A tankless water heater is sized by how much hot water you need at once (flow rate in GPM) and how many degrees it has to heat your incoming water (temperature rise), not by gallons of storage.

Quick Scoop

If you want a fast rule of thumb:

  • Small household (1–2 people, 1 bathroom, one shower at a time):
    • Often fine with about 3–4 GPM gas tankless or a smaller electric unit, depending on climate.
  • Average household (3–4 people, 2 bathrooms, maybe 2 showers at once):
    • Aim for around 6–7 GPM gas tankless in moderate climates.
  • Larger family (4–6 people, 2–3+ bathrooms, frequent overlap like 2 showers + sink or dishwasher):
    • You’re usually in the 8–10 GPM gas range, sometimes even 2 units for big homes or cold areas.

But to do it right , you size it like a pro.

Step 1: Count What’s Running at the Same Time

Tankless sizing is all about your peak simultaneous use , not the whole day.

Ask yourself: on a busy morning, what’s realistically on at once? Typical example scenarios:

  • One-bedroom / couple:
    • 1 shower + maybe kitchen sink or dishwasher.
  • Family with 2 baths:
    • 2 showers at once + a sink running occasionally.
  • Big household:
    • 2–3 showers + dishwasher or clothes washer.

You do not size for “every fixture in the house on at once”; you size for a realistic peak that actually happens in your life.

Step 2: Add Up Flow Rates (GPM)

Each fixture uses a certain gallons per minute (GPM). You can check the label, manual, or use typical values.

Common ballpark GPMs:

  • Shower: about 2.0–2.5 GPM (older heads can be higher, ultra-low-flow can be 1.5 GPM).
  • Bathroom sink: about 0.5–1.0 GPM.
  • Kitchen sink: about 1.5–2.0 GPM.
  • Dishwasher: about 1.0–1.5 GPM.
  • Clothes washer: about 2.0 GPM.

Example story:

Picture a Saturday morning: two teens showering upstairs while someone rinses dishes downstairs. That’s 2 showers (~2.0 GPM + 2.0 GPM) plus a kitchen sink (~2.0 GPM), for a total of ~6 GPM. Your heater must comfortably supply those 6 GPM of hot water at your desired temperature.

Rough “GPM by number of fixtures” shortcuts:

  • 1–2 fixtures at once → about 3.5 GPM.
  • 2–3 fixtures at once → about 5 GPM.
  • 3–4 fixtures at once → about 7 GPM.

These are starting points, not exact numbers, but they’re great for quick sizing.

Step 3: Figure Out Temperature Rise

Temperature rise = how many degrees you need to heat the incoming cold water.

  • Desired hot water temp: many people like around 110–120°F at the tap.
  • Incoming (groundwater) temp:
    • Cold/northern climates: can be around 40–50°F.
    • Moderate climates: often around 50–60°F.
    • Warm/southern climates: around 60–70°F.

So:

  • Northern U.S. example:
    • Incoming 45°F → target 115°F → 70°F rise.
  • Southern U.S. example:
    • Incoming 65°F → target 115°F → 50°F rise.

The colder your incoming water, the bigger the heater you need for the same GPM.

Step 4: Match GPM to a Real Unit

Every tankless model lists its maximum GPM at different temperature rises.

You want a unit that can provide:

  • At least your calculated GPM
  • At your local temperature rise

Example:

  • You calculated you need 6 GPM at a 60–65°F temperature rise.
  • You look at units and pick one that can do ≥ 6.5 GPM at 60–65°F to give yourself a bit of cushion.

One guide suggests:

  • Around 10 GPM for northern regions (colder incoming water).
  • Around 7 GPM for southern regions (warmer incoming water).

Handy Household Cheat Sheet (Gas Tankless, Typical Use)

Here’s a rough idea of what many people end up with (always check the exact specs and your climate):

[6][3] [1][3] [5][3] [9][5]
Household Type Bathrooms Typical Peak Use Suggested Tankless Size (GPM) Notes
Small apartment / couple 1 1 shower + occasional sink 3–4 GPM Electric may work in warm climates; gas preferred in colder areas.
Average home, 3–4 people 2 2 showers at once, light sink use 6–7 GPM Very common gas size for typical suburban homes.
Larger family home 2–3 2–3 showers + dishwasher 8–10 GPM May need high-capacity gas or even 2 units in cold regions.
Luxury home with soaking tub 3+ Tub filling or multiple showers 10+ GPM / multiple units Always check manufacturer sizing tools for large homes.

Extra Things That Matter

Beyond “what size tankless water heater do I need,” real-world details can push you up or down a size:

  • Climate:
    • Colder area = higher temperature rise = need more capacity.
  • Fuel type:
    • Gas (natural/propane) can deliver very high GPM; electric often tops out lower and needs big electrical service.
  • Home infrastructure:
    • Gas line size, venting route, or electrical panel capacity can limit what model you can install.
  • Future plans:
    • Adding bathrooms, finishing a basement, or installing a big tub may justify stepping up a size.
  • Budget vs comfort:
    • Too small → lukewarm or cold showers at peak times.
    • Too big → higher upfront cost but some flexibility if you expand later.

Latest Tools, Forums, and “Trending” Angle

In the last couple of years, a lot of homeowners and DIYers have gravitated toward online sizing calculators and quick quizzes that ask about bathrooms, climate, and usage to give a custom GPM recommendation. Pros and content creators are also sharing step-by-step sizing guides and downloadable spreadsheets for BTU and GPM calculations, making the process more transparent for non‑experts.

On home-improvement forums, you’ll see a few recurring themes:

  • People often undersize their first tankless, then complain about lukewarm water with two showers running.
  • Others intentionally oversize a bit for comfort and future-proofing, as long as their gas or electric service can handle it.
  • Many threads end with: “Use the manufacturer’s sizing tool for your exact brand and local temperatures,” which is still the gold standard.

Putting It All Together for You

To decide what size tankless water heater you need, here’s the simple, practical flow:

  1. List what realistically runs at the same time (showers, sinks, appliances).
  2. Assign each fixture a GPM and add them up for your peak demand.
  3. Estimate your temperature rise (desired hot temp minus incoming water temp).
  4. Pick a unit that delivers at least that GPM at that temperature rise, with a bit of safety margin.
  5. Double‑check your gas/electric capacity and, if possible, confirm with the brand’s online sizing tool.

If you tell me:

  • How many bathrooms you have
  • How many people live there
  • Your state/region (for climate)
  • Whether you use gas or electric

I can walk through a quick, custom example and suggest a target GPM range tailored to your home.