Water Heater Sizing - BTU

Water Heater Sizing Calculator helps you calculate required BTU/hour capacity for any water heating application. Instantly determine heater size for gas (natural gas or propane), electric, or heat pump water heaters based on flow rate and temperature rise. Support efficiency percentages for fuel types: 80% for natural gas/propane, 95% for electric resistance, 250% COP for heat pump systems. Works 100% offline on the job site. Designed for sizing water heaters to meet simultaneous hot water demand.

FEATURES
• 4 Fuel Types: Natural gas (80% efficiency), propane (80% efficiency), electric resistance (95% efficiency), and heat pump systems (250% coefficient of performance)
• Temperature Input: Fahrenheit or Celsius selection for global compatibility
• Flow Rate Support: GPM (US gallons per minute) and L/min input options
• Temperature Rise Calculation: Automatic calculation from inlet and outlet temperatures
• kW Conversion: Automatic kilowatt conversion for electric heater sizing
• Simultaneous Demand: Account for multiple hot water draws simultaneously
• Code Note: Result shows the GPM x deltaT x 500 basis and the fuel efficiency applied

HOW IT WORKS
1. Select fuel type (natural gas, propane, electric, or heat pump)
2. Enter water flow rate (GPM) required by fixtures
3. Enter inlet and outlet temperatures or temperature rise
4. App calculates required output BTU/hour and the fuel-adjusted input BTU/hour
5. Export sizing recommendation to PDF for equipment selection

PERFECT FOR
• Plumbers and HVAC technicians sizing water heaters
• Plumbing contractors and estimators
• Mechanical engineers designing hot water systems
• Home builders selecting appropriate equipment
• Facilities managers replacing water heaters
• Plumbing inspectors verifying system design

OPTIONAL PRO UPGRADE — $2.99 ONE-TIME IN-APP PURCHASE
The following features require a separate one-time in-app purchase. They are not required to use the calculator:
• Unlimited saved calculation history
• PDF export of results
• Dark theme
• Ad-free experience

STANDARDS & COMPLIANCE
Standard BTU/h formula: BTU/h = GPM x Temperature Rise (°F) x 500 / Efficiency, Fuel efficiency ratings per manufacturer specifications (80% gas, 95% electric, 250% COP heat pump), Energy Factor (EF) calculations for comparing heater models, Standard residential hot water demand per ASHRAE guidelines.

PERFECT FOR
• Plumbers and HVAC technicians
• Mechanical engineers and estimators
• Plumbing contractors
• Home builders and renovators
• Facilities and building managers
• Plumbing inspectors

OFFLINE & PRIVACY
• Works 100% offline - calculate heater sizes anywhere
• No account required - instant access without signup
• No data collection - all calculations stored locally
• No internet dependency

Part of PlumbTools Suite - professional plumbing and HVAC calculator tools. Calculate water heater BTU requirements accurately, select right-sized equipment, and meet simultaneous hot water demand with precise heater sizing.

Recent changes:
Added a link to TradeTools — the one-app version of this calculator and 61 more, for HVAC, electrical, plumbing, fire protection, solar and carpentry. Nothing else changed.