Tankless Water Heater Size Calculator
Simultaneous fixtures and groundwater to tankless GPM and rise.
What it does
Tankless units never store hot water; they heat what flows. Two numbers pick the box: peak gallons per minute and temperature rise from groundwater to 120 F. Miss either and showers swing cold in January.
The tool above adds simultaneous fixtures at 2.0 gpm per shower and 1.5 per other tap, then subtracts groundwater from the 120 F target. Metric mode mirrors it in liters per minute and Celsius. The unit note carries both figures to the maker chart.
DOE sizing guidance teaches exactly this two-number method: estimate simultaneous demand, then correct for groundwater zones that run cold in the north and mild in the south. EPA WaterSense fixture rates anchor the per-fixture flows, since stamped low-flow heads draw less than old ones.
Plan with winter groundwater, not the yearly average. A unit that sings in August can starve in February when rise doubles. When the math lands between sizes, step up and confirm gas, vent, and electrical needs with a pro.
How to use
- Pick imperial (gpm, F) or metric (L/min, C).
- Count showers running at the same time.
- Count other hot fixtures running with them.
- Enter local groundwater temperature.
- Read peak flow, rise, and the minimum rating.
- Match both numbers on maker charts, then round up.
Formula
- showers
- Showers running at once.
- others
- Other hot fixtures running at once, 1.5 gpm each.
- ground
- Incoming groundwater temp in F (C metric).
Metric uses 7.6 L/min per shower and 5.7 per other fixture with a 49 C target. Counts mean simultaneous use, not every fixture in the house. Makers rate units as flow at a stated rise, so carry both numbers to the chart.
Worked example
Example: 2 showers, 50 F ground
Two showers and one faucet together, 50 F groundwater.
- Two showers plus one other fixture running together.
- Flow: 2 x 2.0 + 1 x 1.5 = 5.5 gpm.
- Groundwater is 50 F, target 120 F.
- Rise: 120 - 50 = 70 F.
- Shop: a unit rated at least 5.5 gpm at 70 F rise.
Reference table
| Zone | Groundwater | Rise |
|---|---|---|
| Deep south | About 65 F | About 55 F |
| South | About 57 F | About 63 F |
| Central | About 50 F | About 70 F |
| North | About 42 F | About 78 F |
| Far north wells | About 35 F | About 85 F |
Limits
- Planning only. Gas, vent, and panel limits need a pro.
- Counts simultaneous use; households vary by habit.
- Well temps swing seasonally; use winter lows.
- Hard water derates real output over time.
Common mistakes
- Adding every fixture instead of simultaneous ones.
- Sizing by GPM while ignoring rise.
- Using summer groundwater for a winter decision.
- Trusting one number on a two-number chart.
- Skipping descaling in hard water.
Try it yourself
Quick checks. Answers sit tucked under each question.
Demand for 3 showers plus 2 fixtures at 40 F ground?
Flow is 3 x 2.0 + 2 x 1.5 = 9.0 gpm. Rise is 120 - 40 = 80 F.
Demand for 3 showers plus 2 fixtures at 5 C ground?
Flow is 3 x 7.6 + 2 x 5.7 = 34.2 L/min. Rise is 49 - 5 = 44 C.
Questions
How is peak GPM figured?
Add the simultaneous fixtures: about 2.0 gpm per shower and 1.5 per faucet in this planner. Real low-flow heads vary, so read the stamped rate and rerun with your numbers.
What is temperature rise?
Target minus groundwater: 120 F minus the incoming temp. Northern winters near 40 F need an 80 F rise; warm states near 60 F need only 60 F. Rise decides the unit class more than flow does.
Why do two same-GPM units differ?
Flow and rise together. A box rated 8 gpm at a 35 F rise may only give 4 gpm at 70 F. Match both numbers on the maker chart, then round up for winter lows.
Do low-flow heads change the answer?
Yes. Standard heads near 2.0 gpm; WaterSense heads near 1.8 or lower. Lower flow trims peak demand and can drop the install a size class.
Gas or electric: does sizing differ?
Gas needs venting and gas line capacity; electric needs heavy amperage and possibly a panel upgrade. Size the energy side with a pro after the GPM math.
How do I keep it performing?
Descale yearly in hard water zones, flush per the maker guide, and check inlet filters. Scale steals both flow and rise on the coldest day.