Tankless Water Heater Sizing Calculator

Size a tankless (on-demand) water heater based on simultaneous fixture usage and desired temperature rise. Compare unit sizes from 5 to 12 GPM across natural gas, propane, and electric fuel types with estimated annual operating costs.

Fuel Type

Simultaneous Fixtures

Flow Rate per Fixture (GPM)

Incoming Water Temp (°F)

Desired Output Temp (°F)

Pricing (edit to match local rates)

Small 5 GPM ($)

Medium 8 GPM ($)

Large 10 GPM ($)

Whole-House 12 GPM ($)

Natural Gas ($/therm)

Required Flow Rate

5.0 GPM

2 fixtures × 2.5 GPM

Temperature Rise

70°F

50°F → 120°F

Required BTU/hr

175,000

Natural gas burner

Est. Annual Fuel Cost

$934.38

First-year total: $1,734.38

Unit Size	Capacity	First-Year Cost
Small (5 GPM)Best value	Handles 5 GPM	$1,734.38
Medium (8 GPM)	Handles 8 GPM	$2,134.38
Large (10 GPM)	Handles 10 GPM	$2,534.38
Whole-House (12 GPM)	Handles 12 GPM	$3,134.38

Recommended: Small (5 GPM) at $1,734.38 first-year costBased on a required flow rate of 5.0 GPM with a 70°F temperature rise, the small (5 gpm) unit is the most cost-effective option that meets your demand. Annual operating cost is estimated at $934.38 for natural gas.

Small Unit: Limited Simultaneous UseA 5 GPM unit handles your current calculated demand but leaves little margin for additional fixtures running at the same time. If household usage patterns change, you may experience temperature drops. Consider a medium unit for more flexibility.

This is an estimate for planning purposes. Actual tankless heater performance depends on incoming water temperature, altitude, gas line sizing, electrical capacity, and installation factors. Flow rates vary by fixture type and manufacturer. Consult a licensed plumber for a site-specific recommendation.