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Tankless water heater sizing: a step-by-step guide
By the VettedHeater team · Last updated October 23, 2026 · How we verify
Tankless units are sized by flow rate in gallons per minute (GPM) at a given temperature rise, not by gallons stored. Most deliver 2 to 5 GPM (U.S. DOE Energy Saver, "Which Water Heater Is Right for You?"). Size for the fixtures you run at the same time.
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Step 1: List fixtures you use at the same time
Write down the most you run at once, such as one shower plus the kitchen tap. Find each fixture's flow rate in GPM, which is often printed on the showerhead or faucet aerator.
Step 2: Add the flow rates
The total is the GPM your unit must deliver.
Step 3: Work out your temperature rise
Subtract your incoming water temperature from the outlet temperature you want. Colder incoming water means a bigger rise and less GPM from the same unit.
Step 4: Match the spec sheet
Manufacturers publish GPM at different temperature rises. Pick a unit rated at or above your total GPM at your rise. Gas units generally give higher flow rates than electric ones (U.S. DOE Energy Saver, "Which Water Heater Is Right for You?"); see gas vs electric.
Get it confirmed
Undersized units leave showers lukewarm when a second tap opens. Ask a licensed tankless installer to confirm sizing, gas or electrical capacity and the permit (City of San Diego, Plumbing/Gas Permit). Compare what to look for in a unit.
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Related tankless guides
- Tankless installation in San Diego
- Tankless water heater cost
- Gas vs electric tankless
- How to choose a tankless water heater
- Tankless vs tank
FAQs
What size tankless water heater do I need?
Add the GPM of fixtures you run at once, work out your temperature rise, then pick a unit rated for that GPM at that rise.
How many GPM does a tankless water heater deliver?
DOE says demand water heaters typically provide 2 to 5 gallons per minute.
Sources
- U.S. DOE Energy Saver, "Which Water Heater Is Right for You?" (checked October 23, 2026)
- City of San Diego, Plumbing/Gas Permit (checked October 23, 2026)