Tonnage isn’t weight
“1 ton” AC doesn’t weigh a ton—it moves 12,000 BTU/h, the cooling needed to melt one ton of ice in 24 hours.
Ice origin
HVAC BTU Calculator – Cooling & Heating Load, AC Tonnage
Calculator
Room or combined area to be cooled.
Default rules assume 8 ft. We’ll scale for your height.
Affects solar heat gain.
Simplified envelope quality.
We add ~600 BTU/h per person above 2.
Larger glazed area increases load.
Kitchens add a lump sum; equipment adds per-watt heat.
Cooling results will appear here.
Area to be heated.
We scale to 8 ft baseline.
Used to scale the rule-of-thumb BTU/ft².
Envelope quality multiplier.
Air leakage affects heating load.
Used lightly in scaling; this is not a Manual J.
Heating results will appear here.
Use cooling BTU/h for AC sizing.
Add capacity buffer for hot spells/growth.
AC sizing will appear here.
How the HVAC BTU Calculator Works
Cooling load starts from a simple rule of thumb around 20 BTU/h per ft² at 8 ft ceilings. We scale for ceiling height and apply gentle multipliers for sun exposure, insulation/tightness, window area, people, and internal gains (kitchens, equipment).
Heating load uses typical ranges (≈25–60 BTU/h per ft²) depending on climate, insulation, and infiltration, then scales for ceiling height. This is an educational estimate—not a substitute for a Manual J heat-loss calculation.
AC tonnage converts BTU/h to tons: \(1~\text{ton} = 12{,}000~\text{BTU/h}\), then applies optional headroom for sizing.
Formulas (simplified)
- Cooling baseline: \( Q_\text{cool} \approx 20~\frac{\text{BTU}}{\text{h·ft}^2} \times A \times \frac{H}{8~\text{ft}} \times M \) where \(M\) is the product of adjustment multipliers.
- Heating baseline: \( Q_\text{heat} \approx R_\text{ft²} \times A \times \frac{H}{8~\text{ft}} \times M \), with \(R_\text{ft²}\) chosen from climate/insulation presets.
- Watts to BTU/h: \( 1~\text{W} \approx 3.412~\text{BTU/h} \).
- Tonnage: \( \text{tons} = \dfrac{\text{BTU/h}}{12{,}000} \).
Results are approximations for quick planning. For equipment selection or design, consult a professional for a Manual J and local code requirements.
5 Fun Facts about HVAC BTU & Tonnage
People are little heaters
A person at rest adds ~600 BTU/h (≈175 W). A room full of guests can feel like a small space heater.
Human load
Sun vs shade swings load
South/west sun can boost cooling load by 10–25% compared with shade. Window orientation matters more than wall colour.
Solar gain
Right-size beats oversize
Oversized ACs short-cycle, wear faster, and leave rooms clammy because humidity isn’t removed. A modest 10–20% buffer is plenty.
Goldilocks sizing
Watts become BTU
Every indoor watt turns into heat. A 1,000 W PC is ~3,412 BTU/h—like adding a small heater to the room.
Hidden heat
Understanding HVAC BTU, Cooling & Heating Loads
When people talk about “how big” an air conditioner or heater needs to be, they’re really asking about load—the rate at which a room or home gains heat in summer (cooling load) or loses heat in winter (heating load). Load is commonly expressed in BTU per hour (BTU/h) or kilowatts (kW). Our calculator uses simple, well-known heuristics to give a fast, educational estimate. It is not a substitute for a professional Manual J (residential) or heat-loss analysis, but it’s perfect for planning, sense-checking quotes, and comparing options.
Key Terms in Plain English
- BTU/h (British Thermal Units per hour): A measure of heating or cooling capacity. Higher BTU/h means more capacity.
- Ton of refrigeration: 1 ton = 12,000 BTU/h. A 2-ton AC unit is ~24,000 BTU/h.
- Rule-of-thumb baselines: Cooling often starts near 20 BTU/h per ft² at 8 ft ceilings; heating spans about 25–60 BTU/h per ft² depending on climate and envelope quality.
- Envelope: The walls, roof, windows, doors, and air-tightness that separate indoors from outdoors. Better envelopes lower required BTU/h.
What Drives Cooling Load?
Cooling is affected by solar gain (sun hitting walls and glass), internal gains (people, lights, appliances), and ventilation/infiltration (hot air sneaking in). Our calculator starts with area and ceiling height, then applies gentle multipliers for:
- Sun exposure: South- and west-facing glass increases load; shaded rooms need less.
- Insulation & tightness: Good construction tampers down heat flow and drafts.
- Window area: More glass ≈ more solar and conductive gain.
- People & equipment: Humans give off heat (~600 BTU/h per extra person above two in a typical room). Electronics and appliances add wattage that converts directly to heat (1 W ≈ 3.412 BTU/h).
- Kitchens: Ovens, hobs, and fridges add a notable bump, so we include a simple lump sum.
What Drives Heating Load?
Heating depends on how quickly the building loses heat to the outside, which is set by climate (how cold it gets), insulation levels, window performance and area, and air leakage. We provide climate presets so you can pick what feels closest to your location (mild/coastal, cool, cold, very cold). We also scale for ceiling height and apply multipliers for insulation and infiltration (leaky homes need more heat).
AC Tonnage & Sizing Headroom
Once you estimate a cooling load, sizing an air conditioner is straightforward: divide BTU/h by 12,000 to get tons. We also show a headroom option, which adds a small capacity buffer for hot spells, internal-gain spikes, or future changes. Avoid large oversizing: it can cause short cycling, poor humidity control, and unnecessary cost. A modest buffer (e.g., 10–20%) is often enough.
Metric vs Imperial
Prefer metric? Use m² and metres in the inputs. The core relationships are the same: 1 kW ≈ 3,412 BTU/h. The tool converts between units and reports clearly so you can compare like-for-like.
When You Need More Than a Rule of Thumb
These estimates are designed for education and early planning. For new builds, major renovations, ductwork changes, heat pumps in cold climates, or code/permitting, consider a professional Manual J or detailed heat-loss model. That process accounts for exact window U-values, shading geometry, ventilation rates, moisture (latent) loads, distribution losses, and local weather data.
Quick Tips
- Reduce load first: shading, air sealing, insulation, and efficient windows can downsize equipment and cut bills.
- Think humidity: in humid regions, proper dehumidification matters as much as temperature control.
- Check the whole system: duct sizing, airflow, and setpoints impact comfort as much as BTU/h.
This calculator provides engineering approximations for educational purposes only. Always verify critical decisions with a qualified HVAC professional.