Tankless water heater sizing Last reviewed June 30, 2026
What Size Tankless Water Heater for a Family of 5—?
By the WhatSize editorial team 4 min read Standards-reviewed
A family of 5–6 needs 9–11 GPM gas (250k BTU or two 199k units) for 3+ simultaneous showers. Top commercial unit: Rinnai RU199iN (~11 GPM at 35°F rise / ~6 GPM at 65°F rise, 199k BTU).
Why large families need serious tankless capacity
Large families (5–6 people) have peak demand that can reach 9–11 GPM: 3 simultaneous showers (6–7.5 GPM per ENERGY STAR fixture rates) + 2 kitchen/bath sinks (~2 GPM) + dishwasher (~1.3 GPM) = ~10 GPM. This is at the upper limit of single-unit gas tankless (most top out at 11 GPM at 35°F rise).
For cold climates (inlet <50°F): the same household needs 11+ GPM, which often exceeds single-unit capacity. Solutions:
- Single commercial-grade 11 GPM unit: ENERGY STAR lists Rinnai RU199iN (199k BTU, ~11 GPM at 35°F rise, ~6 GPM at 65°F rise, UEF ~0.97) and Rinnai RU180iN (180k BTU, ~10 GPM at 35°F rise) as the top commercial condensing units.
- Two parallel 199k BTU units: Two Noritz NCC199CDV or Bradford White RTGR199 units, one per floor or zone. Each handles 5–6 GPM.
For warm climates (inlet 65°F+): a single 9–10 GPM unit suffices (Noritz NCC199CDV handles this with margin at 5.5 GPM at 67°F rise, up to 10 GPM at lower rises).
Top picks for this size
These Amazon-available picks meet the calculated spec. For region-specific or licensed contractor-installed options, consult a local professional. See our Disclaimers.
When to use multi-unit or commercial-grade
- 5–6 person, moderate climate, no simultaneous dishwasher: Single 250k BTU unit (10 GPM at 67°F rise).
- 5–6 person, cold climate: 11+ GPM or two 199k BTU units in parallel (Rinnai RU199iN + RU180iN pair).
- 5–6 person + teenager showers: Commercial-grade 11+ GPM unit (Rinnai RU199iN, Bosch 9400 series).
- 6+ person: Two 199k BTU units (one per floor) is more reliable than one large unit.
Frequently asked questions
What size tankless for a family of 5?
A family of 5 needs 9–10 GPM gas tankless (199–250k BTU) or two 199k BTU units in parallel for cold climates. Electric tankless (36 kW) is at the upper limit. Top unit for cold climates: Rinnai RU199iN (~11 GPM at 35°F rise).
Can one tankless handle 3 showers at once?
A single 11 GPM gas tankless (commercial-grade) can handle 3 simultaneous showers (6 GPM) + kitchen (1.5 GPM) = 7.5 GPM peak. For residential units (8–10 GPM max), 3 simultaneous showers is at the limit. For larger households, consider Rinnai RU199iN (199k BTU commercial condensing) or two parallel 199k units.
Should I get two tankless water heaters?
For 6+ person households, two 199k BTU units (one per floor or zone) is often more reliable than one large unit. Two smaller units also provide redundancy — if one fails, the other still provides partial hot water. Total installed cost: $3,500–$6,000.
How much is a commercial-grade tankless?
Commercial-grade 11+ GPM gas tankless units cost $1,500–$3,500 plus installation ($2,500–$5,000 total installed). They have higher BTU ratings and commercial-grade heat exchangers for durability under sustained high demand. Top pick: Rinnai RU199iN / Noritz NCC199 commercial condensing.
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