Introduction
Behind every cooling load number sits a chain of physics: heat conducts through walls, sunlight streams through glass, people and appliances add warmth, and humid air carries hidden energy. The cooling load calculator runs all of this math for you, but understanding the formulas — even qualitatively — makes you a far better judge of the result. You will spot suspicious outputs, ask sharper questions of contractors, and see exactly which upgrades shrink your number.
This guide breaks the calculation into its four building blocks: sensible conduction load, solar gain, internal gains, and latent load — then shows how they combine into tons of refrigeration.
The Master Equation
At its simplest, the cooling load is a sum:
Total load (BTU/hr) = Envelope conduction + Solar gain + Internal gains + Latent load + Infiltration
Each term is computed separately, in BTU per hour, and the grand total is divided by 12,000 to get tons. In metric, multiply BTU/hr by 0.29307 to get watts, or multiply tons by 3.51685 to get kilowatts — the two paths must agree. Let us take each term apart.
1. Envelope Conduction: Q = U × A × ΔT
Heat flows through any building surface in proportion to its area, its thermal transmittance, and the temperature difference across it. The formula is:
Q = U × A × ΔT, where U is the assembly’s U-value (1 ÷ R-value) in BTU/(hr·ft²·°F), A is the area in square feet, and ΔT is the indoor/outdoor temperature difference in °F.
Example: a 1,000 sq ft wall with R-13 insulation (U ≈ 0.077) facing a 25°F temperature difference (100°F outside, 75°F inside) conducts 0.077 × 1,000 × 25 = 1,925 BTU/hr. Repeat for every wall, the ceiling, the floor or slab edge, and each door, then add them up. Because U-values shrink as insulation improves, this formula also shows precisely why upgrading from R-13 to R-21 walls cuts that wall’s contribution by about a third — a relationship our materials guide explores in practical terms.
2. Solar Gain Through Windows
Windows conduct heat like any surface, but they also admit solar radiation directly — and the solar part usually dwarfs the conduction part. The solar gain formula is:
Q_solar = A × SHGC × Solar irradiance × Orientation factor
SHGC (solar heat gain coefficient) is the fraction of incident sunlight that becomes indoor heat: 0.25 for good low-E glass, up to 0.70+ for clear single pane. Typical peak solar irradiance values and orientation effects look like this:
| Orientation | Peak irradiance (BTU/hr·ft²) | Relative gain |
|---|---|---|
| West | ~200–250 | Highest — low afternoon sun strikes glass nearly head-on |
| East | ~180–220 | High — morning sun, cooler outdoor air offsets slightly |
| South | ~120–160 | Moderate — high summer sun angle is easier to shade |
| North | ~40–60 | Lowest — diffuse skylight only |
Example: 60 sq ft of west-facing glass with SHGC 0.40 at 220 BTU/hr·ft² gives 60 × 0.40 × 220 = 5,280 BTU/hr — nearly half a ton from one wall of windows. Move that same glass to the north side and the gain drops below 1,500 BTU/hr. This is why measuring window area by orientation, as detailed in our estimating guide, is non-negotiable for an honest result, and why ignoring orientation is one of the costliest errors in our mistakes guide.
3. Internal Gains: People, Lights, Appliances
Everything inside the conditioned space that uses energy or metabolizes food becomes part of the cooling load. Standard allowances:
- People: ~400 BTU/hr sensible + ~200 BTU/hr latent per resting adult (about 600 BTU/hr total); a moderately active person adds roughly 50% more sensible heat.
- Lighting: wattage × 3.412 BTU/hr per watt. A room with 200 W of lighting adds ~680 BTU/hr.
- Appliances: nameplate wattage × 3.412 × usage factor. A refrigerator running a 40% duty cycle at 150 W contributes about 205 BTU/hr; cooking equipment is far larger but intermittent.
Manual J applies diversity and usage schedules rather than assuming everything runs at once — the oven is not on 24 hours a day. Still, in well-insulated modern homes, internal gains can rival the envelope load, which surprises many homeowners.
4. Latent Load: The Hidden Half of Humidity
Latent load is the energy required to condense moisture out of the air, and it comes from two sources: occupants (each person exhales and perspires roughly 200 BTU/hr worth of moisture) and humid outdoor air entering through infiltration or ventilation. The moisture formula is:
Q_latent = 0.68 × CFM × Δ grains of moisture
where CFM is the airflow of moist incoming air and Δ grains is the humidity ratio difference between outdoors and indoors (grains of water per pound of dry air, from psychrometric data). In a humid climate with leaky construction, latent load alone can exceed 5,000 BTU/hr — nearly half a ton of capacity that does nothing to lower the thermostat but everything to keep the house from feeling clammy. Undersized latent capacity is the technical reason oversized equipment feels worse: it cools the air quickly and shuts off before wringing out the moisture.
Deriving Tonnage: Putting It All Together
Add the five terms, and suppose the total is 42,000 BTU/hr for a 2,100 sq ft two-story home in a hot climate:
- Tons = 42,000 ÷ 12,000 = 3.5 tons
- Metric: 42,000 × 0.29307 = 12,309 W ≈ 12.3 kW
- Cross-check: 3.5 × 3.51685 = 12.31 kW ✓
Equipment is then selected per Manual S to meet — but not greatly exceed — this figure, with the sensible heat ratio (sensible ÷ total) matched to the equipment’s rated sensible capacity. A load that is 75% sensible needs equipment whose sensible capacity covers the sensible portion; nominal tonnage alone is not enough information, which is exactly the nuance a quality professional calculation provides and a rule of thumb never does.
Related Guides
- Complete Cooling Load Guide — the full overview with a worked 1,800 sq ft example.
- Estimating Guide — how to measure windows, insulation, and internal gains.
- Common Cooling Load Mistakes — where the math most often goes wrong.
- Cooling Load Calculation Checklist — every input, in order, so nothing is missed.
- Cooling Load FAQ — twelve detailed answers to the questions readers ask most.
Get Your Number
Formulas are satisfying, but nobody wants to hand-calculate every wall and window. Enter your measurements into the cooling load calculator and it will run the conduction, solar, internal, latent, and infiltration math for you — in BTU/hr, tons, and kW.
These formulas and examples are planning estimates for educational purposes. Real cooling loads depend on site-specific construction, shading, occupancy schedules, and climate data — verify your results with a licensed HVAC professional before purchasing equipment, and consult your local building authority for any code-related decisions.
