Tankless Water Heater Calculator — GPM by Temperature Rise
A tankless unit has no reserve — whatever it can heat as the water passes through is all you get. That output depends entirely on temperature rise, and the gpm figure on the box assumes a rise most of the country never sees. Enter the real inlet temperature for your coldest month and see what the unit actually delivers.
Size your tankless unit
199,000 is the common ceiling before commercial venting rules apply.
Condensing ~95%, non-condensing ~82%.
Simultaneous fixtures only, not the whole house.
Coldest month: ~70 Gulf coast, ~40 northern.
120 °F is the usual domestic delivery setting.
Deliverable flow
Covers the 4.0 gpm you asked for with 1.40 gpm to spare at this rise.
Temperature rise
70 °F
BTU for your target flow
147,309 BTU/h
Short of your target flow
At this temperature rise the unit cannot deliver what you asked for. Raise the input, lower the target temperature, or plan on fewer simultaneous fixtures — a tankless has no reserve to fall back on.
See the breakdown
Planning estimate. Minimum activation flow, modulation range, cold-water sandwich and gas supply capacity are not modelled — check the unit's own spec, and confirm the gas branch can carry the input. A licensed plumber has final say.
The formula, explained in plain English
One equation, run in both directions, with everything hanging on the temperature rise.
Inverse, not linear-ish
Flow is inversely proportional to rise. Going from a 50 °F rise to 80 °F is a 60% increase in rise and a 38% cut in output — the same unit, the same gas, a different month.
No reserve, ever
A tank has gallons already hot, so a brief heavy draw costs it nothing. A tankless is capped at its flow rate for every second of every draw. Burst demand is the one thing a tank does better.
Count simultaneous, not total
Three bathrooms do not mean three showers at once. Size on what genuinely overlaps — usually two fixtures in a family house, and often just one plus a sink.
The gas line is part of the job
A 199,000 BTU unit can exceed the whole rest of the house combined. A 1/2-inch branch that fed a 40,000 BTU tank will not carry it, and the upsize is frequently the largest single line on a conversion quote.
Worked examples
The defaults, the same unit moved north, and sizing backwards from a flow you actually need.
199,000 BTU condensing, 50 °F inlet
gpm = 199,000 × 0.95 ÷ (499.8 × 70) = 189,050 ÷ 34,986
→ 5.40 gpm
= two 2.0 gpm showers with 1.4 gpm to spare
Result: 5.40 gpm. Comfortable for a family house in a temperate climate — two showers, or a shower and a kitchen sink, at once.
The same unit in a northern winter
temperate, 50 °F inlet: rise 70 → 5.40 gpm
northern winter, 40 °F inlet: rise 80 → 4.73 gpm
→ 38% less output for the same appliance
Result: the identical unit that ran two showers and a sink in Texas struggles with two showers in Minnesota. Nothing has broken — the rise went up by 60% and the flow came down accordingly.
Working backwards from 4 gpm
→ 147,309 BTU/h at a 70 °F rise
same 4 gpm at an 80 °F rise = 168,354 BTU/h
non-condensing at 82%: 4 × 499.8 × 70 ÷ 0.82 = 170,663
Result: 4 gpm at a temperate rise needs about 150,000 BTU. Push either the climate or the efficiency the wrong way and the same 4 gpm needs nearly 170,000 — which is how a mid-range unit quietly becomes a top-of-range one.
Output by inlet temperature
A 199,000 BTU condensing unit at 95%, delivering 120 °F. Computed from the same code the calculator runs.
| Inlet | Rise | Deliverable flow | 2.0 gpm showers |
|---|---|---|---|
| 70 °F | 50 °F | 7.57 gpm | 3 |
| 60 °F | 60 °F | 6.30 gpm | 3 |
| 50 °F | 70 °F | 5.40 gpm | 2 |
| 40 °F | 80 °F | 4.73 gpm | 2 |
Output by unit size and climate
All at 95% efficiency. The three columns are a warm-climate rise, a temperate rise, and a northern winter rise — the same appliance, three very different answers.
| Input | 50 °F rise | 70 °F rise | 80 °F rise |
|---|---|---|---|
| 120,000 BTU/h | 4.56 gpm | 3.26 gpm | 2.85 gpm |
| 150,000 BTU/h | 5.70 gpm | 4.07 gpm | 3.56 gpm |
| 180,000 BTU/h | 6.84 gpm | 4.89 gpm | 4.28 gpm |
| 199,000 BTU/h | 7.57 gpm | 5.40 gpm | 4.73 gpm |
Sources & standards: the flow equation is derived from the specific heat and density of water — 8.33 lb/gal × 60 min gives the 499.8 constant used here rather than the rounded 500. Delivery temperature practice follows IPC 607 and ASSE mixing-valve standards; groundwater inlet temperatures vary by region and season, so use a local figure for the coldest month. Gas supply must be sized separately under the IFGC. Efficiency, minimum activation flow and modulation range come from the unit's own listing.
Frequently asked questions
Common questions about tankless sizing, temperature rise, and why the box rating misleads.
What size tankless water heater do I need?
Work from the temperature rise, not the headline gpm. A 199,000 BTU unit at 95% efficiency delivers 5.40 gpm with a 50 °F inlet and a 120 °F target — a 70 °F rise. Those are the defaults. The same unit does 7.57 gpm on a warm 70 °F inlet and only 4.73 gpm at 40 °F. Size for your coldest month.
Why does the advertised gpm never match what I get?
Because the headline figure assumes a temperature rise most of the country never sees. Manufacturers commonly quote flow at a 35 or 45 °F rise, which is a summer inlet in a warm state. At a realistic northern winter rise of 80 °F, the same unit loses 38% of its output against a 50 °F rise. This is the single most common tankless sizing failure and it is entirely predictable.
How do I work out my temperature rise?
Target temperature minus incoming groundwater temperature. Target is usually 120 °F for domestic use. Inlet ranges from about 70 °F along the Gulf coast to 40 °F in the northern states in winter — and it is the winter number that matters, because that is when everyone showers longest.
How many fixtures can one tankless unit run?
Divide the deliverable flow by what the fixtures draw. At 5.40 gpm you get roughly two 2.0 gpm showers with a little left over, or one shower plus a kitchen sink. A modern low-flow shower at 1.5 gpm stretches that to three. Count the fixtures that will genuinely run simultaneously, not the total in the house.
What is the 500 in the formula?
It is 8.33 lb/gal × 60 minutes = 499.8, the BTU needed per hour to raise one gallon per minute by one degree Fahrenheit. Every water-heating calculation in the trade uses it. This calculator carries the exact 499.8 rather than the rounded 500, which is why results sit a fraction of a percent above what a hand calculation gives.
Does a tankless need a bigger gas line?
Usually, and it is the cost that surprises people. A 199,000 BTU tankless can draw more gas than every other appliance in the house combined, and a 1/2-inch branch that fed a 40,000 BTU tank will not carry it. Size the gas line properly — the Gas Line Sizing Calculator handles that. A condensing unit also needs its own sealed vent and a condensate drain.
Is tankless always better than a tank?
Not always. Tankless wins on standby loss and endless supply, and takes far less floor space. A tank wins on burst capacity — it has gallons already hot, so a short heavy draw is easy for it, whereas a tankless is limited to its flow rate no matter how brief the demand. Compare with the Water Heater Size Calculator before deciding.
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