Tankless Water Heater Sizing Guide & Calculator

Use the DOE GPM approach. Sum the GPM of all fixtures you might run simultaneously during peak use (typically 2 morning showers + kitchen sink = 5–7 GPM), then add a 1 GPM buffer. For a 4-person household, plan for 7–9 GPM (gas) or 4–5 GPM (electric, with flow restrictors). Use our tankless calculator.

A family of 4 with average use (2 showers + kitchen + laundry) needs a 7–9 GPM gas tankless (180,000–199,000 BTU) or a 4–5 GPM electric tankless (27–36 kW). For cold climates (inlet water below 50°F), add 20–30% to the GPM target because the heater has to lift water temperature more.

Gas tankless heats faster (180k–199k BTU typical, 8–11 GPM), lower operating cost ($20–40/month), requires gas line + venting. Best for whole-house 3+ person households. Electric tankless (18–36 kW, 4–8 GPM), lower upfront cost ($500–1,500), no venting needed, but slower recovery. Best for point-of-use or small households in warm climates.

A gas tankless costs $20–40/month to operate (natural gas), an electric tankless $40–80/month. Tankless saves 24–34% on water heating vs a standard tank because there's no standby heat loss. Annual savings vs tank: $100–200. Payback on the higher upfront cost: 6–10 years. Lifespan: 20+ years vs 10–15 for tank.

Tankless water heaters last 20–25 years with proper maintenance, vs 10–15 years for tanks. Hard water shortens lifespan by 30–50% — annual descaling (vinegar flush) extends life significantly. Look for error codes, reduced flow, or fluctuating temperature as failure signs.

Yes, but requires verification: (1) gas line size — many homes have 1/2" lines that need upsizing to 3/4" for high-BTU gas tankless; (2) electrical — electric tankless needs 2–4 × 40A 240V breakers (most homes need panel upgrade); (3) venting — direct-vent or power-vent required for gas; (4) condensate drain for high-efficiency units. Budget $1,500–3,500 installed for whole-house conversion.