BTU Requirement Calculator
Calculate BTU heating or cooling requirements for rooms based on size and insulation
Room Dimensions
Required BTU/hr
Room Area
Tonnage
BTU Breakdown
Equipment Size Recommendation
Step-by-Step Calculation
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, BTU/hr measures the heating or cooling capacity of equipment. A higher BTU rating means the system can heat or cool a larger space.
Cooling
- • 1 ton of cooling = 12,000 BTU/hr
- • Window AC: 5,000-25,000 BTU
- • Central AC: 24,000-60,000 BTU
- • Mini-split: 9,000-36,000 BTU
Heating
- • Gas furnace: 40,000-120,000 BTU
- • Space heater: 5,000-40,000 BTU
- • Fireplace insert: 25,000-80,000 BTU
- • Boiler: 50,000-200,000 BTU
Conversions
- • 1 BTU = 1,055 joules
- • 1 kW = 3,412 BTU/hr
- • 1 therm = 100,000 BTU
- • 1 ton = 12,000 BTU/hr
BTU Quick Reference by Room Size
These are general estimates for cooling with average insulation and moderate climate. Actual requirements vary based on insulation, climate, sun exposure, and other factors.
| Room Size (sq ft) | Cooling BTU | Heating BTU | AC Tonnage |
|---|---|---|---|
| 100 - 150 | 5,000 | 4,000 - 6,000 | 0.4 |
| 150 - 250 | 6,000 | 5,000 - 8,000 | 0.5 |
| 250 - 300 | 7,000 | 6,000 - 9,000 | 0.6 |
| 300 - 350 | 8,000 | 7,000 - 11,000 | 0.7 |
| 350 - 400 | 9,000 | 8,000 - 12,000 | 0.75 |
| 400 - 450 | 10,000 | 9,000 - 14,000 | 0.8 |
| 450 - 550 | 12,000 | 10,000 - 16,000 | 1.0 |
| 550 - 700 | 14,000 | 12,000 - 20,000 | 1.2 |
| 700 - 1,000 | 18,000 | 16,000 - 28,000 | 1.5 |
| 1,000 - 1,200 | 21,000 | 20,000 - 34,000 | 1.75 |
| 1,200 - 1,400 | 23,000 | 22,000 - 40,000 | 2.0 |
| 1,400 - 1,500 | 24,000 | 24,000 - 44,000 | 2.0 |
Factors That Affect BTU Requirements
Insulation Quality
Insulation is the single biggest factor in BTU requirements. Poor insulation can increase needs by 30-50%.
- • Poor: Old homes, no wall insulation, single-pane windows
- • Average: Standard fiberglass batts, double-pane windows
- • Good: Upgraded insulation, energy-efficient windows
- • Excellent: Spray foam, triple-pane, air-sealed envelope
Climate Zone
Your geographic location determines the temperature differential between indoor comfort and outdoor extremes.
- • Hot climates need more cooling capacity
- • Cold climates need significantly more heating BTUs
- • Heating typically requires 30-60 BTU/sq ft
- • Cooling typically requires 20-30 BTU/sq ft
Sun Exposure & Windows
Windows are a major source of heat gain (summer) and heat loss (winter).
- • South-facing rooms gain more solar heat
- • Each sq ft of window adds ~800 BTU cooling load
- • Shaded rooms need ~10% less cooling
- • Low-E windows reduce heat transfer by 30-50%
Occupancy & Heat Sources
People and appliances generate heat that adds to cooling load.
- • Each person adds ~600 BTU/hr
- • Kitchen appliances add 1,000-4,000 BTU
- • Computers and electronics add 200-500 BTU each
- • Lighting adds ~3.4 BTU per watt
HVAC Sizing Tips
- Don't oversize: An oversized AC cools quickly but short-cycles, leading to poor humidity control, higher energy bills, and premature wear.
- Don't undersize: An undersized unit runs constantly, struggles to reach set temperature, and increases energy costs.
- Right-size is key: Aim for equipment rated within 10-15% of the calculated load for optimal performance and efficiency.
- Consider SEER ratings: Higher SEER (Seasonal Energy Efficiency Ratio) means lower operating costs. Minimum is 14 SEER; high-efficiency units reach 20+ SEER.
- Professional Manual J: For whole-home systems, a Manual J load calculation by an HVAC professional accounts for every factor and is the gold standard for sizing.
- Ductwork matters: Leaky or poorly insulated ducts can waste 20-30% of conditioned air, effectively increasing your BTU needs.
References
The BTU calculation methods used are based on industry-standard guidelines:
Related Calculators
Note: This calculator provides estimates based on simplified HVAC load calculations. Results are suitable for general planning and equipment comparison. For precise sizing of whole-home systems, a professional Manual J load calculation is recommended, which accounts for exact construction details, local design temperatures, ductwork layout, and building orientation.
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