GPM (gallons per minute)
GPM measures how much hot water a tankless water heater can deliver continuously. A shower uses roughly 2 GPM, a bathroom sink 0.5–1 GPM, and a tub filler 4 GPM — a tankless must sustain the sum of everything running at once.
Unlike a tank, a tankless heater never runs out of hot water — but it caps how much can flow at one time. If the demand exceeds its capacity at the current temperature rise, the water gets cooler or the unit throttles flow.
The certified GPM in the ENERGY STAR record reflects the DOE test conditions — 58 °F water in, a 125 °F setting, about a 67 °F rise — which is why it reads lower than the marketing “up to X GPM” figure. That larger claim usually assumes a mild 35–45 °F rise you would only see in a warm climate.
| Flow formula | GPM = input BTU/h × efficiency ÷ (temperature rise °F × 500) |
|---|---|
| Certified max flow | 3.8–6.0 GPM (median 5.4) across the gas tankless models on this site |
| Certified input rates | 130–199 kBTU/h (median 180) |
| Shower | 1.5–2.5 GPM total; about 1.5 GPM of that is hot water at a 105 °F mix |
| Other fixtures | Bathroom faucet 0.5–1.5 GPM · kitchen faucet 1.5–2.2 · dishwasher ~1 · clothes washer 2–3 · tub filler 4–8 |
| Same unit, two climates | A 199 kBTU/h heater delivers roughly 8.5 GPM at a 45 °F rise and 5.5 GPM at a 70 °F rise |
The certified number and the brochure number
Flow and temperature trade against each other on a fixed budget. Heating a gallon per minute by one degree takes about 500 BTU/h, so a heater’s usable flow is its input rating times its efficiency, divided by the rise you are asking for. Nothing about the appliance changes when the seasons do — only the arithmetic.
That is why one unit carries two very different flow claims. The certified figure reflects the DOE test’s own conditions: 58 °F inlet water heated to a 125 °F setting, a 67 °F rise. Back those numbers out of the 387 certified gas tankless models on this site and the implied rise clusters between 63 and 66 °F, consistent across every manufacturer. The “up to 11 GPM” on the box, by contrast, is computed at a rise of 35–45 °F — real in Florida in August, fictional in Minnesota in January.
Use the certified GPM as your baseline and adjust from there. A 199,000 BTU/h condensing unit rated 5.8 GPM certified will do roughly 8.5 GPM where groundwater arrives at 70 °F, and roughly 5.5 GPM where it arrives at 45 °F. Buy for your coldest month, because that is the month the complaints happen.
Adding up what runs at once
Tankless sizing is a concurrency problem. List the fixtures that could realistically run simultaneously and add their flows: showers at 1.5–2.5 GPM each, a kitchen faucet at 1.5–2.2, a bathroom faucet at 0.5–1.5, a dishwasher at about 1, a clothes washer at 2–3, and a tub filler at anywhere from 4 to 8. A two-bathroom house with both showers and a kitchen tap running is around 5–6 GPM of mixed water.
Only part of that is hot. A 105 °F shower blended from a 120 °F heater and 55 °F cold water is about 77% hot — so a 2.0 GPM shower asks the heater for roughly 1.5 GPM. A tub filling at 4 GPM at 100 °F asks for about 2.8. Sizing against total fixture flow rather than hot-water flow routinely overshoots by 20–30%, which pushes people toward units a size larger and several hundred dollars more expensive than they need.
Two showers plus a sink is the load most households actually need covered, and that is about 3.5 GPM of hot water. Nearly every certified residential gas tankless clears it at a moderate rise. Where sizing breaks down is a cold climate plus a soaking tub, or three bathrooms in genuinely simultaneous use — the cases where two units in parallel, or a tank, is the honest answer.
What else caps the flow you get
The heater is not always the bottleneck. Household water pressure, undersized supply piping, a partially closed shutoff, or a clogged aerator will limit flow before the burner does. A tankless also throttles: when demand exceeds what it can heat, better units restrict flow to protect the outlet temperature rather than delivering lukewarm water, which feels like a pressure drop and is actually the control system doing its job.
At the other end of the range, flow can be too low. Every tankless has a minimum activation flow — typically 0.4 to 0.5 GPM in the certified records, occasionally as high as 1.0 — below which the burner will not light at all. Pairing a high-threshold unit with 1.0 GPM WaterSense bathroom aerators produces the classic complaint of a tap that never goes warm.
Scale is the slow killer. Hard water deposits inside the heat exchanger reduce heat transfer, which shows up first as reduced flow at temperature and eventually as error codes. Annual descaling in hard-water areas, or a softener, protects the flow rating you paid for; without it a five-year-old unit can be materially slower than its certified figure.
Frequently asked questions
How many GPM do I need for a tankless water heater?
Add the hot-water flow of the fixtures that will run at the same time, not every fixture in the house. Two showers plus a sink is about 3.5 GPM of hot water; add a tub filler and you are near 6. Then check that a candidate unit sustains that at your winter temperature rise — the certified GPM assumes about a 67 °F rise, so a colder climate needs a bigger burner for the same flow.
How many GPM does a shower use?
Federal rules cap showerheads at 2.5 GPM at 80 psi, and WaterSense models run at 2.0 or less; many older heads flow closer to 2.5. Of that, roughly three quarters is hot water when you are mixing a 105 °F shower from a 120 °F tank, so budget about 1.5 GPM of hot water per shower.
Is 5 GPM enough for a 2-bathroom house?
For two showers plus a sink, yes — that is about 3.5 GPM of hot water, comfortably inside 5. It stops being enough if you fill a large tub while a shower runs, or if you are in a cold-groundwater region where the same unit’s real output drops from its certified 5 GPM toward 4. Check the flow at your winter temperature rise, not the rating on the box.
What does GPM mean on a water heater?
Gallons per minute — the volume of hot water the unit can deliver continuously at a given temperature rise. It is the primary capacity rating for tankless heaters, which never run out but cap how much can flow at once. Storage tanks are rated instead by first-hour rating, since their limit is stored volume rather than instantaneous heat output.
Why doesn’t my tankless heater deliver its rated GPM?
Almost always because your temperature rise is larger than the one behind the rating. Cold groundwater in winter can cut usable flow by a third against a mild-climate figure. Other causes: low household water pressure, undersized gas supply starving the burner of its full input, or scale build-up in the heat exchanger reducing heat transfer.