How to size a tankless water heater (GPM and temperature rise)

By Francesco Zinghinì

The most common tankless mistake is sizing it like a tank — by gallons or by number of bathrooms. A tankless holds no water; it heats on demand, so its two limits are flow (gallons per minute it can raise) and rise (how many degrees it can raise them). A unit rated “9.8 GPM” only delivers that at a small rise; ask it for a 70 °F rise and the usable flow drops sharply. Size for the honest combination and you avoid the classic complaint that a pricey new tankless “can’t keep up.”

Step 1 — add up simultaneous GPM

List the fixtures that can realistically run at the same time in your busiest moment and add their flows. Planning typicals: a shower ~2.0 GPM, a bath faucet ~2.0, a kitchen faucet ~1.5, a dishwasher ~1.5, a washing machine ~2.0. Two showers plus a kitchen faucet is 2.0 + 2.0 + 1.5 = 5.5 GPM. This is simultaneous flow, not a daily total — the whole point of a tankless is that it does not care about the day’s volume, only the peak instant.

Step 2 — find your temperature rise

The rise is ΔT = desired_output − inlet_water_temp. Most homes target 120 °F. Inlet water is the variable that trips people up: it swings from about 40 °F in a northern winter to 60–77 °F in the warm south. At a 50 °F inlet the rise is 70 °F; at 40 °F it is 80. That 10-degree difference can cut a unit’s usable flow by a fixture’s worth. See the inlet water temperature by region reference and size for your coldest month, not the annual average.

Step 3 — read the unit at your rise

Manufacturers publish a GPM-versus-rise curve. You need a unit whose rated flow at your ΔT meets your simultaneous GPM. For 5.5 GPM at a 70 °F rise you are in whole-house gas tankless territory (roughly 150,000–199,000 BTU/hr). The tankless sizing calculator does this for you: enter the fixtures and the inlet, and it returns the required GPM at your rise.

Worked example, warm vs cold

Same house, two showers and a kitchen faucet = 5.5 GPM. In a warm climate (inlet 65 °F, rise 55) a mid-size gas tankless handles it comfortably. Move the house north (inlet 40 °F, rise 80) and the same unit now tops out around 4 GPM — short by a shower. The fix is either a larger unit, or accepting that in deep winter you stagger the two showers. This is the whole discipline: the unit did not fail, it was sized for the wrong rise.

Edge cases and gotchas

• Low-flow fixtures help. A 1.5 GPM shower head instead of 2.5 shrinks your simultaneous demand and lets a smaller tankless keep up — a cheaper fix than upsizing the heater.
• Electric tankless is thirsty. A whole-house electric tankless at a high rise can need multiple 40–60 amp circuits; many older panels cannot carry it. Gas is usually the practical whole-house choice in cold climates.
• Minimum activation flow. Tankless units need a threshold flow (often ~0.5 GPM) to fire — a trickle at a faucet may not trigger heating.
• Point-of-use vs whole-house. A small point-of-use tankless at a remote sink can be smarter than upsizing the whole-house unit for one far fixture.

Parallel vs point-of-use, and recirculation

When one whole-house unit cannot cover the peak — common in cold climates or big homes — you have two escape hatches the brochures rarely explain. The first is parallel (manifolded) units: two tankless heaters plumbed together share the load and fire only as many as the flow demands, which both covers a high peak and idles efficiently at low draw. The second is a point-of-use unit at a far or occasional fixture — a small electric tankless under a guest bath sink can be smarter than upsizing the main unit and running hot water a long way through cold pipe. Speaking of that pipe run: a tankless does nothing to shorten the wait for hot water at a distant tap. A recirculation loop fixes the wait but adds cost, a small standby penalty, and for a tankless usually a built-in or add-on pump and a return line — plan it in rather than bolt it on later.

Altitude and gas type are the quiet gotchas on the combustion side. A gas tankless derates at high elevation (thinner air, less combustion), so a unit that hits your GPM at sea level may fall short in the mountains — check the manufacturer’s derate table. Propane versus natural gas changes the orifice and the available BTU, so confirm the unit matches your fuel. And size the gas line and meter for the unit’s full input (often 150,000–199,000 BTU/hr) — an undersized line starves the burner and you never reach the rated flow, which feels exactly like an under-sized heater even though the sizing math was right. These are a licensed plumber and gas fitter’s calls; the point here is to know they exist before you buy on GPM alone.

What to measure first

Before sizing, bucket-test your shower flow (fill a gallon jug and time it), list which fixtures truly overlap, and measure or look up your winter inlet temperature. Then size to the peak simultaneous GPM at that cold-month rise, and confirm the unit’s published curve and its gas or electrical requirements. Gas line, venting and combustion are a licensed plumber’s job — this guide sizes flow and heat, not the install.