Cooling Load Calculator
Calculate sensible and latent cooling loads for HVAC system sizing including occupancy, lighting, equipment,
Category: Hvac
Cooling Load Calculator Inputs
Cooling Load Calculator Formula
Equation
Q_total = Q_sensible + Q_latent | Q_sensible = U×A×ΔT + infiltration + internal gains
Excel Formula
=Q_total=Q_sensible+Q_latent|Q_sensible=U×A×ΔT+infiltration+internalgains
Variables
- Unit System — Choose between Imperial (ft, °F, BTU/h) or SI (m, °C, kW) units
- Floor Area (ft² or m²) — Imperial: ft² | SI: m²
- Ceiling Height (ft or m) — Imperial: ft | SI: m
- Number of Occupants — Enter the Number of Occupants value used by the Cooling Load Calculator.
- Lighting Power Density (W/ft² or W/m²) — Imperial: W/ft² (Office: 1.0-1.3, Retail: 1.5-2.0) | SI: W/m² (Office: 10-14, Retail: 16-22)
- Equipment Power (kW) — Enter the Equipment Power (kW) value used by the Cooling Load Calculator.
- Outdoor Design Temperature (°F or °C) — Imperial: °F | SI: °C
- Indoor Design Temperature (°F or °C) — Imperial: °F | SI: °C
- Outdoor Relative Humidity (%) — For latent load calculation
- Indoor Relative Humidity (%) — Target indoor humidity level
- Exterior Wall Area (ft² or m²) — Imperial: ft² | SI: m²
- Window Area (ft² or m²) — Imperial: ft² | SI: m²
- Wall U-Value (BTU/(h·ft²·°F) or W/(m²·K)) — Lower is better insulation. Imperial: BTU/(h·ft²·°F) | SI: W/(m²·K)
- Window U-Value (BTU/(h·ft²·°F) or W/(m²·K)) — Imperial: BTU/(h·ft²·°F) | SI: W/(m²·K)
- Window SHGC (Solar Heat Gain Coefficient) — Dimensionless coefficient - lower value blocks more sun
- Infiltration Air Changes (ACH) — Tight construction: 0.3-0.5, Average: 0.5-1.0
How the Cooling Load Calculator Works
Cooling load calculation determines the rate of heat removal required to maintain desired indoor conditions. It includes sensible heat (temperature change) and latent heat (moisture removal). Accurate load calculations are critical for proper equipment sizing - undersizing leads to inadequate cooling, while oversizing causes short cycling, poor dehumidification, and wasted energy.
The core relationship is Q_total = Q_sensible + Q_latent | Q_sensible = U×A×ΔT + infiltration + internal gains. Typical inputs include Unit System, Floor Area (ft² or m²), Ceiling Height (ft or m), Number of Occupants.
Enter your values in the cooling load calculator above, review the step-by-step solution, and compare against the worked examples below so you can see how each input changes the result. This free online hvac tool is built for homework, design checks, and professional verification.
Cooling Load Calculator Theory & Explanation
Heat Gain Components
Total cooling load consists of external and internal heat gains:
**External Loads**:
1. **Conduction through Building Envelope**:
Q_wall = U · A · (T_outdoor - T_indoor) · CLTD
where CLTD = Cooling Load Temperature Difference accounting for thermal mass.
2. **Solar Heat Gain through Windows**:
Q_solar = A_window · SHGC · SHGF · CLF
where: - SHGC = Solar Heat Gain Coefficient - SHGF = Solar Heat Gain Factor (BTU/h·ft²) - CLF = Cooling Load Factor
3. **Infiltration**:
Q_inf = \rho · c_p · Q_air · Δ T + \rho · h_fg · Q_air · Δ W
**Internal Loads**:
1. **Occupants**:
Sensible: 70-100 W/person (seated office work) Latent: 50-70 W/person
2. **Lighting**:
Q_lights = W_installed · F_use · F_ballast · CLF
3. **Equipment**:
Q_equip = W_installed · F_use · F_load · CLF
Sensible vs Latent Heat
**Sensible Heat**: Changes air temperature without phase change.
Q_sensible = \dotm · c_p · Δ T = 1.08 · CFM · Δ T_°F
Or in SI units:
Q_sensible = \rho · Q · c_p · Δ T = 1.2 · Q_m^3/s · 1.005 · Δ T_K
**Latent Heat**: Changes moisture content (phase change) without temperature change.
Q_latent = \dotm_air · h_fg · Δ W = 0.68 · CFM · Δ W_gr/lb
Or:
Q_latent = \dotm_air · 2501 · Δ W_kg/kg
where h_fg = latent heat of vaporization ≈ 2501 kJ/kg.
**Sensible Heat Ratio (SHR)**:
SHR = \fracQ_sensibleQ_sensible + Q_latent
Typical SHR: - Dry climates: 0.85-0.95 - Moderate climates: 0.75-0.85 - Humid climates: 0.65-0.75 - High occupancy spaces: 0.60-0.70
Cooling Load Calculation Methods
**Simplified Method** (for quick estimates):
Q_total = Area × Load\,Factor
Typical load factors: - Residential (moderate climate): 25-30 BTU/h·ft² (80-95 W/m²) - Office (moderate internal gains): 30-40 BTU/h·ft² (95-125 W/m²) - Retail (high occupancy): 40-50 BTU/h·ft² (125-160 W/m²) - Restaurant: 50-70 BTU/h·ft² (160-220 W/m²) - Data center: 150-300 BTU/h·ft² (470-950 W/m²)
**ASHRAE Cooling Load Calculation** (detailed):
Uses Radiant Time Series (RTS) method or Transfer Function Method (TFM) to account for: - Thermal mass effects - Time lag in heat transfer - Solar radiation variation - Intermittent internal gains
**Block Load vs Room-by-Room**:
- Block load: Single calculation for entire building/zone - Room-by-room: Individual calculations, sum for total - Room-by-room is more accurate for equipment selection - Block load useful for preliminary sizing
Safety Factors and Diversity
**Safety Factors**:
Add 10-20% to calculated load for: - Calculation uncertainties - Future expansion - Extreme weather events
Avoid excessive safety factors (>25%) as they lead to oversizing.
**Diversity Factors**:
Not all loads occur simultaneously:
Load_actual = Σ (Load_i × DF_i)
Typical diversity factors: - Lighting: 0.8-0.9 (not all lights on) - Occupancy: 0.7-0.9 (not all spaces occupied) - Equipment: 0.5-0.8 (not all equipment running) - Peak coincidence: 0.85-0.95
**Rule of Thumb Conversions**:
- 1 ton cooling = 12,000 BTU/h = 3.517 kW - 400-600 ft²/ton (residential, moderate climate) - 200-400 ft²/ton (commercial, high internal loads) - 1 person ≈ 400-600 BTU/h total (sensible + latent)
Cooling Load Calculator Worked Examples
Worked Example
Inputs
- unitSystem: imperial
- floorArea: 1000
- ceilingHeight: 9
- occupants: 10
- lightingPower: 1.2
- equipmentPower: 15
- outdoorTemp: 95
- indoorTemp: 75
- outdoorHumidity: 60
- indoorHumidity: 50
- wallArea: 800
- windowArea: 150
- wallUValue: 0.35
- windowUValue: 0.45
- windowSHGC: 0.35
- infiltrationACH: 0.5
Result: Total Cooling Load: 85,450 BTU/h (7.1 tons, 25.0 kW) | Sensible: 67,200 BTU/h | Latent: 18,250 BTU/h | SHR: 0.786
Explanation
For a 1000 ft² office space with 10 occupants:
**Step 1: Envelope Load (Walls)** Q_wall = U · A · Δ T = 0.35 × 800 × (95 - 75) = 5,600 BTU/h
**Step 2: Window Conduction** Q_window,cond = 0.45 × 150 × 20 = 1,350 BTU/h
**Step 3: Solar Heat Gain (Windows)** Assuming peak solar: 200 BTU/h·ft²: Q_solar = 150 × 0.35 × 200 = 10,500 BTU/h
**Step 4: Infiltration** Volume: 1000 × 9 = 9,000 ft³ Airflow: 9,000 × 0.5 / 60 = 75 CFM Sensible: 1.08 × 75 × 20 = 1,620 BTU/h Assuming Δ W = 30 grains/lb: Latent: 0.68 × 75 × 30 = 1,530 BTU/h
**Step 5: Occupant Load** Sensible: 10 × 250 = 2,500 BTU/h Latent: 10 × 200 = 2,000 BTU/h
**Step 6: Lighting Load** Q_lights = 1000 × 1.2 × 3.41 = 4,092 BTU/h (all sensible)
**Step 7: Equipment Load** Q_equip = 15 × 3412 = 51,180 BTU/h (assume 80% sensible, 20% latent) Sensible: 51,180 × 0.8 = 40,944 BTU/h Latent: 51,180 × 0.2 = 10,236 BTU/h
**Step 8: Total Loads** Q_sensible = 5,600 + 1,350 + 10,500 + 1,620 + 2,500 + 4,092 + 40,944 = 66,606 BTU/h Q_latent = 1,530 + 2,000 + 10,236 = 13,766 BTU/h Q_total = 66,606 + 13,766 = 80,372 BTU/h
**Step 9: Add Safety Factor (10%)** Q_design = 80,372 × 1.10 = 88,409 BTU/h ≈ 7.4 tons
**Step 10: Calculate SHR** SHR = 66,606 / 80,372 = 0.83 (moderate humidity removal needed)
**Equipment Selection**: Select 7.5 ton unit (90,000 BTU/h) with SHR capability matching 0.80-0.85.
Second Scenario
Inputs
- unitSystem: imperial
- floorArea: 1251
- ceilingHeight: 9
- occupants: 10
- lightingPower: 1.2
- equipmentPower: 15
- outdoorTemp: 95
- indoorTemp: 75
- outdoorHumidity: 60
- indoorHumidity: 50
- wallArea: 800
- windowArea: 150
- wallUValue: 0.35
- windowUValue: 0.45
- windowSHGC: 0.35
- infiltrationACH: 0.5
Result: Total Cooling Load: 85,450 BTU/h (7.1 tons, 25.0 kW) | Sensible: 67,200 BTU/h | Latent: 18,250 BTU/h | SHR: 0.786
Explanation
This scenario uses different inputs (unitSystem = imperial, floorArea = 1251, ceilingHeight = 9, occupants = 10, lightingPower = 1.2, equipmentPower = 15, outdoorTemp = 95, indoorTemp = 75, outdoorHumidity = 60, indoorHumidity = 50, wallArea = 800, windowArea = 150, wallUValue = 0.35, windowUValue = 0.45, windowSHGC = 0.35, infiltrationACH = 0.5) to show how changing one variable affects the cooling load result. Run the calculator above with these values to get the exact updated output with step-by-step work.
Common Cooling Load Calculator Use Cases
- HVAC load and equipment sizing
- Comfort and indoor air quality analysis
- Energy audit support
- Lighting
- Equipment
Cooling Load Calculator FAQs
How accurate is the simplified ft²/ton method for sizing?
The ft²/ton rule of thumb (400-600 ft²/ton residential, 200-400 ft²/ton commercial) provides rough estimates only, accurate within ±30%. It assumes typical construction, occupancy, and internal gains. Use for preliminary budgeting, not final equipment selection. Factors that invalidate ft²/ton: 1) High ceilings (>10 ft), 2) Large windows (>15% wall area), 3) High occupancy density, 4) Significant equipment loads, 5) Poor insulation, 6) Extreme climates. Always perform detailed load calculations per ASHRAE or ACCA Manual J for actual equipment sizing. Oversizing by >25% causes short cycling, poor humidity control, and comfort issues. Undersizing leads to inadequate cooling and high energy costs.
Why is sensible heat ratio (SHR) important for equipment selection?
SHR determines the balance between temperature control (sensible) and moisture removal (latent). Standard AC equipment has fixed SHR (typically 0.75-0.80). If building SHR does not match equipment: 1) Building SHR > Equipment SHR: Overcooling occurs, space too cold while maintaining humidity. 2) Building SHR < Equipment SHR: Poor dehumidification, space feels clammy even at setpoint. Humid climates and high-occupancy spaces need lower SHR equipment (0.65-0.75). Dry climates need higher SHR (0.85-0.95). Solutions for SHR mismatch: 1) Select equipment with adjustable SHR, 2) Use dedicated dehumidification, 3) Consider enthalpy wheels for high latent loads, 4) Variable-speed equipment provides better SHR control. Calculate building SHR accurately - occupants and ventilation air contribute significant latent loads.
How do diversity factors affect cooling load calculations?
Diversity factors recognize that not all heat gains occur simultaneously. Applying diversity avoids oversizing: 1) Lighting: In open office, assume 80-90% diversity (some lights off, dimmers, occupancy sensors). 2) Occupancy: Use design occupancy (not maximum capacity): offices 70-80%, conference rooms 85-95%. 3) Equipment: Office equipment averages 50-70% of installed capacity (computers sleep, printers idle). 4) Peak coincidence: Solar gain peaks afternoon; occupancy may peak midday. Total load is NOT sum of individual peaks. However, be conservative with diversity: 1) Use 100% for critical loads (server rooms, data centers), 2) Code-required ventilation uses 100% occupancy, 3) Avoid excessive diversity (>30% reduction) without measured data, 4) Document all diversity assumptions. For multi-zone systems, diversity allows smaller central equipment than sum of zone peaks.
What are the differences between Imperial and SI unit calculations?
This calculator supports both Imperial (IP) and SI (metric) units. In Imperial: temperatures in °F, areas in ft², cooling loads in BTU/h and tons (1 ton = 12,000 BTU/h). In SI: temperatures in °C, areas in m², cooling loads in Watts and kW (1 ton = 3.517 kW). Key conversions: 1 ft² = 0.0929 m², °C = (°F - 32) × 5/9, 1 BTU/h = 0.293 W. U-values differ: Imperial uses BTU/(h·ft²·°F), SI uses W/(m²·K), conversion: IP × 5.678 = SI. Solar heat gain: ~200 BTU/h·ft² (IP) = ~630 W/m² (SI). Occupant loads: ~450 BTU/h per person (IP) = ~130 W per person (SI). Both systems use the same fundamental physics equations, just different units. Always verify your input units match the selected unit system. Equipment manufacturers provide specifications in both systems; always check which system the spec sheet uses to avoid errors that could lead to significant over/undersizing.
What does the Cooling Load Calculator calculate?
It applies the formula on this page to your inputs and returns the primary result plus any supporting values shown in the output panel.