Tankless Water Heater Sizing Calculator
Find the exact GPM and BTU/hr you need based on simultaneous fixture use and your climate zone.
| Fixture | Typical GPM | Hot Water % | Effective GPM |
|---|---|---|---|
| Shower | 1.5 – 2.5 (std 2.0) | 80% | 1.60 |
| Kitchen Faucet | 1.5 – 2.2 (std 1.8) | 50% | 0.90 |
| Bathroom Faucet | 0.5 – 1.5 (std 1.0) | 50% | 0.50 |
| Dishwasher | 1.0 – 1.5 (std 1.2) | 100% | 1.20 |
| Clothes Washer | 1.5 – 3.0 (std 2.0) | 25% | 0.50 |
| Bathtub Fill | 3.0 – 5.0 (std 4.0) | 70% | 2.80 |
| Utility Sink | 1.5 – 2.5 (std 2.0) | 60% | 1.20 |
| Size Class | GPM Capacity | BTU/hr (Gas) | kW (Electric) | Typical Application |
|---|---|---|---|---|
| Point-of-Use | 0.5 – 2.0 | 19,000 – 40,000 | 3 – 6 kW | Single fixture, handwashing |
| Small Whole-Home | 2.0 – 5.0 | 40,000 – 100,000 | 8 – 13 kW | Studio, 1-bath, mild climate |
| Medium Whole-Home | 5.0 – 8.0 | 100,000 – 160,000 | 14 – 24 kW | 2-bath home, moderate climate |
| Large Whole-Home | 8.0 – 11.0 | 160,000 – 220,000 | 24 – 36 kW | 3-bath home, cold climate |
| High-Capacity | 11.0 – 14.0 | 220,000 – 280,000 | 36 – 48 kW | Large family, peak demand |
| Commercial / Cascaded | 14.0+ | 280,000+ | 48+ kW | Multi-unit, small office, cascaded |
| Climate Zone | Region | Typical Inlet Temp | Temp Rise to 110°F |
|---|---|---|---|
| Cold | Northern US, Canada, Mountain West | 40 – 45°F | 65 – 70°F rise |
| Moderate | Midwest, Mid-Atlantic, Pacific NW | 50 – 55°F | 55 – 60°F rise |
| Warm | Southern US, California | 60 – 65°F | 45 – 50°F rise |
| Hot | Florida, Desert Southwest | 70 – 77°F | 33 – 40°F rise |
Choosing the right size tankless water heater is very serious. If the device is too small for several fixtures at once, only some of them will have full flow. Then the users will notice that the water pressure drops.
Because of that irritation, right sizing is important
How to Choose the Right Size Tankless Water Heater
You measure tankless water heaters by the maximum temperature they can reach at a set flow rate. So, to choose the size, you must count the necessary flow and temperature for your target, whether it is the whole house or only one place like the bathroom. The higher the BTU rating, the bigger flow the device can prodcue.
To find the right model, use the highest needed flow and temperature.
A typical home usually requires 6 to 8 GPM to use a shower and tap at once. Guessing commonly leads to cold water. According to standard guidelines, you count around 2.5 gallons for a shower, 1.5 for a sink, and so on.
You add those values to find your total peak gallons each minute.
Unlike tanks, tankless water heaters do not have “recovery”; they must be chosen exactly according to the flow rate. Usually, gas models can raise the temperature by 70 degrees at a flow of 5 gallons each minute, while electric models do that at 2 gallons. It is very serious to know the temperature of the cold inlet water during the coldest day in your region.
For instance, if you require 4 GPM in Florida, the minimal temperature rise is 35 degrees. That means you require a heater capable of 4 GPM at that temperature. You also can look at your water bills of the summer to estimate the usage, because then you most commonly bathe and shower.
Electrical models require big power. Most require 120 to 180 amps to operate well and give warm water to several outlets. If your electrical setup is not mighty enough, you could consider a gas heater or a heat pump.
For instance, some person chose between models of 3.75 and 4.68 gallons each minute; he liked the smaller one because of the electrical demands, but wanted to ensure that it will suffice.
Tankless water heaters also save a lot of space, which is a big reason why folks choose them. Online calculators can help determine the size, considering the number of simultaneous spots and the temperatures. Be smart and plan for the future, especially if the family will grow.
