Tankless water heater sizing: real GPM at your groundwater temperature
Updated · Reviewed against the current HotWaterAtlas dataset
Short answer: ignore the marketing "up to X GPM" — it assumes warm
groundwater. Compute your real flow: GPM = BTU/h × efficiency ÷ (temperature rise ×
500). In a northern winter (70 °F rise) even the largest certified unit sustains
about 5.4 GPM — roughly two showers at once.
Step 1: find your temperature rise
Your temperature rise is the delivery temperature (typically 110–120 °F) minus your groundwater temperature — and groundwater follows climate:
| Region | Groundwater | Winter rise to 120 °F |
|---|---|---|
| Florida, Gulf Coast | 70–75 °F | 35–50 °F |
| Southern California, Texas | 60–70 °F | 50–60 °F |
| Mid-Atlantic, Pacific Northwest | 50–60 °F | 60–70 °F |
| Midwest, New England | 40–50 °F | 70–80 °F |
Step 2: compute the flow a unit can sustain
The physics is fixed: heating one gallon per minute by 1 °F takes about 500 BTU/h. Take the unit's gas input rating and efficiency (its UEF is a good stand-in) and divide. Using the largest certified unit — the A. O. Smith ACTH-199M-(N,P) 1 at 199 kBTU/h input, UEF 0.95 — as the worked example:
| Temperature rise | Sustained flow | 2-GPM showers at once |
|---|---|---|
| 45 °F | 5.8 GPM | 2 |
| 55 °F | 5.8 GPM | 2 |
| 70 °F | 5.4 GPM | 2 |
| 77 °F | 4.9 GPM | 2 |
Every tankless model page on this site shows this exact table for that unit — the calculation and its assumptions are documented in the methodology.
Step 3: match against your simultaneous demand
Add up what actually runs at once (shower ≈ 2 GPM, sink ≈ 0.5–1, dishwasher ≈ 1.5, tub filler ≈ 4 — full table in the sizing guide) and require the unit to clear that number at your winter rise, not the brochure's. This is why ENERGY STAR's certified GPM figures — measured at a 77 °F rise — look "low" next to marketing claims: they are the honest winter number for the northern half of the country.
The two gotchas after sizing
- Gas supply. A 199 kBTU/h unit draws several times a storage heater's input (~40 kBTU/h); the gas line and meter must support it. This — not the unit price — is often the surprise cost in a tank-to-tankless conversion.
- Low-flow behavior. Below its minimum activation flow a tankless won't fire at all. Check that spec if your fixtures are aggressively water-saving.
Browse tankless by certified flow tier — 4 GPM, 5 GPM, 6 GPM — or start from your household on the best-by-household pages.