Guides
Water heater sizing guides
How to size tank, tankless and heat pump water heaters, with worked numbers from the calculator and citations to the federal test procedure and codes.
- 01What size water heater do I need?Size a water heater by peak-hour demand, not by guesswork: how to count your busiest hour, read First Hour Rating, and size tankless by GPM at your rise.
- 02First Hour Rating explainedFirst Hour Rating is the hot water a full tank delivers in one hour, measured in a federal test with 58°F water. How to find it and use it to size a tank.
- 03Size by householdWorked water heater sizing for 2 to 6 people: the First Hour Rating and tankless GPM each busiest hour needs with cold, average and warm incoming water.
- 04Size by bathroomsMinimum First Hour Rating by bathrooms and bedrooms from Uniform Plumbing Code Table 501.1(2), and why bathroom count is only a rough stand-in for demand.
- 0540 vs 50 gallon40 or 50 gallon water heater? Compare real First Hour Ratings, which busiest hours each covers, and why a 40-gallon gas tank can beat a 50-gallon electric.
- 06Hot water per showerA typical 8-minute shower at 2.5 gpm draws about 15.7 gallons of hot water from a 120°F heater, not 20. Calculate your own with the hot-fraction formula.
- 07Incoming water temperatureHow to measure your incoming (mains) water temperature, why to size tankless on the coldest month, and estimated mains temperatures for U.S. cities.
- 08Tankless temperature riseTankless flow ratings are measured at a 67°F rise. Find your rise, convert a rated GPM to your conditions, and read a manufacturer's flow curve correctly.
- 09Whole-house tanklessTankless sizing for 1, 2 or 3 showers at once: GPM, BTU/hr, gas input and electric kW at cold, average and warm inlet water, and when one unit is not enough.
- 10Tank vs tankless sizingTank and tankless capacity are rated differently: gallons in the first hour vs. gallons per minute at a rise. How to compare them for the same household.
- 11Heat pump water heatersSize a heat pump water heater by First Hour Rating like any tank, then account for heat-pump-only mode, slower recovery and ENERGY STAR's advice to upsize.
- 12Sizing chartsWater heater sizing charts: First Hour Rating for common busiest hours and tankless flow at each temperature rise, every cell calculated from stated inputs.
Water Heater Sizing Calculator for iPhone
Take the calculator to the job site
The app runs the same sizing engine as this site, offline. Basic tank and tankless sizing is free. Lifetime Pro, a one-time $4.99 purchase, adds tank vs. tankless comparison, custom fixtures, saved jobs with notes and PDF reports you can send to a customer.
Coming soon to theApp Store
