Two quotes, two different units, $1,400 apart. One specs 7.5 gallons per minute, the other 11, and neither explains why. That is the usual starting point, and it makes sizing look like a judgment call you have to take on trust.
It is not. It is arithmetic you can do in about two minutes, and once you have your own number you can tell which quote fits your house.
The number on the box is not your number
A tankless heater stores nothing. It heats water as it passes through, so its capacity is measured in gallons per minute rather than gallons, and that figure only means something alongside a temperature rise: how far the unit has to lift the incoming water to reach your tap.
Warm water in means a small rise and high flow. Cold water in means a big rise and much lower flow. Manufacturers print the flattering condition on the box, usually a 35°F rise, because it produces the largest number.
The trade is close to linear, so you can convert any advertised figure to your own conditions:
GPM at your rise = (rated GPM × rated rise) ÷ your rise
A unit advertised at 9.8 GPM at a 35°F rise delivers roughly 5.7 GPM at a 60°F rise. Same unit, same house, honest number. That gap is where most disappointed tankless owners live.
Step 1: your peak, in gallons per minute
Household size is the wrong input. Four people showering one after another need less capacity than two people running a shower and a tub at once. You are sizing for your worst realistic minute.
| Fixture | Typical flow |
|---|---|
| Shower head | 1.5 to 2.5 GPM |
| Bathroom faucet | 0.5 to 1.5 GPM |
| Kitchen faucet | 1.0 to 2.2 GPM |
| Dishwasher | 1.0 to 1.5 GPM |
| Clothes washer | 1.5 to 3.0 GPM |
| Large soaking tub filler | 4.0 to 8.0 GPM |
Pick the fixtures that actually overlap on a weekday morning and add them. For most three bedroom, two bath homes here that is two showers, about 4 to 5 GPM. Add the kitchen sink and you are near 6. A large tub filler can exceed the rest of the house combined, which is why it drives so many sizing decisions on its own.
If you do not know what your shower uses, the flow rate is usually stamped on the head. Otherwise run it into a bucket for fifteen seconds and multiply the gallons by four.
Step 2: your temperature rise, in winter
Your rise is your target temperature minus whatever comes out of the cold tap in January. Size for winter. A unit sized for August will let you down in the same eight weeks every year.
Shower temperature is typically 105 to 110°F. Incoming water in the Cape Fear region is mild by national standards, and supply drawn from surface water swings further through the seasons than well water does. For winter design here, plan on a rise of roughly 60°F, against 70 to 80°F in the upper Midwest.
That works in your favor, but it is a smaller advantage than tankless marketing suggests. Sizing off a 35°F rating because the water here is warm is how people end up with a unit that is comfortable in October and thin in February.
Step 3: match the two
Gas units are sold by burner input in BTU. Worked out at a 60°F winter rise for a condensing unit, the common whole house sizes land here:
| Burner input | At 60°F rise (winter) | At 45°F rise (mild) |
|---|---|---|
| 140,000 BTU | 4.4 GPM | 5.9 GPM |
| 160,000 BTU | 5.1 GPM | 6.8 GPM |
| 180,000 BTU | 5.7 GPM | 7.6 GPM |
| 199,000 BTU | 6.3 GPM | 8.4 GPM |
Calculated figures for planning, not manufacturer ratings. Non condensing models deliver roughly 15 percent less at the same input, so a non condensing 199,000 BTU unit is closer to 5.4 GPM in winter. That is part of the price gap between the two types, not only the running cost.
Now compare. If your peak is 4.5 GPM, a 160,000 BTU condensing unit covers it with margin to spare, and an 11 GPM headline unit is capacity you are unlikely to reach.
Electric sizing is capped by your panel
Electric tankless is rated in kilowatts, and the conversion is less forgiving. A 27 kW unit draws about 112 amps and delivers roughly 3.0 GPM at a 60°F rise. A 36 kW unit draws about 150 amps for roughly 4.0 GPM.
Three gallons per minute is one strong shower. And 112 amps is most of a 200 amp service before the rest of the house is counted. The sizing looks fine at 35°F, and the panel will not carry the unit that covers a winter morning. Our guide to electric vs gas tankless covers the load calculation to run before pricing one.
What each mistake feels like
- Undersized. Hot water turns lukewarm the moment a second fixture opens, and it is worst in January. There is no adjustment that fixes it later.
- Oversized. Every tankless unit has a minimum activation flow, usually around 0.4 to 0.5 GPM. A large burner paired with low flow bathroom faucets can switch on and off during a draw, sending alternating warm and cold slugs down the line. You also paid for capacity you will not use.
- Right sized with one gap. If a soaking tub is the only fixture pushing you up a size, a second small unit for that bathroom is often cheaper than upsizing the whole house.
Four questions before you sign
- At what temperature rise is the GPM figure on this quote?
- What peak flow did you assume, and which fixtures does that cover?
- Is this unit condensing or non condensing?
- Do the existing gas line and vent support this size, and is any upgrade in the price?
A quote that cannot answer the first question was not sized, it was picked. For what installation involves locally, see our Wilmington tankless installation page, or call and describe the fixtures you run at once.
Frequently Asked Questions
What size tankless water heater do I need for a 2 bathroom house?
Two showers running at once is about 4 to 5 gallons per minute. In this region, sizing for a 60°F winter temperature rise, that points at a 160,000 to 180,000 BTU condensing gas unit. If the two showers are never used at the same time, 140,000 BTU covers it. The fixture count matters less than how many of them actually overlap.
Why does my tankless water heater go cold when the dishwasher starts?
Because the combined flow has passed what the burner can heat. The unit keeps supplying water, it just cannot raise all of it to temperature, so everything downstream goes lukewarm. It happens most in winter, when the same burner has a larger temperature rise to cover. This is what an undersized unit looks like in practice, and it cannot be corrected after installation without a larger unit.
Is a 199,000 BTU tankless unit overkill for a small home?
Often, yes. A large burner costs more, may need the gas line upsized to feed it, and brings no benefit if your peak demand is 3 gallons per minute. There is also a low flow drawback: a big unit paired with low flow bathroom faucets can cycle on and off, which produces alternating warm and cold slugs at the tap. Size to your peak, with a little margin, not to the largest unit available.
Can I install two tankless units instead of one large one?
Yes, and it is a common answer to a single oversized fixture. If a large soaking tub is the only thing that pushes you into the next unit size, a second smaller unit serving that bathroom is often cheaper than upsizing the whole house, and it shortens the hot water run at the same time. Units can also be linked in parallel for genuinely high demand homes.
Does a recirculation pump change what size I need?
It changes the specification more than the size. A recirculation loop asks the heater for small, frequent draws, which is the condition tankless units handle worst. If you want instant hot water at a far bathroom, choose a model built for recirculation, usually one with an internal buffer tank, and tell the installer at quoting time rather than adding the pump later.