HVAC Sizing Guide
Choose the right capacity for mini splits, multi-zone systems, central ducted heat pumps, dual fuel, packaged units, and AC-only condensers. Built for clarity, speed, and confidence—by Freedom HVAC Supply.
How to Think About Sizing
Sizing is about matching equipment output (BTU/hr or tons) to your home’s design heating and cooling loads. The most accurate method is a Manual J load calculation. When you can’t run one, use our Quick Calculator, then confirm with the tables below.
Key Inputs
- Square footage & ceiling height (volume)
- Climate (hot/mild/cold), sun exposure, window area
- Insulation & air-sealing quality
- Occupancy & internal gains (kitchen, electronics)
Right-Sizing Goals
- Avoid oversizing: short cycling, humidity issues, higher cost
- Avoid undersizing: won’t hit setpoint in extremes
- Prefer inverter equipment with good turndown
Tip: 12,000 BTU/hr ≈ 1 ton. Typical comfort airflow is ~350–450 CFM per ton.
Quick Sizing Calculator
Get a ballpark BTU/hr. This does not replace a Manual J—use it to shortlist sizes before you pick equipment.
Rule-of-Thumb BTU per Sq Ft (Cooling)
Use these to sanity-check the calculator. Values assume average internal gains and typical window area.
| Climate | Good Shell | Average Shell | Poor/Leaky Shell |
|---|---|---|---|
| Mild | 16–20 | 20–24 | 24–30 |
| Hot/Humid | 20–24 | 24–30 | 30–36 |
| Cold (cooling) | 17–21 | 21–26 | 26–32 |
| Very Cold (cooling) | 16–20 | 20–24 | 24–30 |
For heating, very cold climates can run 25–50 BTU/sq ft+ depending on envelope and design temp; verify heat pump low-temp capacity and consider auxiliary/dual fuel.
Single-Zone Mini Splits
Match one indoor unit to one outdoor unit. Ideal for targeted rooms, additions, or garage conversions.
Common Sizes & Typical Coverage
| Nominal Size | BTU | Typical Coverage | Notes |
|---|---|---|---|
| 6k | 6,000 | 150–300 sq ft | Small bedrooms, offices |
| 9k | 9,000 | 250–400 sq ft | Bedrooms, medium offices |
| 12k | 12,000 | 350–600 sq ft | Living rooms, larger beds |
| 18k | 18,000 | 600–900 sq ft | Great rooms, big studios |
| 24k | 24,000 | 900–1,200 sq ft | Open concept areas |
Always check the actual capacity curves: many inverter systems deliver more/less than nameplate at high/low outdoor temps.
Selection Checklist
- Confirm wall/ceiling cassette airflow and throw meet room geometry
- Verify heating capacity at design outdoor temp
- Dehumidification performance (latent capacity) in humid climates
- Electrical: voltage, MCA/MOP, breaker size
Multi-Zone Mini Splits
One outdoor condenser serving multiple indoor heads (wall, ceiling cassette, slim-ducted). Great for homes without full ductwork.
How to Size
- Calculate the load for each zone/room (use Quick Calculator per room as a proxy).
- Sum the indoor unit BTUs you intend to install.
- Apply a diversity factor (commonly 0.75–0.9) if not all zones peak together.
- Select an outdoor unit whose rated capacity meets the diversified sum at your design temp.
Example Diversified Sizing
| Zones | Indoor BTUs | Sum | Diversity (0.85) | Pick Outdoor |
|---|---|---|---|---|
| 3 | 9k + 9k + 12k | 30k | 25.5k | ~27–30k |
| 4 | 6k + 9k + 9k + 12k | 36k | 30.6k | ~30–36k |
| 5 | 9k + 9k + 9k + 12k + 12k | 51k | 43.4k | ~42–48k |
Don’t oversize the outdoor unit far above the diversified sum—low turndown + many small heads can cause short cycling.
Multi-Zone Tips
- Each head must meet its room’s minimum load even when other zones are off.
- Check minimum connected capacity rules and port combinations in the manufacturer tables.
- Long line set runs and elevation changes affect capacity—account for them.
Central Ducted Heat Pumps
Whole-home comfort via existing or new ductwork. Inverter ducted systems provide great efficiency and comfort if ducts are sized and sealed properly.
Whole-Home Sizing (Quick)
| Home Size | Typical Cooling | Typical Heating (cold climates) | Common Nominal |
|---|---|---|---|
| 800–1,200 sq ft | 1.5–2.0 tons | 24k–36k BTU | 18k–24k / 2.0 ton |
| 1,200–1,800 sq ft | 2.0–3.0 tons | 36k–48k BTU | 24k–36k / 2–3 ton |
| 1,800–2,400 sq ft | 3.0–4.0 tons | 48k–60k BTU | 36k–48k / 3–4 ton |
| 2,400–3,000 sq ft | 4.0–5.0 tons | 60k–72k BTU | 48k–60k / 4–5 ton |
Confirm cooling and low-temperature heating capacity at your design outdoor temp. Balance point and defrost strategy matter.
Ducted Heat Pump Checklist
- Airflow: target 350–450 CFM/ton; verify blower curves vs. external static pressure (ESP)
- Ducts: size, seal (mastic), and insulate; keep total ESP ≤ 0.5 in. w.c. (typical)
- Heat strips (aux): size to cover shortfall below balance point (e.g., 5–15 kW depending on climate)
- Thermostat/controls: match staging, dehumidify and heat strip lockouts
Central Ducted Dual Fuel (Heat Pump + Furnace)
Best of both: heat pump handles mild/cool weather; gas furnace takes over in very cold temps or for backup.
Sizing Approach
- Size the heat pump to meet most cooling load and a significant share of heating load above the economic switchover temperature.
- Size the furnace to meet the design heating load at the coldest design temp.
- Use controls to set switchover based on outdoor temp/utility rates.
| Home Size | HP (Cooling Nominal) | Furnace Input (typical) | Notes |
|---|---|---|---|
| 1,200–1,800 sq ft | 2–3 ton | 60–80k BTU | High altitude may require derate |
| 1,800–2,400 sq ft | 3–4 ton | 80–100k BTU | Match cased coil & blower |
| 2,400–3,200 sq ft | 4–5 ton | 100–120k BTU | Balance duct sizing & ESP |
Confirm coil match for AHRI ratings (efficiency + capacity) and proper TXV/orifice for heat pump mode.
Central Ducted Package Units (Heat Pump / Dual Fuel / AC)
All-in-one rooftop or pad units. Great for tight mechanical spaces or simple replacements.
Quick Selection
| Application | Typical Range | Notes |
|---|---|---|
| Packaged Heat Pump | 2–5 ton | Verify low-ambient heating tables; defrost & strip heat sizing |
| Packaged Gas/Electric (Dual Fuel) | 2–5 ton / 60–120k BTU | Match gas input to design heat; confirm economizer options |
| Packaged AC-Only | 2–5 ton | Ensure coil/airflow for latent removal in humid climates |
AC-Only Condensers (Split Systems)
Cooling-only systems paired with a matching indoor coil and air handler or furnace.
Sizing by Area (Cooling)
| Home Size | Typical Tons | Airflow Target | Notes |
|---|---|---|---|
| 800–1,200 sq ft | 1.5–2.0 | 525–900 CFM | High humidity → choose higher SHR coils/controls |
| 1,200–1,800 sq ft | 2.0–3.0 | 700–1,350 CFM | Dehumidify mode or reheat helpful |
| 1,800–2,400 sq ft | 3.0–4.0 | 1,050–1,800 CFM | Verify return grille capacity |
| 2,400–3,000 sq ft | 4.0–5.0 | 1,400–2,250 CFM | Ducts often the limiting factor |
Matching Coils & Furnaces
- Use AHRI-matched coil for stated SEER2/EER2/Capacity
- Furnace blower must deliver required CFM at expected ESP
- Consider TXV vs. fixed orifice for better part-load control
Ducting & Airflow Essentials
Targets
- 350–450 CFM per ton (cooling), 400 CFM/ton is a common design point
- Total ESP: aim ≤ 0.5 in. w.c. unless equipment allows higher
- Return grille area: ~2 sq in per CFM (rule of thumb)
Best Practices
- Short, straight runs; wide, smooth radius elbows
- Seal with mastic; insulate in unconditioned spaces
- Balance dampers for room-by-room comfort
Electrical & Breaker Considerations
- Match nameplate voltage/phasing (e.g., 208/230V single-phase)
- Use manufacturer MCA (Minimum Circuit Ampacity) and MOP for wire/breaker
- Heat strips add significant amperage—size feeders accordingly
Always follow the unit’s installation manual and local electrical code.
Need a Second Set of Eyes?
Send us your square footage, climate, insulation level, and any plans (rooms/zones). Freedom HVAC Supply can validate your picks and suggest compatible kits.
Frequently Asked Questions
Do I really need a Manual J?
For best results, yes. Our calculator and tables are for screening. A Manual J accounts for windows, orientation, infiltration, and local design temperatures, giving the most accurate answer.
How do I size multi-zone mini splits without oversizing?
Start with room loads, pick heads that meet each room’s minimum/maximum, sum them, then apply a diversity factor (often 0.75–0.9). Choose an outdoor unit that meets the diversified sum at design conditions and supports your port combination.
What’s the difference between nameplate and actual capacity?
Inverter equipment can modulate. Actual capacity changes with outdoor temperature, indoor setpoints, and airflow. Check the manufacturer’s capacity tables (cooling & heating) for your design temps.
How much airflow do I need?
Aim for 350–450 CFM per ton in cooling. Verify your blower tables at expected ESP. If you can’t deliver the airflow, reduce external static (better ducts/filters) or select equipment with higher blower capability.
Should I choose dual fuel?
In very cold climates or where gas is inexpensive, dual fuel offers comfort and cost control. Let the heat pump run above the switchover temperature; furnace handles deep cold and faster recovery.
Any quick coverage rules of thumb?
- Mini split 9k: ~250–400 sq ft, 12k: ~350–600, 18k: ~600–900 (varies by climate/shell)
- Whole-home: 2–3 tons for ~1,200–1,800 sq ft (verify!)
- Heating in cold climates often governs size more than cooling—check low-ambient tables
Disclaimer: This guide provides general guidance only and does not replace a licensed design or Manual J/JS/D. Always follow manufacturer instructions and local codes.