Water Heater Sizing Calculator

What size water heater do I need? Get a clear answer for tank and tankless water heaters in seconds, before you buy, quote or install. Water Heater Sizing Calculator turns real hot-water use into the numbers that matter: the minimum First Hour Rating (FHR) for a tank, and the flow in GPM at your temperature rise for a tankless unit. Basic sizing is free, with no account, no ads and no subscription. Built for plumbers, HVAC technicians, contractors, builders, property managers and homeowners who want a better answer than guessing by tank size. TANK WATER HEATER SIZING • Build your busiest hour from showers, faucets, tubs, dishwashers, clothes washers and more • See peak-hour hot-water demand in gallons • Get the minimum First Hour Rating to look for on the EnergyGuide label or spec sheet Two 50-gallon tanks can deliver very different amounts of hot water in an hour. First Hour Rating includes recovery, so it is a better guide than tank capacity alone. TANKLESS WATER HEATER SIZING • Choose the fixtures that may run at the same time • Enter your incoming water and outlet temperatures • See the flow you need in GPM at your temperature rise • See the heat load in BTU/hr • Get the performance point to check on the manufacturer's flow curve Advertised maximum GPM is measured at a 67°F rise. Where incoming water is colder, the rise is higher and a tankless unit delivers less flow. The app shows the flow you need at your actual rise. HOW IT WORKS 1. Pick fixtures from the built-in library 2. Enter your incoming water and heater temperatures 3. Read your result, with a plain-language explanation LIFETIME PRO: ONE PURCHASE A one-time upgrade for the professional workflow: • Compare tank vs. tankless for the same job • Add custom fixtures and edit flow, duration and use temperature • Adjust design margin and heater efficiency • Estimate gas input in BTU/hr or electric input in kW • Check a specific water heater against the requirement • See every step of the calculation • Save jobs with notes, then reopen, edit and duplicate them • Create PDF reports for customers, inspectors or your files Lifetime Pro is not a subscription. Buy once, and restore it on any iPhone signed in with the same Apple Account. BUILT FOR THE JOB SITE • Works offline • Large, easy-to-tap controls • Dark mode and larger text support • Your data stays on your iPhone: no account, no ads, no tracking TRANSPARENT METHOD Every formula, default and assumption is explained in the app. The method draws on U.S. Department of Energy test procedures, federal fixture flow limits, EPA WaterSense and ASHRAE guidance. IMPORTANT Water Heater Sizing Calculator is an estimating and planning tool. Always confirm your final choice against manufacturer specifications, installation instructions, applicable codes and local requirements. Performance varies by model, fuel type, temperature rise and installation. Terms of Use: https://gabrielwpetersen.github.io/water-heater-sizing-calculator/terms.html Privacy Policy URL: https://gabrielwpetersen.github.io/water-heater-sizing-calculator/privacy.html