Tankless Water Heater Calculator

A tankless heater has no reservoir, so it has no capacity in gallons. What it has is a rate: how many degrees it can lift the water at a given flow. The flow figure printed on the box was measured at some rise the manufacturer chose, and the rise your house needs depends on how cold the ground is where you live. A unit rated at five gallons a minute in Texas delivers under three in Minnesota in January, at the same setting, with nothing wrong with it. This calculator works from the rise you actually need, and shows what the gas line or the electrical service has to deliver.

Burner input required: 170,732 BTU/hr 4 gpm × 70 °F rise × 500 = 140,000 BTU/hr of heat into the water, ÷ 82% efficiency to get burner input. The rise is what sizes it — the gallons per minute figure on a box means nothing without knowing at what rise.
Temperature rise: 70 °F 50 °F incoming to 120 °F delivered. Groundwater temperature is the variable nobody accounts for: it tracks the local annual average air temperature and falls further in winter in a cold climate. This is the single number that decides what a tankless heater can do in your house.
The same unit in a warm climate: 5.6 gpm With 70 °F incoming water, the same heat output delivers 5.6 gpm instead of 4. That is why manufacturer flow ratings are meaningless without the rise they were measured at, and why a unit that satisfies a family in Texas fails the same family in Minnesota.
Gas line and venting: 171 MBH A tankless burner is typically three to five times the input of the tank heater it replaces, so the existing gas line is very often too small — half inch pipe that fed a 40,000 BTU tank will not feed a 199,000 BTU tankless. Venting is category III or IV stainless or PVC depending on the model, and the old B-vent is not reusable.
Simultaneous demand: 1.9 showers at once 4 gpm at 2.1 gpm per shower. A tank heater degrades gracefully — it runs cool before it runs out. A tankless unit does not: past its capacity the temperature drops immediately and stays down for as long as the demand lasts. Size for the worst simultaneous case, not the average.
Minimum activation flow: 0.5 gpm Below this the unit will not fire at all. It matters with low-flow fixtures, with a bathroom tap opened slightly for hand washing, and with any fixture on a long branch that gets throttled. A trickle of hot water from a tankless system is cold water.
The cold water sandwich: Inherent to the format Stop and restart the flow — turning a shower off to soap — and the slug of water sitting in the heat exchanger arrives hot, followed by cold while the burner relights and comes up to temperature. Recirculation and small buffer tanks reduce it; nothing removes it entirely.
What it does save: Standby loss A tank heater keeps forty or fifty gallons hot around the clock whether anyone uses it or not. A tankless heats only what flows. The saving is real but modest in absolute terms, and the honest case for tankless is endless hot water and floor space rather than the energy bill.
Descaling is maintenance, not an option: Annually on hard water A tankless heat exchanger has narrow passages and runs the water hot and fast, which is the ideal condition for scale. On hard water the capacity falls measurably within a year without flushing, and manufacturers make the warranty conditional on it.

Rise is the whole calculation

Heating water is arithmetic with no room in it. Raising one gallon per minute by one degree Fahrenheit takes five hundred BTU per hour, and that is the entire relationship: multiply flow by rise by five hundred and you have the heat that must go into the water.

What varies between houses is the rise, and the variable that drives it is groundwater temperature. Incoming water tracks the annual average air temperature at your location, moderated by depth, and it drops further through winter in a cold climate.

Seventy degree water in a warm climate needs a fifty degree rise to reach a hundred and twenty. Forty degree water in a northern winter needs eighty. That is a sixty percent increase in the heat required for the same shower, which shows up directly as reduced flow from a given unit.

This is why comparing units by their headline gallons-per-minute figure is meaningless. The only comparable number is flow at a stated rise, and the only relevant rise is the one your worst month produces.

The infrastructure is the real constraint

A tankless heater has to deliver in minutes what a tank heater delivers over hours, so its instantaneous input is enormous by comparison.

A gas tankless is typically a hundred and fifty to two hundred thousand BTU against the forty thousand of the tank heater it replaces. The existing gas line is very often too small — half inch pipe that comfortably fed the tank will not feed the tankless, and the pipe sizing has to be checked back to the meter with every other appliance considered. Venting is a different category entirely and the old flue is not reusable.

An electric whole-house tankless is worse. Four gallons a minute at a seventy degree rise is over forty kilowatts, which is a hundred and seventy amps at two hundred and forty volts. That is a service upgrade in most houses and an impossibility in many, and it is routinely discovered after the unit has been bought.

This is why electric tankless makes sense at the point of use — a single sink, a workshop — and rarely for a whole house. Gas tankless works for a whole house provided the gas supply and venting are done properly, which is most of the installation cost.

How it behaves when it runs out

A tank heater degrades gracefully. As it empties it delivers progressively cooler water, and it recovers between draws. Two showers back to back work even when two simultaneous showers would not.

A tankless unit does not degrade. Up to its capacity the temperature is exactly what you asked for; past it, the outlet temperature falls immediately and stays down for as long as the demand continues. There is no reserve to draw on and no recovery period, because there is nothing stored.

That makes the worst simultaneous case the design case rather than an occasional inconvenience. Two showers and a dishwasher is around six gallons a minute, and a unit that cannot do six gallons a minute at your winter rise will fail that morning every time it happens.

The two other behaviours to know about are the minimum activation flow — below which the unit does not fire at all, so a barely opened tap runs cold — and the cold water sandwich, where restarting a flow delivers a slug of hot, then cold while the burner relights. Recirculation loops and small buffer tanks reduce both; neither eliminates them.

What this is based on

Sensible heat of water — 500 BTU/hr per gpm per °F of rise
Electrical conversion — 3,412 BTU per kWh

A capacity estimate. It does not size gas piping, which must be calculated back to the meter with all appliances considered, and it does not address venting, combustion air, condensate disposal or the electrical load calculation for the panel. Groundwater temperature varies with location, depth and season — use a local figure for the coldest month.

Frequently asked questions

What size tankless water heater do I need?

Enough capacity for your simultaneous flow at your winter temperature rise. Multiply gallons per minute by the rise in degrees Fahrenheit by 500 to get the heat required. The gallons-per-minute figure on the box is meaningless without knowing what rise it was measured at.

Why does my tankless heater work worse in winter?

Because the incoming water is colder, so the rise is larger. Going from 60 °F groundwater to 40 °F increases the required heat by a third for the same shower, which shows up directly as reduced flow.

Can I run an electric tankless in my house?

For a whole house, usually not. Four gallons a minute at a 70 degree rise is over 40 kW, which is around 170 amps at 240 volts. Electric tankless suits point-of-use applications; whole-house tankless is generally gas.

Will my existing gas line work?

Very often not. A tankless burner is typically three to five times the input of the tank heater it replaces, and the pipe has to be sized back to the meter with every other appliance considered. Venting is a different category too — the old flue is not reusable.

What is the cold water sandwich?

Stop and restart the flow and you get the hot water sitting in the heat exchanger, then cold while the burner relights and comes back up. It is inherent to the format. Recirculation and small buffer tanks reduce it; nothing removes it.