Tankless Water Heater Size Calculator
Determine the ideal size of your tankless water heater based on household size, temperature rise, and flow rate.
List of the Best Tankless Water Heater:
How to Use This Tankless Water Heater Size Calculator
This tankless water heater size calculator is designed to help homeowners, plumbers, and contractors determine the correct size of a tankless unit based on water demand and temperature rise. Tankless water heaters provide hot water on demand, which means sizing your unit correctly is crucial to ensure a steady, uninterrupted hot water supply.
Why Size Matters for Tankless Water Heaters
Unlike traditional tank water heaters that store and heat a limited amount of water, tankless units heat water as it flows through the device. This means the size of a tankless water heater is primarily determined by the number of fixtures it needs to supply hot water to at the same time and how much the incoming water temperature must be raised. A tankless water heater that is too small will lead to low water pressure, temperature drops, and poor performance during peak demand.
Understanding Flow Rate (GPM)
Flow rate, measured in gallons per minute (GPM), represents the total volume of hot water your household will require at any given time. Each fixture or appliance that uses hot water has a specific GPM rating. For instance, a shower may use 2.5 GPM, a dishwasher around 1.5 GPM, and a washing machine approximately 2 GPM.
To properly size your tankless water heater, calculate the combined GPM for all fixtures and appliances that could be used simultaneously. This calculator simplifies the process by letting you enter the number of bathrooms and hot water appliances, then estimating the required total GPM for you.
| Fixture | Average Flow Rate (GPM) |
|---|---|
| Shower | 2.0–2.5 |
| Bathtub | 4.0 |
| Bathroom Sink | 0.5–1.0 |
| Kitchen Sink | 1.0–1.5 |
| Dishwasher | 1.5 |
| Washing Machine | 2.0–2.5 |
What is Temperature Rise?
Temperature rise is the difference between the incoming groundwater temperature and your desired hot water temperature. For example, if the incoming water is 50°F and you want a shower at 120°F, the unit needs to raise the temperature by 70°F. The greater the temperature rise required, the lower the GPM capacity of your tankless water heater will be. This is why climate and ground temperature play a big role in sizing your unit.
Incoming Water Temperature by Region (U.S. Average)
| Region | Average Incoming Temp (°F) |
|---|---|
| Northern States (e.g., MN, WI) | 40–50°F |
| Central States (e.g., KS, IL) | 50–60°F |
| Southern States (e.g., TX, FL) | 60–70°F |
| Western States (e.g., CA, AZ) | 55–65°F |
How to Calculate Tankless Water Heater Size
This tankless water heater size calculator works by taking the number of bathrooms and appliances, estimating a total GPM, and combining that with the required temperature rise. The result is a recommendation based on standard sizing practices. For example, a home with two bathrooms and three simultaneous hot water appliances might need a unit that delivers at least 8 GPM with a 70°F temperature rise.
Basic Sizing Rules of Thumb
- 1 bathroom = 2.5 GPM
- 1 appliance (dishwasher, laundry, etc.) = 1.5 GPM
- Target output temperature: 120°F (recommended)
Recommended Tankless Water Heater Sizes
Based on your total hot water demand and local climate, you’ll need to choose a tankless unit with sufficient flow capacity at the needed temperature rise:
| Total Flow Rate (GPM) | Temperature Rise | Recommended Unit Size |
|---|---|---|
| 4–5 GPM | 60–70°F | Small Unit |
| 6–8 GPM | 60–70°F | Medium Unit |
| 8+ GPM | 60–70°F | Large Unit / Dual System |
Frequently Asked Questions
Can a tankless water heater support an entire house?
Yes, if sized correctly. Whole-house tankless water heaters are commonly installed and can handle the hot water needs of large homes, provided the GPM capacity and temperature rise are matched to your demand.
What size tankless water heater do I need for a family of 4?
Most families of 4 require 6–8 GPM depending on climate and how many fixtures may be used at once. Use this tankless water heater size calculator for an exact recommendation based on your needs.
Can I use more than one tankless unit?
Yes, high-demand households often install two or more tankless units in parallel. This setup is also used for zoning (e.g., one for upstairs, one for downstairs).
What’s better: gas or electric tankless water heaters?
Gas tankless units offer higher flow rates and are better for whole-house applications. Electric models are more energy-efficient and suitable for point-of-use or small homes.
Does a tankless water heater require maintenance?
Yes, regular descaling (every 6–12 months) is necessary to prevent mineral buildup, especially in hard water areas. This ensures efficient operation and extends unit life.
Installation Tips & Best Practices
- Check your gas line or electric panel capacity before choosing a unit
- Place the heater close to the point of use to reduce lag time
- Use a water softener in hard water areas to prevent scale buildup
- Hire a licensed plumber or HVAC contractor for installation
- Install isolation valves for easy maintenance
Conclusion: Get the Right Size the First Time
Using this tankless water heater size calculator can save you from costly mistakes, cold showers, and wasted energy. With the right size unit, you'll enjoy endless hot water exactly when and where you need it. Whether you're retrofitting a home or building new, accurate sizing is the key to comfort, efficiency, and satisfaction.
Bookmark this page and use it anytime you need to estimate tankless water heater size — whether for yourself, your clients, or your next home improvement project.