BTU Calculator
Calculate the heating or cooling capacity (BTU/hr) your space needs. Enter square footage or room dimensions, adjust for climate conditions, ceiling height, windows, and occupancy to get a practical starting point for HVAC sizing.
BTU Calculator
Calculate the heating or cooling capacity (BTU) needed for your space
Used for ceiling height adjustment only
Subtracts 600 BTU per person above 2
⚠️ Important Note: These calculations are based on normal conditions and provide estimates only. Every house and climate can result in different requirements. We recommend getting a proper inspection and professional assessment done for accurate sizing.
This BTU calculator uses proven contractor shortcuts to size heating and cooling capacity. It accounts for room dimensions, ceiling height, climate conditions, windows, occupancy, and kitchen loads so you can quote quickly and spot oversizing or undersizing before investing in equipment.
When Should You Use a BTU Calculator?
Use this calculator whenever you're scoping a heating or cooling job, creating a quick quote, or validating whether existing equipment is in the right range. It's perfect for early conversations with homeowners and small commercial clients.
Combine these BTU estimates with equipment specs, ductwork checks, and a professional load calculation (like Manual J for cooling or Manual S for heating) for final sizing. The goal is to avoid undersized systems that can't keep up and oversized systems that short cycle, waste energy, and struggle with comfort.
How Does This BTU Calculator Work?
For Heating: The calculator starts with a base formula of length × width × 35 BTU. It then adjusts for colder climates or shade (+10%), warmer climates or sunlight (-10%), ceilings higher than 8 feet (+10%), more than 2 windows or large windows/doors (+10%), and subtracts 600 BTU per person above 2 occupants.
For Cooling: The calculator uses length × width × 25 BTU as the base. It adjusts for lots of sunlight or warm weather (+10%), shaded or colder climates (-10%), ceilings higher than 8 feet (+10%), adds 600 BTU per person above 2 occupants, and adds 4000 BTU if the space is a kitchen.
The calculator supports both imperial (square feet) and metric (square meters) units. Results are shown in BTU/hr. For cooling, you can convert to tons by dividing BTU by 12,000. Always confirm with a full professional load calculation for final sizing.
💡 From Sizing to Closeable Proposals
Use these BTU estimates to anchor your recommendations, then present a professional estimate that spells out equipment options, SEER/SEER2 ratings, duct adjustments, and warranty details. Clear proposals help homeowners understand trade-offs and move forward faster.
Learn how to build professional contractor estimates →Dave helps you turn quick sizing into polished proposals—complete with photos, line items, and payment terms—so you can win jobs while staying confident in your recommendations.
Understanding BTU for Heating and Cooling
BTU/hr measures how much heat a system can add (heating) or remove (cooling) each hour. One cooling ton equals 12,000 BTU/hr. Knowing both units makes it easier to match equipment specs to what a space actually needs.
📋 Example Heating Calculation
20' × 25' room (4,500 cubic feet), 9' ceiling, average climate, 2 windows, 2 occupants:
Base: 20 × 25 × 35 = 17,500 BTU
Ceiling adjustment (9' > 8'): 17,500 × 1.1 = 19,250 BTU
Estimated need: ~19,250 BTU/hr. Final selection should consider ducts, insulation, windows, and climate.
📋 Example Cooling Calculation
20' × 25' room (500 sq ft), 9' ceiling, average sun, 3 occupants, kitchen:
Base: 20 × 25 × 25 = 12,500 BTU
Ceiling adjustment (9' > 8'): 12,500 × 1.1 = 13,750 BTU
Occupant adjustment: +600 BTU (one extra person) = 14,350 BTU
Kitchen adjustment: +4,000 BTU = 18,350 BTU
Estimated need: ~18,350 BTU/hr (about 1.5 tons). Final selection should consider ducts, insulation, windows, and climate.
Remember: rules of thumb are a starting point. Window area, insulation levels, infiltration, and equipment efficiency all influence comfort and energy use. Always get a professional assessment for accurate sizing.
Factors That Affect BTU Requirements
Use these factors to fine-tune your estimate and explain why your recommended size makes sense.
Insulation and Air Sealing
Better insulation and tight envelopes reduce heat gain, often lowering required BTU compared to older or leaky homes.
Windows and Orientation
Large west- or south-facing glass increases solar load. Shading, low-E glass, and exterior covers can reduce BTU needs.
Climate and Humidity
Hot, humid climates often require higher capacity and proper latent (moisture) control. Verify SEER/SEER2 and sensible heat ratios for local conditions.
Ductwork
Restrictive or leaky ducts waste capacity. Balancing, sealing, and proper sizing help the delivered BTU match the design target.
Occupancy and Appliances
More people, cooking, and electronics add heat. Adjust BTU upward for crowded spaces, kitchens, or home offices with equipment.
Frequently Asked Questions
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