Water heater sizing comes down to one question: can it deliver hot water to all the things running at the same time on your busiest morning? For tank heaters that's called the First Hour Rating (FHR). For tankless it's gallons per minute (GPM) at the temperature rise your inlet needs. Get either wrong and you run cold partway through showers. Here's the math, the standard sizes, and what changes by climate.

Tank vs tankless — the actual decision

Tank water heaters store 30-80 gallons at 120-140°F continuously and refill as you draw down. Cheaper upfront ($500-1,200 installed), shorter life (10-15 years), and they cycle the burner all day to maintain stored temperature — which costs money on the fuel bill.

Tankless water heatersheat water on demand as it flows through the unit. Higher upfront ($1,500-3,500 installed), longer life (20+ years), and only run when there's actual hot water draw — saving 20-30% on energy in moderate-use households. The constraint is flow rate: a unit rated 7 GPM handles one shower (2.5 GPM) plus a kitchen sink (1.5 GPM) simultaneously. Past that, the temperature drops.

Tankless wins for: small/medium households (1-4 people), tight mechanical rooms, vacation homes, gas-line upgrades. Tank wins for: large households (5+ people), heavy simultaneous use, cold-inlet climates that derate tankless GPM, and tight budgets.

Tank sizing — the FHR rule

First Hour Rating is the gallons of hot water a tank can deliver in one hour starting from a full tank. It depends on tank capacity, recovery rate (how fast the burner refills hot water), and burner BTU. The DOE Energy Saver guideline:

Household FHR target Tank size 1-2 people 45 gal 40 gal 3 people 60 gal 50 gal 4 people 70 gal 60 gal 5 people 80 gal 75 gal 6+ people 90 gal 80 gal

Notice tank size is smaller than FHR — that's because gas tanks recover ~30 gallons of hot water per hour while the burner runs, so the FHR exceeds the static capacity. Electric tanks recover slower (~20 gal/hr), so you size up by ~10 gallons for the same household.

The water heater sizing calculator handles both gas and electric assumptions.

Tankless sizing — GPM at temperature rise

Tankless capacity is rated as “X GPM at Y°F rise.” The rise is the difference between cold inlet temperature and desired output temperature. Standard hot water output is 110- 120°F at the fixture. Cold inlet varies by season and region:

  • Southern US, summer: 70°F inlet → 50°F rise to 120°F output
  • Northern US, winter: 40°F inlet → 80°F rise
  • Cold-climate well water, winter: 35°F inlet → 85°F rise

Manufacturers rate units at 70°F rise as the standard. A unit labeled 7 GPM at 70°F rise delivers only ~5 GPM at 90°F rise — which means a Boston winter starves a unit that worked fine in Houston. Size for your worst-case rise.

Required GPM by household:

  • 1-2 people: 5-6 GPM (one shower + sink)
  • 3-4 people: 7-8 GPM (one shower + dishwasher + sink)
  • 5+ people: 9-11 GPM (two showers simultaneously)

Worked example — 4-person family in Buffalo

Tank option: 60 gallon gas tank, FHR ~70 gal. Handles morning peak (two showers + dishwasher running back-to-back) without running out. Lifecycle cost over 12 years: ~$8,000 (equipment + gas).

Tankless option: 8 GPM tankless rated at 70°F rise. In Buffalo winter (40°F inlet, 80°F rise), effective output drops to ~7 GPM — still handles one shower (2.5) + dishwasher (1.0) + sink (1.5) = 5 GPM peak. Lifecycle cost over 20 years (longer service life): ~$11,000 (equipment + gas, 25% less than tank).

Tankless wins on lifecycle in this case — the longer service life and lower fuel use offset the higher upfront cost. In a 6-person household, the math flips because peak GPM exceeds what a single residential tankless can handle.

What pros do differently

Verify gas line capacity before tankless. A 199,000 BTU tankless on a long gas run from the meter may starve at peak fire — the line either needs upsizing or a dedicated 1-inch run from the meter. Quote includes line assessment.

Keep tank water heater drained low for safety. Set thermostat to 120°F not 140°F. Above 130°F is the legionella safety threshold but also a major scald risk — water at 140°F causes third-degree burns in 5 seconds. Anti- scald valves at fixtures are required by IRC R310.

Run a recirculation loop on long hot-water runs. If the bathroom is 50+ feet of pipe from the heater, a recirculation pump or on-demand recirc system saves the gallons that get wasted waiting for hot water. Pays for itself in under 3 years on water bills alone.

Quick FAQ

What size water heater for a family of 4? 60 gallon gas tank or 8 GPM tankless at 70°F rise. Adjust tankless up to 9-10 GPM if you're in a cold inlet region.

Can I run two showers on one tankless? Yes if the unit is rated 7+ GPM at your design rise. Two showers at 2.5 GPM each = 5 GPM, plus any other simultaneous use.

Heat pump water heaters — different math? Same FHR sizing as electric tank, with a recovery rate bonus when ambient air is warm (heat pumps pull heat from the mechanical room). In cold basements they switch to backup resistance and lose the efficiency advantage.

Need to size the supply line too? Use the water supply pipe size calculator to confirm your incoming water service can deliver the GPM your new heater needs.