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Browse ProductsBTU Calculator: What Size AC or Heater Do I Need?
How many BTUs do you need? As a starting point, plan on roughly 20 BTU per square foot of living space for cooling and 30–60 BTU per square foot for heating, depending on your climate. A 500 sq ft room needs about 10,000 BTU of cooling (a 1-ton system) and 15,000–30,000 BTU of heating. Adjust up for high ceilings, lots of sun, poor insulation, or a cold climate.
That single rule of thumb is enough to ballpark almost any space, but getting the size right matters more than most people realize. An air conditioner or furnace that’s too small never catches up on a hot or cold day. One that’s too big short-cycles, leaves rooms clammy, wears out faster, and costs more upfront and to run. This guide gives you the charts, formulas, and adjustment factors to size cooling and heating equipment correctly, whether you’re shopping for a ductless mini split, a central air conditioner, a window unit, or a gas furnace.
What Is a BTU?
A BTU (British Thermal Unit) is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. In HVAC, BTUs measure capacity, or how much heat a system can move per hour. You’ll usually see it written as BTU/h (BTUs per hour), though most people just say “BTU.”
- For air conditioners and heat pumps, BTU describes how much heat the unit can remove from a room.
- For furnaces and heaters, BTU describes how much heat the unit can add.
The bigger the BTU number, the more heating or cooling power. Your job is to match the BTU output of the equipment to the heat load of the space, not too little, not too much.
The Base Rule: ~20 BTU per Square Foot (and Why It’s Only a Starting Point)
The most common BTU calculator shortcut for cooling is:
Key Formula
Square footage × 20 = base cooling BTU
So a 600 sq ft room starts at about 12,000 BTU. This rule comes from decades of field experience and works reasonably well for an average, single-story room with 8-foot ceilings, moderate sun, and decent insulation.
But “average” rarely describes a real room. A sun-drenched top-floor bedroom with cathedral ceilings needs far more capacity than a shaded basement of the same size. That’s why the base rule is a starting point, not a final answer. Below, we give you a room-size chart, a heating chart, the tonnage reference, and the adjustment factors that turn a rough estimate into a confident size.
Cooling BTU Chart by Room Size
Use this AC BTU chart to find a baseline cooling capacity for window units, portable ACs, mini splits, and central systems. These figures assume standard 8-foot ceilings and average conditions; apply the adjustment factors in the next section.
| Room / Area Size | Recommended Cooling BTU | Typical Equipment |
|---|---|---|
| 100–150 sq ft | 5,000 BTU | Small window unit |
| 150–250 sq ft | 6,000 BTU | Window unit / small mini split |
| 250–350 sq ft | 8,000 BTU | Window unit / 9k mini split |
| 350–450 sq ft | 10,000 BTU | Mini split |
| 450–550 sq ft | 12,000 BTU (1 ton) | 12k mini split |
| 550–700 sq ft | 14,000 BTU | Mini split |
| 700–1,000 sq ft | 18,000 BTU (1.5 ton) | Mini split / small central |
| 1,000–1,200 sq ft | 21,000 BTU | Central AC / multi-zone |
| 1,200–1,500 sq ft | 24,000 BTU (2 ton) | Central AC |
| 1,500–2,000 sq ft | 30,000–36,000 BTU (2.5–3 ton) | Central AC |
| 2,000–2,500 sq ft | 42,000 BTU (3.5 ton) | Central AC |
| 2,500–3,000 sq ft | 48,000–60,000 BTU (4–5 ton) | Central AC |
Shopping for a whole-home system? Browse central air conditioners and compare capacities side by side.
Adjustment Factors: Fine-Tuning Your BTU Estimate
Take your base BTU from the chart, then adjust for the realities of your space. These are the same variables a pro accounts for:
- Ceiling height: The base rule assumes 8-foot ceilings. For every foot above 8, add roughly 10–12% more BTU. A room with 10-foot ceilings needs about 20–25% more capacity.
- Sun exposure: Add ~10% for rooms that get heavy direct sun (south- and west-facing, large windows). Subtract ~10% for heavily shaded rooms.
- Climate zone: Hot, humid regions push cooling loads higher; cold-dominated regions push heating loads higher. See the heating chart below.
- Insulation and air sealing: Poorly insulated or drafty rooms (older homes, sunrooms) can need 10–20% more. Tight, well-insulated rooms need less.
- Occupancy: People give off heat. Add about 600 BTU for each person beyond the first two who regularly occupy the space.
- Kitchens: Add roughly 4,000 BTU to a kitchen to account for heat from the stove and oven.
Example
A 450 sq ft west-facing living room (base 10,000 BTU) with 10-foot ceilings (+20%) and full afternoon sun (+10%) lands closer to 13,000 BTU, so you’d size up to a 14,000 BTU unit rather than a 12,000.
BTU to Tonnage: The Conversion You Need for Central AC and Mini Splits
Central air conditioners and many mini splits are sized in tons, not BTUs. The conversion is simple and worth memorizing:
Key Formula
12,000 BTU/h = 1 ton of cooling
The term “ton” is historical, it’s the cooling power of melting one ton of ice over 24 hours. Use this reference when matching the BTU you calculated to equipment tonnage:
| Tonnage | Cooling BTU/h | Roughly Cools (avg. conditions) |
|---|---|---|
| 1 ton | 12,000 BTU | 450–550 sq ft |
| 1.5 ton | 18,000 BTU | 700–1,000 sq ft |
| 2 ton | 24,000 BTU | 1,200–1,500 sq ft |
| 2.5 ton | 30,000 BTU | 1,500–1,800 sq ft |
| 3 ton | 36,000 BTU | 1,800–2,100 sq ft |
| 3.5 ton | 42,000 BTU | 2,100–2,400 sq ft |
| 4 ton | 48,000 BTU | 2,400–2,800 sq ft |
| 5 ton | 60,000 BTU | 2,800–3,300 sq ft |
Note that efficiency matters too. New AC and heat pump equipment is rated by SEER2 (Seasonal Energy Efficiency Ratio 2), the current federal test standard that replaced SEER in 2023. A higher SEER2 means lower running costs at the same BTU capacity, but SEER2 measures efficiency, not size. Always size first by BTU/tonnage, then shop for the highest SEER2 you can afford. For ENERGY STAR efficiency guidance, see the ENERGY STAR central AC criteria.
Sizing a Mini Split
Ductless mini splits are sized in BTU just like other AC equipment, and the room-size chart above applies. Two things make mini split sizing a little different:
Zone-by-zone, not whole-home
Each indoor head serves one zone, so you size each room individually. A single-zone 9,000 BTU unit suits a bedroom; a multi-zone outdoor unit can pair several heads (e.g., 9k + 12k + 12k) for a whole floor.
They heat too
Most modern mini splits are heat pumps, so the same unit cools in summer and heats in winter. Check the unit’s heating BTU rating separately, it may differ from the cooling rating, especially in cold climates where you’ll want a cold-climate (hyper-heat) model.
Because mini splits modulate (ramp output up and down), they tolerate being slightly oversized better than single-stage central systems, but resist the urge to dramatically oversize. Compare ductless mini split systems by zone count and BTU to match your layout.
Heating BTU Chart by Climate Zone
Heating loads vary far more by region than cooling loads, so heating is calculated as BTU per square foot by climate zone. The colder your zone, the more BTU per square foot you need. The chart below uses the U.S. heating climate regions; the U.S. Department of Energy is a good resource for matching heating systems to your region.
| Climate Zone | Region Examples | Heating BTU per sq ft | 1,500 sq ft Home |
|---|---|---|---|
| Zone 1 (Hot) | South FL, South TX, Hawaii | 30–35 BTU | ~45,000–52,500 BTU |
| Zone 2 (Warm) | GA, AL, coastal Carolinas, AZ | 35–40 BTU | ~52,500–60,000 BTU |
| Zone 3 (Moderate) | VA, TN, NC, central CA | 40–45 BTU | ~60,000–67,500 BTU |
| Zone 4 (Mixed/Cold) | NY, PA, OH, KS, OR | 45–50 BTU | ~67,500–75,000 BTU |
| Zone 5 (Cold) | New England, Upper Midwest, MT | 50–60 BTU | ~75,000–90,000 BTU |
Color-coded climate zone map used to help determine the correct HVAC equipment for different regions.
Furnace BTU Sizing (and Why Output Beats Input)
When you shop for a gas furnace, you’ll see a BTU input rating (how much fuel it burns) and an AFUE rating (Annual Fuel Utilization Efficiency, the percentage of fuel converted to usable heat). What heats your home is the output:
Key Formula
Furnace output BTU = Input BTU × AFUE
So a 100,000 BTU input furnace at 95% AFUE delivers 95,000 BTU of actual heat. To find the input rating you need, divide your heating load by the AFUE. A 60,000 BTU heating load on a 95% AFUE furnace requires an input of about 63,000 BTU (round up to the nearest available size, typically 70,000 or 80,000 BTU). Modern high-efficiency furnaces range from 90% to 98.5% AFUE.
Try the BTU Calculator here.
Enter your room size and conditions below for an instant BTU and tonnage recommendation.
When to Get a Manual J Load Calculation
The charts and calculator here are excellent for shopping, comparing, and ballparking. For a permanent whole-home installation, especially central AC or a furnace, the gold standard is a Manual J load calculation. This is the industry-standard method (from ACCA, the Air Conditioning Contractors of America) that accounts for your exact windows, insulation R-values, orientation, infiltration, and local design temperatures.
Get a Manual J when:
- You’re replacing or installing a central system or furnace for the whole house.
- Your home has unusual features, lots of glass, additions, vaulted ceilings, or a major renovation.
- You want to right-size rather than copy the old (often oversized) equipment.
Contractors buying equipment in volume can pair Manual J results with NAW’s wholesale HVAC pricing for contractors to spec and quote jobs efficiently.
How many BTUs do I need per square foot?
For cooling, plan on about 20 BTU per square foot as a starting point. For heating, plan on 30–60 BTU per square foot depending on your climate zone, warmer regions are at the low end, cold northern regions at the high end. Then adjust for ceiling height, sun, insulation, and occupancy.
How many BTU is a 1-ton air conditioner?
One ton equals 12,000 BTU per hour. So a 2-ton unit is 24,000 BTU, a 3-ton unit is 36,000 BTU, and so on. Tonnage is just BTU divided by 12,000.
What size air conditioner do I need for a 1,500 sq ft house?
A 1,500 sq ft home typically needs about 24,000–30,000 BTU of cooling, or roughly a 2 to 2.5-ton central air conditioner under average conditions. Sun exposure, ceiling height, and climate can move that up or down, so confirm with a load calculation before installing.
Is it bad to oversize an air conditioner?
Yes. An oversized AC cools the air quickly but shuts off before it removes enough humidity, leaving rooms cold and clammy. It also short-cycles, which wears out the compressor faster and wastes energy. Correct sizing beats bigger.
How do I size a furnace by BTU?
Multiply your home’s square footage by the heating BTU per square foot for your climate zone (see the heating chart) to get your heating load. Then divide by the furnace’s AFUE efficiency to find the input BTU rating you need, and round up to the nearest available furnace size.
Your growth is the mission.
A good BTU calculation is the difference between a system that quietly keeps you comfortable for 15 years and one that struggles, short-cycles, and runs up your bills. National Air Warehouse offers mini splits, central air conditioners, heat pumps, and furnaces at manufacturer-direct wholesale pricing with free shipping. Visit the National Air Warehouse homepage to browse the full catalog or contact our team to match your number to the right system.