What Size Mini-Split Do I Need? A Room-by-Room Sizing Guide

Mini-split sizing starts with one number: heating load in BTU per hour. In Colorado, you size for heating, not cooling. Denver's summer highs rarely exceed 95°F, but winter design temperatures hit 5°F. A system sized for cooling will be undersized for heating, and that means cold rooms in January.

Three inputs determine your heating load per room:

• Square footage: the floor area of the room. A 200 sq ft bedroom has a very different load than a 600 sq ft basement.
• Insulation quality: older Denver homes (pre-1980) with single-pane windows and minimal insulation need 40 BTU per square foot. New construction with spray foam and triple-pane windows needs only 20 BTU per square foot.
• Climate zone: Denver metro (design temp 5°F) is the baseline. Mountain communities need 25% more capacity. Eastern plains are slightly milder.

Once you know each room's heating BTU, you round up to the next standard indoor head size. Mitsubishi makes heads in 6,000, 9,000, 12,000, 15,000, 18,000, and 24,000 BTU. Sum all heads to get your outdoor unit tonnage (12,000 BTU = 1 ton).

Quick Reference: BTU by Room Size (Average Insulation, Denver)

Add your rooms below and our zone planner calculates the right indoor head size for each one, recommends an outdoor unit, and estimates your total system cost.

Zone Planning Tips for Denver Homes

One head per room is the standard approach: each room with a door gets its own indoor unit for independent temperature control. Open floor plans are different: a kitchen/living/dining area that flows together can often be served by a single larger head positioned centrally.

Common Denver Layouts

• Ranch (1,200–1,800 sq ft): 2–3 zones. Living area + master bedroom, optional third zone for a secondary bedroom or basement.
• Two-story (1,800–2,500 sq ft): 3–4 zones. Living area + master bedroom + 1–2 secondary bedrooms. Upstairs runs warmer. Prioritize those rooms.
• Split-level (1,500–2,000 sq ft): 3 zones. Upper level + main level + lower level. Each level has distinct heating needs.

When One Zone Covers Multiple Rooms

Large open areas often need just one head. A kitchen/living/dining space that flows together can be served by a single larger unit. In homes with a hallway connecting bedrooms, a wall-mount in the hallway can condition adjacent rooms if doors stay open. This works best for secondary bedrooms that don't need precise temperature control.

Denver's ASHRAE design temperature is 5°F (some weather stations record -10°F). Standard mini-split heat pumps lose significant heating capacity below 30°F and effectively stop heating around 15–20°F. At Denver's design conditions, a standard unit delivers maybe 40–60% of its rated capacity.

Cold climate mini-splits, like the Mitsubishi Hyper-Heat series and Bosch Climate 5000, use enhanced inverter compressors and vapor injection to maintain heating capacity down to -13°F. At 5°F (Denver's design temp), a good cold climate unit retains 75–85% of its rated capacity. No electric backup heat strips needed.

For Colorado, cold climate is almost always the right choice if you're using the mini-split for heating. The exceptions: cooling-only applications (garages, server rooms, sunrooms where you have separate heating), or mild-climate vacation properties along the Front Range corridor.

Cold climate models also qualify for larger rebates. Xcel Energy's heat pump incentives require cold climate certification, and the per-ton rebate amounts are higher for cold-climate-rated equipment.

Mini-splits come in several form factors, and you can mix types on a single outdoor unit:

• Wall-mount (ductless): The most common and most affordable option. Mounts high on an interior wall. Visible, but modern units are sleek (Mitsubishi's MSZ-FH is 11" tall). Best for bedrooms, offices, and living rooms.
• Ceiling cassette: Flush-mounts into a drop ceiling. Clean, commercial look with 360° airflow. Requires ceiling space and is more expensive to install. Best for kitchens, open-plan areas, and commercial-style homes.
• Ducted (concealed): The air handler hides in a closet, attic, or above a ceiling and connects to short duct runs. You only see small supply grilles, no visible unit on the wall. Costs $1,000–$2,000 more per zone for the duct fabrication. Best for homeowners who want invisible HVAC.
• Floor-mount: Sits at baseboard height. Good for rooms with limited wall space, large windows, or low ceilings. Delivers warm air low. Natural convection helps distribute heat.
• Mixed/hybrid: Combine different head types on one outdoor unit. Ducted in the hallway, wall-mount in the master, cassette in the kitchen. Mitsubishi MXZ outdoor units support up to 8 mixed heads.

Decision Matrix

Getting the size right matters. Both oversizing and undersizing create problems, and the wrong choice for Colorado's climate can leave you uncomfortable and overpaying.

1. Oversizing

An oversized mini-split short-cycles: it reaches the set temperature too quickly, shuts off, then turns on again minutes later. This causes temperature swings, poor humidity control (the system doesn't run long enough to dehumidify), and premature wear on the compressor. Variable-speed inverter systems handle mild oversizing better than single-speed units, but a 24,000 BTU head in a 150 sq ft bedroom is still too much.

2. Sizing for Cooling Instead of Heating

This is the most common mistake in Colorado. Online BTU calculators default to cooling-load assumptions (20 BTU/sq ft). Denver needs 30–40 BTU/sq ft for heating. If you size a system based on cooling load, it'll handle summer fine but leave you cold in winter. Xcel Energy's Quality Installation requirements specifically mandate sizing for the heating load.

3. Ignoring Altitude

Denver sits at 5,280 feet. Air is 17% less dense than at sea level, which slightly reduces both heating and cooling capacity. The effect is minor (3–5% derating), but it compounds with other factors. Manufacturer specifications are rated at sea level. A professional load calculation accounts for altitude.

4. Not Accounting for Solar Gain

West-facing rooms in Denver take a beating from afternoon sun, especially in summer. Large west-facing windows can add 20–30% to a room's cooling load. Conversely, south-facing rooms benefit from passive solar gain in winter, reducing heating needs. A good load calculation accounts for window orientation.

5. Using Generic Online Calculators

Most online BTU calculators use national averages. They don't account for Denver's altitude, dry climate, intense solar radiation, or the 30°F daily temperature swings that are normal here. Our zone planner above uses Colorado-specific BTU factors, but a professional Manual J load calculation is the gold standard.

A Manual J load calculation is the industry standard for HVAC sizing. It accounts for every factor: square footage, insulation R-values, window types and orientation, air infiltration rates, occupancy, and local climate data. Our comfort advisors perform a simplified version during every free estimate.

During a UniColorado estimate, your comfort advisor will:

• Measure each room and note ceiling heights
• Assess insulation quality and window types
• Check for air leaks and drafts
• Note sun exposure and shading patterns
• Discuss your comfort priorities (which rooms matter most?)
• Recommend specific equipment with transparent pricing
• Identify all eligible rebates and calculate net cost

The estimate is free, takes 15–30 minutes, and comes with zero pressure. We email you the options afterward so you can review at your own pace. Many homeowners schedule estimates in multiple rooms to compare targeted-zone vs whole-home approaches.

Learn more about our mini-split installation process or see current mini-split pricing.