Tankless Flow Calculator
Work out the flow rate and temperature rise a tankless water heater has to deliver for your home, and see whether that is a small, standard, large or two-unit setup.
A real person answers. Free estimate, no obligation.
Data: DOE water heater guidance (120°F setpoint) · federal and EPA WaterSense fixture flow rates · groundwater temperature bands from local winter climate (Daymet 2015–2024) · Updated September 2026
How tankless sizing works
Flow: what runs at the same time
A tank stores hot water; a tankless unit makes it as you use it, so it is sized by gallons per minute, not gallons. Add up the fixtures that realistically run together: a showerhead is about 2.5 GPM, a faucet about 1.8, a dishwasher or washer about 2. Two showers at once already means 5 GPM.
Rise: how cold the water comes in
The unit has to lift incoming water to the setpoint, and DOE recommends 120°F. Ground water in a cold-winter state can arrive in the low 40s in January, a rise of nearly 80°F; in a warm climate it is in the 60s and the rise is closer to 55°F. Same house, very different job.
Flow times rise is the size
Heat needed per hour is GPM times rise times 500 (from the weight of a gallon of water). A tankless unit can trade one for the other: the same unit that gives 8 GPM at a small rise gives 4 or 5 GPM at a big one, which is why manufacturer charts are plotted by rise.
Gas versus electric capacity
Gas units go up to about 199,000 BTU per hour input, enough for a whole house in most climates, and can be linked in pairs. Electric units top out near 36 kW, about 123,000 BTU per hour, and need a very large electrical service, so in cold-groundwater areas they cover roughly one shower at a time.
Common questions
Usually the flow for two showers at once, about 5 GPM, at your local temperature rise. In a warm climate that is a standard unit; in a cold-winter state it lands on a large unit or two linked units. The calculator gives the class for your climate.
A large gas unit can, in most climates. Two showers is about 5 GPM, and at a 55 to 70°F rise that is within reach of one high-capacity unit. At the 78°F rise of a cold-groundwater state it is at the edge, which is where two units or low-flow showerheads come in.
The difference between the incoming water temperature and the setpoint. With 120°F out and 45°F in, the rise is 75°F. Every tankless flow rating is quoted at a rise, so a unit rated 9 GPM at 35°F rise gives far less in a cold climate.
Rarely in cold climates. The largest common electric units, about 36 kW, deliver around 3 GPM at a 78°F rise, roughly one shower. In warm-groundwater areas the same unit reaches 4 to 5 GPM. Either way it needs a very large electrical service, which an electrician has to confirm.
Often. A 199,000 BTU per hour unit draws several times what a tank heater does, and the existing gas line and venting may not be sized for it. That is one of the first things a plumber checks on a tankless conversion.
Get an exact answer from a local pro
Free to call — get connected with a local pro in your area.
Tap to Call(855) 321-3426disclaimer: Delmarva Plumbers is a free service to assist homeowners in connecting with local service providers. All contractors/providers are independent and Delmarva Plumbers does not warrant or guarantee any work performed. It is the responsibility of the homeowner to verify that the hired contractor furnishes the necessary license and insurance required for the work being performed. All persons depicted in a photo or video are actors or models and not contractors listed on Delmarva Plumbers.
Same-day and 24/7 emergency services are subject to provider participation, location, technician availability, and demand. Availability is not guaranteed and may vary by market and appointment capacity.
We use cookies and session-based analytics, including session replays (with form inputs masked), to improve this site. See our Privacy Policy.
About · Contact · Privacy · Terms · Disclosure · Sitemap
© 2026 Delmarva Plumbers. All rights reserved.