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Tool-first calculator

Water heater sizing: tank gallons or tankless GPM.

Pick a household profile to estimate the right tank capacity or tankless flow rate. Real sizing depends on incoming water temperature, fixture count, and simultaneous use, but this gets you in the right ballpark before a plumber quotes a replacement.

Independent FatBook v3 cost indexVerified permit/source data where availableReviewed by Leonard "Chuck" Thompson
Recommended size
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Local installed cost

What an installer charges for your scope.

The numbers above are material counts and sizing. Below shows what a contractor typically charges in your city for an installed version of this scope, scaled from FatBook's cost index of BLS wages + Craftsman labor hours + permits + materials.

Pick the cost-index service that matches the scope you're scaling. The default is the trade's headline service.
National Average
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How to use this result
  • Tank size in gallons is one number to match. First-hour rating (FHR) is the more honest one. Confirm both on the model spec sheet.
  • Tankless GPM is the peak flow. Cold climates require a higher GPM unit than warm climates for the same household.
  • Electric tank water heaters with the same gallon rating recover slower than gas. If two showers run back-to-back, size up one tier.
Cost-index version: 2026.Q2. Read the FatBook methodology, or use the bid and true cost calculators for a water heater install bid check.
Data Sources Used On This Page
  • Independent FatBook v3 cost index for water heater calculator for Atlanta.
  • BLS OEWS wage inputs and FRED PPI material inflation references.
  • Craftsman labor-hour references and contractor overhead benchmarks.
  • Verified permit/source data from PermitCalculator.com and permits_compiled where available.
Cost-index version: 2026.Q2
Updated: May 2026
Sources: BLS, Craftsman, FRED
Reviewed by: Leonard "Chuck" Thompson
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Cost index built by David Olson, Founder of TheFatBook · Methodology reviewed by Leonard "Chuck" Thompson, LC Thompson Construction Co., Owner (retired) · 2026.Q1