What Size Tankless Water Heater Do I Need?
Tankless units are sized by GPM (gallons per minute) at peak simultaneous demand. Get the size right by counting fixtures you actually run at the same time, not square footage. The calculator below does the math.
The biggest sizing mistake is choosing by household size. A 5-person home that never runs two showers simultaneously needs less GPM than a 2-person home that does. Peak simultaneous demand is what matters, adjusted for your climate zone.
GPM Sizing Calculator
Select how many hot water fixtures you run simultaneously at peak usage.
Flow Rate by Fixture
Add together every fixture you might run at the same time during peak use.
| Fixture | Flow Rate (GPM) | Notes |
|---|---|---|
| Shower (low-flow) | 1.5–2.0 | WaterSense certified; newer fixtures |
| Shower (standard) | 2.0–2.5 | Most common in US homes built before 2010 |
| Shower (rain/luxury) | 2.5–5.0 | Rainfall showerheads, body sprays |
| Bathtub fill | 4.0 | Filling a full tub; shorter duration |
| Kitchen faucet | 1.0–1.5 | Modern aerator-equipped faucets |
| Bathroom faucet | 0.8–1.2 | WaterSense models at lower end |
| Dishwasher | 1.0–1.5 | Varies by cycle phase |
| Washing machine | 1.5–2.5 | Hot water per cycle |
Household Size Quick Guide
Useful starting reference, but always check against your actual peak demand.
| Household | Recommended GPM | Recommended BTU | Notes |
|---|---|---|---|
| 1–2 people | 6–8 | 120K–150K | 2 fixtures simultaneously max |
| 3–4 people | 8–10 | 150K–199K | 3 fixtures simultaneously |
| 5–6 people | 10–12 | 199K+ | Consider 2 units in parallel |
| 7+ people | 12+ | Multiple units | Two units strongly recommended |
Climate Zone Adjustments
Cold inlet water means more energy per gallon, which means lower effective GPM at the same unit capacity.
Whole-House vs Point-of-Use
Two completely different products, two different sizing approaches.
Whole-house unit
Single unit serves the entire home. Sized by total peak GPM across all simultaneous fixtures. Typical: 6 to 11 GPM.
Use when: replacing a whole-house tank, planning new construction, or the existing tank already serves all fixtures.
Point-of-use unit
Small electric unit at a single fixture. Sized by that fixture's GPM. Typical: 1.5 to 3 GPM, 3 to 11 kW.
Use when: remote bathroom with long pipe runs, garage or detached ADU, kitchen-only hot water, supplementing existing tank.
Oversizing wastes money
A 199K BTU unit costs $200 to $500 more than a 150K BTU unit but does not save more energy if you do not need the capacity. Many contractors default to the largest unit because complaints about undersizing are louder than complaints about overspending. Run our calculator first and push back if your installer recommends a unit larger than you need.