Insulation quality: Average", "Sun exposure: Moderate", "Typical occupants: 2" ], "outputs": [ "Recommended unit size: 9,000", "Calculated load: 7,800", "Smallest unit still oversized? (1=yes): 0" ] }, "how_to": { "title": "How to use this", "steps": [ "Enter room/zone area (sq ft).", "Enter ceiling height (ft).", "Enter insulation quality.", "Enter sun exposure.", "Enter typical occupants.", "Read your recommended unit size on the right — it updates as you type.", "Hit Share to keep the scenario or send it to someone." ] }, "scenarios": [ { "name": "Conservative", "description": "Lower-end numbers — what if things land soft?", "values": { "sqft": 210, "ceilingHeight": 7, "insulationQuality": "average", "sunExposure": "moderate", "occupants": 1 } }, { "name": "Typical", "description": "Defaults — the most common real-world setup.", "values": { "sqft": 350, "ceilingHeight": 8, "insulationQuality": "average", "sunExposure": "moderate", "occupants": 2 } }, { "name": "Ambitious", "description": "Higher-end numbers — what if things really pop?", "values": { "sqft": 560, "ceilingHeight": 12.8, "insulationQuality": "average", "sunExposure": "moderate", "occupants": 3 } } ], "limitations": [ "Estimates assume standard, average conditions — local rules, pricing, and materials vary.", "Results are rounded for readability; add a buffer before ordering, booking, or committing.", "Double-check anything with a real cost attached against a local quote." ], "faq": [ { "q": "Why is oversizing a mini split actually bad?", "a": "An oversized unit satisfies the thermostat quickly and shuts off before it completes a full dehumidification cycle, leaving the room cool but clammy. It also cycles on/off more frequently, which increases wear and reduces the seasonal efficiency (SEER2) you actually experience versus the rated number." }, { "q": "Should I size for heating or cooling load?", "a": "Size for whichever is larger in your climate — cooling load in the South, heating load in cold climates, especially since heat pump heating capacity derates as outdoor temperature drops. Check the manufacturer's capacity table at your area's 99% design temperature, not just the nameplate rating at 47°F." }, { "q": "Does this replace a Manual J calculation?", "a": "No — this is a solid rule-of-thumb sizing method fine for a single-room DIY mini split decision, but a whole-home multi-zone system, a room with unusual heat gains (large west-facing glass, a home office full of servers), or a load calc for permit purposes should use a proper Manual J from an HVAC contractor." }, { "q": "What's the smallest mini split size available?", "a": "9,000 Btu/hr (3/4 ton) is the smallest common single-zone size, and it's often oversized for a well-insulated bedroom or small office under 300 sq ft — in that case, expect some short-cycling and don't be surprised if the unit runs a low fan speed most of the time rather than cycling fully off." } ], "related": [ "https://www.revenuelab.fyi/toolbox/heat-pump-vs-furnace", "https://www.revenuelab.fyi/toolbox/whole-home-load-calc" ], "license": "CC-BY-4.0", "citation": "RevenueLab — Mini Split Heat Pump Sizing Calculator (https://www.revenuelab.fyi/toolbox/mini-split-sizing)" }