British Thermal Unit (BTU) - Unit Information & Conversion

Quick Answer

1 BTU = 1,055.06 joules ≈ 0.293 Wh

The energy to heat 1 pound of water by 1°F. Standard in US for AC capacity (12,000 BTU/h = 1 ton), furnaces, natural gas billing, and energy content ratings.

Quick Comparison Table

Application	BTU Value	Metric Equivalent	Description
Small room AC	5,000 BTU/h	1.46 kW	150-250 sq ft
Medium room AC	10,000 BTU/h	2.93 kW	400-450 sq ft
1 ton AC	12,000 BTU/h	3.52 kW	Standard unit
Residential furnace	60,000-100,000 BTU/h	17.6-29.3 kW	Typical home
Natural gas therm	100,000 BTU	105.5 MJ	Billing unit
Gallon of gasoline	114,000 BTU	120 MJ	Energy content
Cubic foot nat. gas	1,030 BTU	1.09 MJ	Average heat value
Food calorie	3.97 BTU	4.184 kJ	Dietary energy

Note: BTU/h (BTU per hour) measures power (rate of energy), while BTU alone measures total energy.

Definition

The British Thermal Unit (symbol: BTU or Btu) is a unit of energy defined as the amount of heat required to raise the temperature of exactly one pound-mass of water by one degree Fahrenheit, measured at or near water's maximum density temperature (approximately 39.2°F or 4°C).

Official Definition (IT BTU)

The International Table BTU (IT BTU) is the standard BTU used in modern applications:

1 BTU (IT) = 1,055.05585262 joules (exactly)

This definition, established through international steam table conventions, provides an exact conversion to SI units.

Historical BTU Variants

Historically, several BTU definitions existed based on different water temperature ranges:

• 39°F BTU: Using water at its maximum density point (39.2°F) = 1,059.67 J
• 60°F BTU: Using water at 60°F (common in British engineering) = 1,054.68 J
• Mean BTU: Average over range 32-212°F = 1,055.87 J
• Thermochemical BTU: Based on thermochemical calorie = 1,054.35 J
• IT BTU (International Table): Modern standard = 1,055.05585262 J (exact)

Modern Usage: The IT BTU is now universal in US engineering, HVAC, and energy applications unless otherwise specified.

BTU vs. BTU/h (Power vs. Energy)

Critical Distinction:

• BTU = unit of ENERGY (total heat)
• BTU/h (BTU per hour) = unit of POWER (rate of energy transfer)

Example:

• A furnace rated at 80,000 BTU/h delivers 80,000 BTU of heat per hour
• Running for 3 hours delivers 240,000 BTU total energy
• This is analogous to watts (power) vs. watt-hours (energy)

Common Notation:

• BTU/h, BTU/hr, BTUh, Btu/h, or BTUH all mean BTU per hour (power)
• HVAC equipment ratings almost always use BTU/h despite sometimes omitting "/h"

Relationship to Other Units

Energy:

• 1 BTU = 1,055.06 joules (J)
• 1 BTU = 1.05506 kilojoules (kJ)
• 1 BTU = 0.293071 watt-hours (Wh)
• 1 BTU = 0.000293071 kilowatt-hours (kWh)
• 1 BTU = 0.251996 kilocalories (kcal)
• 1 BTU = 251.996 calories (thermochemical)

Power (BTU/h to watts):

• 1 BTU/h = 0.293071 watts (W)
• 1,000 BTU/h = 293.071 W ≈ 0.293 kW
• 12,000 BTU/h = 3,516.85 W ≈ 3.52 kW (1 ton AC)

History

The BTU's 170-year history reflects the evolution of thermodynamics, industrial engineering, and American energy infrastructure.

Industrial Revolution Origins (1840s-1860s)

The Problem: Mid-19th century British engineers needed standardized measurements to:

• Compare coal qualities and heating values
• Calculate steam engine efficiency
• Design boilers and heating systems
• Establish fuel pricing based on energy content

Imperial Unit Framework: Early thermodynamics (Joule, Kelvin, Rankine) used British imperial units:

• Pounds (mass)
• Fahrenheit (temperature)
• Foot-pounds (mechanical work)

BTU Development (1850s): The "British thermal unit" emerged as the heat equivalent to raising one pound of water by one degree Fahrenheit, providing an intuitive unit for British engineers familiar with imperial measurements.

Early Usage:

• Coal industry: BTU per pound for fuel quality
• Steam power: BTU input vs. mechanical work output (efficiency)
• Industrial heating: Boiler and furnace capacity ratings

American Adoption (1870s-1920s)

Late 19th Century: American industrial expansion embraced the BTU for:

• Railroad locomotive coal consumption
• Factory steam heating systems
• Urban gas lighting and heating (coal gas)
• Refrigeration industry development

Refrigeration "Ton" (1880s-1900s): The refrigeration industry defined cooling capacity based on ice production:

• 1 ton of refrigeration = melting 1 ton (2,000 lbs) of ice in 24 hours
• Ice melting heat: 144 BTU/lb × 2,000 lbs = 288,000 BTU per day
• Rate: 288,000 BTU ÷ 24 hours = 12,000 BTU/h
• This became the standard unit for air conditioning capacity

Natural Gas Industry (1900s-1920s): As natural gas distribution networks developed, utilities billed customers by BTU content:

• Gas heat value: approximately 1,000 BTU per cubic foot
• Therm (1920s): Defined as 100,000 BTU for convenient billing
• Still used in US natural gas bills today

Standardization (1920s-1960s)

International Steam Tables (1929-1956): International conferences standardized thermodynamic properties of water and steam, establishing:

• IT calorie (International Table calorie) = 4.1868 joules (exact)
• IT BTU = 1,055.05585262 joules (derived from IT calorie)

Post-WWII HVAC Boom (1940s-1960s): Residential air conditioning explosion in the United States:

• Room air conditioners rated in BTU/h (5,000, 8,000, 10,000, 12,000 BTU/h)
• Central AC systems in tons (1-ton, 2-ton, 3-ton, 5-ton)
• Furnaces and boilers rated in BTU/h input/output

UK/Commonwealth Metrication (1960s-1980s): Britain and Commonwealth nations transitioned to SI units:

• Joules (J) replaced BTU for energy
• Watts (W) replaced BTU/h for power
• Celsius replaced Fahrenheit

Modern Era: US Retention (1970s-Present)

1975 Energy Policy and Conservation Act: Mandated energy efficiency labeling for appliances using BTU-based metrics:

• EER (Energy Efficiency Ratio) = BTU/h output ÷ watts input
• AFUE (Annual Fuel Utilization Efficiency) = BTU output ÷ BTU input
• SEER (Seasonal Energy Efficiency Ratio) = BTU cooling ÷ watt-hours seasonal

Persistence in US Markets: Despite federal metrication efforts (1975 Metric Conversion Act), BTU remained dominant:

• HVAC contractors trained in BTU/h calculations
• Building codes specify BTU/h capacities
• Consumer familiarity ("12,000 BTU air conditioner")
• Industry infrastructure built around BTU standards

Current Status (2020s):

• US HVAC industry: 100% BTU-based
• Natural gas utilities: Billing in therms (100,000 BTU)
• Appliance ratings: Water heaters, furnaces, stoves in BTU/h
• Energy statistics: DOE reports in "quads" (quadrillion BTU)

While engineers often convert to SI units for calculations, BTU remains the commercial and consumer-facing standard in American energy markets.

Real-World Examples

HVAC and Air Conditioning

Room Air Conditioners:

• 5,000 BTU/h: Small room (150 sq ft), bedroom
• 8,000 BTU/h: Medium room (350 sq ft), office
• 10,000 BTU/h: Large room (450 sq ft), living room
• 12,000 BTU/h: Extra-large room (550 sq ft), or "1 ton" unit
• 18,000 BTU/h: Multiple rooms or open space (1.5 tons)
• 24,000 BTU/h: Large open space (2 tons)

Central Air Conditioning:

• 1.5-ton (18,000 BTU/h): Small house (900-1,200 sq ft)
• 2-ton (24,000 BTU/h): Medium house (1,200-1,500 sq ft)
• 3-ton (36,000 BTU/h): Large house (1,500-2,100 sq ft)
• 4-ton (48,000 BTU/h): Very large house (2,100-2,400 sq ft)
• 5-ton (60,000 BTU/h): Large home or commercial space (2,400-3,000 sq ft)

Heating Systems:

• Residential furnace: 60,000-100,000 BTU/h (typical home)
• Boiler (small home): 50,000-80,000 BTU/h
• Commercial HVAC rooftop unit: 300,000-600,000 BTU/h

Appliances and Equipment

Water Heaters:

• Electric tank (40-50 gal): 18,000-22,000 BTU/h recovery
• Gas tank (40-50 gal): 30,000-40,000 BTU/h burner
• Tankless gas: 120,000-200,000 BTU/h (high-demand)

Kitchen Appliances:

• Residential stove burner: 5,000-12,000 BTU/h (varies by burner size)
• Professional-style range burner: 15,000-25,000 BTU/h (high-output)
• Grill burner: 10,000-15,000 BTU/h per burner
• Oven: 16,000-18,000 BTU/h

Portable Heaters:

• Small space heater: 4,000-6,000 BTU/h
• Medium space heater: 8,000-10,000 BTU/h
• Propane patio heater: 40,000-50,000 BTU/h

Fuel Energy Content

Natural Gas:

• 1 cubic foot (cf): ~1,030 BTU (varies by composition)
• 1 therm (billing unit): 100,000 BTU = ~97 cubic feet
• 1 MCF (thousand cubic feet): ~1,030,000 BTU

Propane (LPG):

• 1 gallon liquid propane: 91,500 BTU
• 1 cubic foot propane gas: 2,516 BTU
• 100-lb tank (23.6 gallons): 2,159,400 BTU

Petroleum Products:

• 1 gallon gasoline: 114,000-116,000 BTU
• 1 gallon diesel: 128,000-130,000 BTU
• 1 gallon heating oil: 138,500 BTU

Coal and Solid Fuels:

• Anthracite coal: 12,500-13,000 BTU/lb
• Bituminous coal: 10,500-14,000 BTU/lb
• Hardwood (air-dried): 20-30 million BTU per cord

Electricity:

• 1 kWh = 3,412.14 BTU (thermal equivalent)
• Note: Electric heating is ~100% efficient at point of use, but power plant generation is ~35-40% efficient

Human and Dietary Energy

Food Energy:

• 1 food Calorie (kcal) = 3.966 BTU
• 2,000 Calorie daily diet = 7,932 BTU per day
• Average person's daily energy ≈ 94 watts continuous (average power)

Human Body Heat Output:

• Resting: 250-300 BTU/h (sitting, sleeping)
• Light activity: 400-500 BTU/h (office work)
• Moderate activity: 600-800 BTU/h (walking, housework)
• Heavy activity: 1,000-1,500 BTU/h (construction, sports)

Common Uses

HVAC System Sizing and Selection

HVAC professionals calculate heating/cooling needs in BTU/h:

Cooling Load Calculation:

• Square footage: 20-25 BTU/h per sq ft (rule of thumb)
• Heat gain factors: Sun exposure, insulation, occupancy, appliances
• Room-by-room Manual J calculations (detailed engineering method)

Example: 1,500 sq ft home

• Basic: 1,500 × 20 = 30,000 BTU/h minimum (2.5 tons)
• With factors: 36,000 BTU/h recommended (3 tons)

Heating Load:

• Climate zone based (heating degree days)
• Insulation R-values
• Air infiltration rates
• Manual J detailed calculations

Natural Gas Billing and Pricing

US natural gas utilities bill in therms (100,000 BTU):

Typical Bill:

• Winter heating month: 150-300 therms (15-30 million BTU)
• Summer low usage: 10-30 therms (1-3 million BTU, cooking/water heating)

Pricing:

• Residential: $0.80-$2.00 per therm (varies by region, season)
• Commercial: $0.50-$1.50 per therm
• Industrial: $0.30-$1.00 per therm (bulk rates)

Appliance Energy Efficiency Ratings

EER (Energy Efficiency Ratio):

• Formula: EER = Cooling capacity (BTU/h) ÷ Power input (watts)
• Example: 12,000 BTU/h AC using 1,200 watts = EER 10.0
• Higher EER = more efficient
• Typical range: 8-13 for room ACs, 10-16 for central AC

SEER (Seasonal Energy Efficiency Ratio):

• Seasonal average efficiency = BTU cooling ÷ watt-hours consumed
• Minimum US standard: SEER 14 (as of 2023 for northern states)
• High-efficiency units: SEER 18-25
• Calculation accounts for varying outdoor temperatures

AFUE (Annual Fuel Utilization Efficiency):

• Percentage of fuel BTU converted to useful heat
• Minimum US standard: 80% AFUE for gas furnaces (mid-efficiency)
• High-efficiency: 90-98% AFUE (condensing furnaces)
• Example: 100,000 BTU/h input, 95% AFUE = 95,000 BTU/h heat output

Energy Content and Fuel Comparison

Industries compare fuels by BTU per unit:

Cost per Million BTU:

• Natural gas at $1.00/therm = $10.00 per million BTU
• Propane at $2.50/gallon = $27.32 per million BTU
• Heating oil at $3.50/gallon = $25.27 per million BTU
• Electricity at $0.12/kWh = $35.16 per million BTU

This allows consumers to compare heating costs across fuel types.

National and Industrial Energy Statistics

US Department of Energy uses "quad" (quadrillion BTU) for national statistics:

2022 US Energy Consumption (approximate):

• Total: 100 quads (100 × 10¹⁵ BTU)
• Petroleum: 36 quads
• Natural gas: 33 quads
• Coal: 10 quads
• Nuclear: 8 quads
• Renewables: 13 quads

Building Codes and Regulations

US building codes specify BTU requirements:

• Minimum ventilation: CFM (cubic feet per minute) and BTU/h heat loss
• Furnace sizing: BTU/h capacity requirements by climate zone
• Water heater recovery: Gallons per hour at ΔT or BTU/h rating

Conversion Guide

BTU to SI Energy Units

BTU to Joules:

• 1 BTU = 1,055.06 J = 1.05506 kJ
• Example: 50,000 BTU = 52.75 MJ

BTU to Kilowatt-Hours:

• 1 BTU = 0.000293071 kWh
• 1 kWh = 3,412.14 BTU
• Example: 100,000 BTU = 29.3 kWh

BTU to Calories:

• 1 BTU = 252.0 calories (thermochemical)
• 1 BTU = 0.252 kilocalories (food Calories)
• Example: 2,000 kcal diet = 7,936 BTU

Power Conversions (BTU/h to Watts)

BTU/h to Watts:

• 1 BTU/h = 0.293071 W
• 1,000 BTU/h ≈ 293 W
• 3,412 BTU/h = 1 kW (approximately)

Common AC Conversions:

• 12,000 BTU/h = 3.517 kW (1 ton)
• 18,000 BTU/h = 5.275 kW (1.5 tons)
• 24,000 BTU/h = 7.034 kW (2 tons)
• 36,000 BTU/h = 10.551 kW (3 tons)

Fuel Energy Conversions

Natural Gas:

• 1 therm = 100,000 BTU = 29.3 kWh = 105.5 MJ
• 1 CCF (hundred cubic feet) ≈ 103,000 BTU ≈ 1 therm

Propane:

• 1 gallon = 91,500 BTU = 26.8 kWh
• 1 pound = 21,548 BTU

Electricity:

• 1 kWh = 3,412 BTU (thermal equivalent)
• Note: Does not account for generation losses

HVAC "Ton" Conversions

Refrigeration Ton:

• 1 ton = 12,000 BTU/h = 3.517 kW = 3,024 kcal/h
• 0.5 ton = 6,000 BTU/h
• 1.5 tons = 18,000 BTU/h
• 2 tons = 24,000 BTU/h
• 3 tons = 36,000 BTU/h
• 4 tons = 48,000 BTU/h
• 5 tons = 60,000 BTU/h

Common Conversion Mistakes

1. Confusing BTU with BTU/h

Wrong: Using BTU when BTU/h is needed

• "My AC is 12,000 BTU" (missing the rate)
• Technically incorrect, though colloquially understood

Right: Specify BTU/h for power ratings

• "My AC is 12,000 BTU/h" (cooling power)
• Or: "My AC is 1 ton" (equivalent cooling capacity)

2. Incorrect Square Footage Calculations

Wrong: Using oversimplified 1 ton per 500 sq ft rule

• Ignores climate, insulation, sun exposure
• Can lead to undersized or oversized systems

Right: Use proper load calculations

• Manual J calculations account for all factors
• Climate zone, R-values, windows, occupancy
• Typical: 400-600 sq ft per ton depending on conditions

3. Forgetting Electricity-to-BTU Is Thermal Equivalent Only

Wrong: Assuming 3,412 BTU/kWh for all comparisons

• Valid for heating comparison (electric vs. gas)
• NOT valid for comparing AC efficiency to furnace efficiency

Right: Use appropriate efficiency metrics

• Electric heat: ~100% at point of use (but power plant ~35% efficient)
• Gas furnace: 80-98% AFUE
• Heat pump: 200-400% effective (COP 2-4, moves heat rather than generates)

4. Mixing Up therm and MCF

Wrong: Treating 1 therm = 1 MCF (thousand cubic feet)

• 1 MCF ≈ 1,030,000 BTU (varies by gas composition)
• 1 therm = 100,000 BTU (exact)

Right:

• 1 MCF ≈ 10.3 therms (approximately)
• Check utility bill for exact heat content (BTU per cubic foot)

5. Using Wrong BTU Definition

Wrong: Mixing historical BTU variants

• 39°F BTU (1,059.67 J) ≠ IT BTU (1,055.06 J)
• ~0.4% error, significant in precision work

Right: Use IT BTU (International Table) unless historical context requires otherwise

• Modern HVAC, energy calculations: IT BTU
• Legacy documents: Check which definition was used