A free, browser-based BTU calculator and AC sizing tool powered by ACCA Manual J mathematics. Estimate heating loads, cooling loads, infiltration rates, and equipment tonnage for residential buildings across 235+ US climate zones — no sign-up required.
ACCA Manual J is the gold-standard methodology for residential HVAC system sizing in the United States. Rather than guessing based on square footage alone, a proper Manual J calculator considers every thermal factor: wall and roof insulation R-values, window U-values and solar heat gain coefficients (SHGC), design temperatures for your specific climate zone, air infiltration rates, internal heat from occupants and appliances, and duct system losses.
The result is a precise BTU/hr requirement for both heating and cooling — which directly converts to HVAC tonnage (1 ton = 12,000 BTU/hr). An oversized system wastes energy and short-cycles. An undersized system can't keep up on extreme weather days. CoreLoad's free Manual J calculator helps you find the correct size in under 2 minutes.
HVAC tonnage measures the cooling capacity of an air conditioning system. One ton equals 12,000 BTU/hr of cooling power. To calculate the correct tonnage for your home, you need the total cooling load — which includes heat gain through walls, roof, windows, infiltration, and internal sources like occupants and appliances.
The formula is simple: Tonnage = Total Cooling BTU/hr ÷ 12,000. But calculating the Total Cooling BTU accurately requires a full block load analysis. A 2,000 sq ft home in Miami with poor insulation might need 4.5 tons, while the same home in Portland with good insulation might only need 2.0 tons. Climate, insulation, and windows make all the difference — which is exactly what this HVAC tonnage calculator computes for you.
BTU (British Thermal Unit) is the standard measure of thermal energy in HVAC. One BTU is the energy needed to raise one pound of water by one degree Fahrenheit. Your home's heating BTU load represents how much heat must be added to maintain 70°F when it's coldest outside. Your cooling BTU load represents how much heat must be removed to maintain 75°F when it's hottest outside.
This BTU calculator computes both loads using the engineering formula Q = U × A × ΔT for each building component, then adds infiltration loads (Q = V × ACH × 0.018 × ΔT) and solar gains through windows. The result is a complete BTU breakdown showing exactly where your home gains and loses the most heat.
Correctly sizing an air conditioner is critical for comfort, efficiency, and equipment lifespan. The old rule of thumb — "1 ton per 500 square feet" — is dangerously inaccurate because it ignores climate, insulation, window area, and building tightness. A proper AC sizing calculation requires a Manual J load analysis.
Why oversizing is bad: An oversized AC cools the house too quickly without running long enough to remove humidity. This causes clammy air, mold risk, higher energy bills, and compressor short-cycling that reduces equipment life by 30-50%.
Why undersizing is bad: An undersized AC runs constantly on extreme days and never reaches the set temperature. This wastes energy and leaves rooms uncomfortable. The correct size runs 80-90% of the time on the hottest day of the year.
This table provides rough estimates. Actual tonnage varies significantly by climate, insulation, and window area. Always use a Manual J calculator for accurate sizing.
| Home Size | Mild Climate | Hot Climate | Cold Climate |
|---|---|---|---|
| 800 sq ft | 1.0 ton | 1.5 tons | 1.0 ton |
| 1,200 sq ft | 1.5 tons | 2.0 tons | 1.5 tons |
| 1,500 sq ft | 2.0 tons | 2.5 tons | 2.0 tons |
| 2,000 sq ft | 2.5 tons | 3.5 tons | 2.5 tons |
| 2,500 sq ft | 3.0 tons | 4.0 tons | 3.0 tons |
| 3,000 sq ft | 3.5 tons | 5.0 tons | 3.5 tons |
How does CoreLoad compare to other methods of sizing an HVAC system?
| Feature | Rule of Thumb | Manual Spreadsheet | CoreLoad |
|---|---|---|---|
| Accuracy | ❌ Poor | ✅ High | ✅ High |
| Speed | ✅ Instant | ❌ Hours | ✅ 2 minutes |
| Climate Data | ❌ None | ⚠️ Manual lookup | ✅ 235 zones built-in |
| Infiltration | ❌ Ignored | ⚠️ If you know ACH | ✅ Automatic |
| Latent Load | ❌ Ignored | ❌ Usually skipped | ✅ Full modeling |
| Duct Losses | ❌ Ignored | ⚠️ Extra work | ✅ Built-in |
| Cost | ✅ Free | ✅ Free | ✅ Free |
Calculates winter heat loss through walls, roof, and windows using Q = U × A × ΔT. Includes sensible infiltration load based on building tightness (ACH method). Uses 99% winter design dry-bulb temperatures from ASHRAE data.
Computes summer heat gain including solar radiation through windows (SHGC × orientation multiplier), conductive gain through the envelope, sensible and latent infiltration loads, and internal gains from occupants and appliances. Outputs total cooling BTU/hr and recommended AC tonnage.
Models duct system losses based on location (conditioned space, unconditioned basement, or vented attic) and insulation level (R-4 through R-8). Infiltration uses the ACH (Air Changes per Hour) method with tight, average, and leaky building classifications.
CoreLoad includes ACCA/ASHRAE design temperatures for 235+ cities. Here are some of the most searched locations and their design conditions for HVAC sizing:
| City | Winter 99% | Summer 1% | Grains |
|---|---|---|---|
| New York, NY | 15°F | 89°F | 35 gr |
| Houston, TX | 33°F | 96°F | 55 gr |
| Phoenix, AZ | 38°F | 109°F | -15 gr |
| Chicago, IL | -3°F | 91°F | 38 gr |
| Los Angeles, CA | 43°F | 89°F | 5 gr |
| Miami, FL | 47°F | 91°F | 57 gr |
| Denver, CO | 1°F | 93°F | -25 gr |
| Seattle, WA | 27°F | 85°F | 5 gr |
| Atlanta, GA | 22°F | 92°F | 40 gr |
| Dallas, TX | 22°F | 100°F | 45 gr |
| Minneapolis, MN | -12°F | 89°F | 38 gr |
| Las Vegas, NV | 30°F | 108°F | -30 gr |
Can't find your city? CoreLoad's database includes 235+ locations. Search for your city in the calculator above, or use the custom temperature override to enter your own ASHRAE design conditions.
Divide the total cooling load in BTU/hr by 12,000. One ton of air conditioning equals 12,000 BTU/hr. For example, a 36,000 BTU/hr cooling load requires a 3-ton AC system.
The common "1 ton per 500 sq ft" rule is inaccurate. A 1,500 sq ft home in Phoenix might need 3.5 tons due to extreme heat, while the same home in Seattle might only need 1.5 tons. Use this calculator for accurate results based on your actual conditions.
These represent extreme outdoor conditions from ASHRAE weather data. The 99% winter value means temperatures are at or above it 99% of the year. The 1% summer value means temperatures exceed it only 1% of the year. HVAC systems are sized to handle these extremes.
Q = U × A × ΔT, where Q is heat loss (BTU/hr), U is the U-value (1 ÷ R-value), A is wall area (sq ft), and ΔT is the indoor–outdoor temperature difference (°F).
Infiltration is uncontrolled air leakage through cracks and gaps in the building envelope. Sensible load: Q = V × ACH × 0.018 × ΔT. Latent load: Q = V × ACH × 0.01133 × ΔGrains. In humid climates like Houston or Miami, latent infiltration can account for 20-30% of the total cooling load.
Loading interactive calculator...