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What Size Tankless Water Heater Do I Need?
Quick answer: Most homes need a tankless that delivers peak GPM (often 4–7 GPM) at winter temperature rise (often 50–80°F). A 1–2 bath mild-climate home may be fine with about 5 GPM at a 60°F rise; a 3-bath cold-climate home may need about 8 GPM at a 75°F rise.
Context: size by flow, not square footage
Tank heaters are sized in gallons and loosely by household size. Tankless heaters are sized by two numbers: how much hot water you use at once (GPM) and how much the unit must warm it (temperature rise). House size matters only because bigger homes tend to run more fixtures at once. Getting these two numbers right is the single biggest factor in whether a tankless install feels "endless" or disappointing.
Terms
- Peak GPM: total flow of fixtures used simultaneously.
- Temperature rise: setpoint (often 120°F) minus coldest inlet water.
- Flow-at-rise chart: the manufacturer's table showing real GPM at each rise.
- Headline GPM: the maximum, quoted at a small rise; rarely your winter number.
Process: three-step sizing
- Peak flow. Count fixtures that run together. Use the GPM calculator and the flow table below.
2. Temperature rise. Setpoint minus coldest groundwater. Use the temperature rise calculator.
3. Match a rating. Use the flow-at-rise chart, not the headline GPM. For gas, check BTU/h with the BTU calculator; for electric, kW and amps with the amps calculator.
Or run everything at once in the tankless size calculator.
Sizing by household (example)
| Household | Example peak | Mild climate (50–60°F rise) | Cold climate (70–85°F rise) |
|---|---|---|---|
| 1 bath, 1–2 people | 2.5–3.5 GPM | Electric 18–27 kW or small gas | Gas ~120,000–160,000 BTU/h |
| 2 bath, 3–4 people | 4–5.5 GPM | Gas ~140,000–180,000 BTU/h | Gas ~180,000–199,000 BTU/h |
| 3+ bath, 5+ people | 6–9 GPM | Gas ~199,000 BTU/h | Two linked gas units |
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 2 showers at 2.0 GPM + bathroom sink at 1.0 GPM = 5 GPM; 45°F inlet to 120°F = 75°F rise; condensing gas at 0.95 efficiency.
- Required input: 5 × 75 × 500 ÷ 0.95 ≈ 197,000 BTU/h.
- Look for about 5 GPM at 75°F on the flow-at-rise chart — the 199,000 BTU/h class.
- If an electric unit were considered: 5 × 75 ÷ 6.83 ≈ 55 kW, about 229 A — not practical for most homes.
- Lower-cost alternative: 1.8 GPM showerheads cut peak to 4.6 GPM and required input to about 182,000 BTU/h.
Oversizing vs undersizing
- Undersized: cooler water or reduced flow at peak, more full-fire running, potentially shorter life.
- Oversized: higher unit cost, bigger gas or electrical requirement; with good modulation, little efficiency penalty.
A small margin (about 10%) above your calculated peak is a common planning approach; your installer has the final word.
Mistakes to avoid
- Sizing by bathrooms or square footage alone.
- Using summer inlet temperature.
- Trusting headline GPM.
- Forgetting gas-line or panel capacity.
FAQ
What size for a family of four?
Often 5–6 GPM at your winter rise, frequently a 180,000–199,000 BTU/h gas unit in colder areas.
Can one unit serve a large home?
Up to about 7–9 GPM in mild climates; larger demand may need two linked units.
Should I size for the bathtub?
If you fill a tub while other fixtures run, include its 2–4 GPM.
Is electric big enough?
In warm climates with modest peaks, yes; in cold climates, rarely for whole-home use.
Safety and code reminders
Gas, venting, combustion air, condensate disposal and high-amperage electrical work all carry real safety risks, including carbon monoxide, fire, scalding and shock. They are also governed by local code and permits that differ between towns and provinces or states. Treat every step on this page as preparation for a professional visit, not as instructions to do the work yourself. Keep the manufacturer's manual for your exact model, set water temperature with scald risk in mind (120°F is a common household target), and stop and call a licensed professional whenever something is unclear.
Quick reference: typical fixture flow rates
These example flow rates help you translate everyday use into the GPM numbers used throughout this site. Check the rating stamped on your own fixtures, as older fixtures often run higher.
| Fixture | Example hot-water flow |
|---|---|
| Low-flow showerhead | 1.5–1.8 GPM |
| Standard showerhead | 2.0–2.5 GPM |
| Bathroom faucet | 0.5–1.2 GPM |
| Kitchen faucet | 1.0–1.8 GPM |
| Dishwasher | 0.8–1.5 GPM |
| Washing machine (warm) | 1.0–2.0 GPM |
| Bathtub fill | 2.0–4.0 GPM |
Add only the fixtures that genuinely run at the same time. Running the dishwasher or laundry outside shower hours is the simplest way to stay within a unit's capacity.
Questions to ask a licensed installer
Bring these questions to every quote so you can compare them fairly. Good installers welcome them, and the answers often reveal hidden costs before work starts.
- What peak GPM and temperature rise did you size this unit for, and what inlet water temperature did you assume for winter?
- Is my existing gas line, meter or electrical panel large enough, and if not, what exactly needs upgrading and what does that line item cost?
- What venting route will you use, how long is it, and does it meet the manufacturer's instructions and local code?
- Does the quote include a permit, isolation (service) valves for future flushing, a condensate drain if needed, and removal of the old heater?
- What are the labour warranty and the manufacturer's heat-exchanger and parts warranty terms, and do they require registration or yearly professional service?
- How long will the install take, and will there be a period without hot water?
How to read the numbers on this site
Every figure on TanklessMath is a planning estimate. Ranges reflect typical North American examples as of 2026-10-29 and will differ with your region, home, fuel prices and installer. Equipment ratings vary by model, so always check the specification sheet for the exact unit you are considering. When our example and a licensed installer disagree, trust the installer who has seen your home.
Next steps
Write down your peak fixtures, coldest incoming water temperature, fuel type and possible locations, then ask at least two licensed installers to confirm sizing, venting, gas or electrical capacity, permits and local code. Compare written quotes line by line so the unit, labour, materials and upgrades are separated. Questions about this site: hello@charlandandco.com.
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