A water heater size calculator helps you determine the ideal tank capacity or tankless flow rate for your household. By factoring in the number of people, bathrooms, and which fixtures you run simultaneously, this tool recommends the right water heater size, compares tank vs tankless options, and estimates annual operating costs by fuel type.
Household Details
Water Heater Type & Fuel
Sizing Details
Tankless BTU = GPM × 500 × Temperature Rise
Tank vs Tankless Comparison
| Feature | Tank | Tankless |
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Estimated Annual Operating Cost
Based on average household hot water usage for your household size
Cost estimates are based on national average fuel prices and typical household usage. Actual costs vary by region, utility rates, climate, and usage habits.
How to Use the Water Heater Size Calculator
Choosing the right water heater size is critical for comfort and efficiency. An undersized unit runs out of hot water during peak demand, while an oversized one wastes energy heating water you never use. This calculator analyzes your household's specific needs to recommend the ideal tank capacity or tankless flow rate.
Step 1: Enter Your Household Details
Select how many people live in your home and how many bathrooms you have. These factors determine your baseline hot water demand. A household of 3-4 people typically uses 50-60 gallons during the peak hour (usually mornings), while a couple may only need 30-40 gallons.
Step 2: Select Your Peak Fixtures
Check all the hot water fixtures that might run at the same time during your busiest hour. For example, if someone showers while the dishwasher runs, check both. This is especially important for tankless sizing, which must deliver enough flow (GPM) to handle all simultaneous demand without a temperature drop.
Step 3: Choose Heater Type and Fuel
Select "Both" to compare tank and tankless side by side. Choose your fuel type to get accurate operating cost estimates. Natural gas is the most common and usually cheapest to operate. Electric is simpler to install. Propane is common in rural areas. Solar with electric backup offers the lowest long-term operating cost.
Step 4: Review Your Results
The calculator shows your recommended tank size in gallons, tankless flow rate in GPM, BTU rating needed, and first-hour rating. The comparison table breaks down upfront cost, efficiency, lifespan, and other factors to help you decide between tank and tankless. Annual cost estimates let you compare fuel types and calculate long-term savings.
Understanding First-Hour Rating
The first-hour rating (FHR) is the most important spec when shopping for a tank water heater. It measures how many gallons the unit can deliver in the first hour when starting with a fully heated tank. Look for a water heater with an FHR that meets or exceeds your peak hour demand. The FHR is always higher than the tank capacity because the burner or element continues heating fresh water as hot water is drawn out.
Frequently Asked Questions
Is this water heater sizing calculator really free?
Yes, completely free with no signup required. All calculations run locally in your browser — no data is sent to any server. Use it as many times as you need to compare tank vs tankless options and different fuel types.
Is my data safe when using this tool?
Absolutely. Everything runs entirely in your browser using JavaScript. No household information or personal data is transmitted anywhere. You can even use it offline once the page loads.
How do I know what size water heater I need?
Water heater size depends on your household size and peak demand. For tank heaters, a family of 3-4 typically needs a 50-60 gallon tank. For tankless, you need to calculate the total GPM of all fixtures running simultaneously and ensure the unit can deliver that flow rate with the required temperature rise.
What is a first-hour rating?
The first-hour rating (FHR) measures how many gallons of hot water a tank water heater can deliver in the first hour of use, starting with a full tank. It accounts for both stored hot water and the recovery rate. Your FHR should meet or exceed your peak hour demand.
Should I choose a tank or tankless water heater?
Tank heaters cost less upfront ($500-1,500) and are simpler to install. Tankless units cost more ($1,000-3,500 installed) but last 20+ years, save 20-30% on energy, and provide unlimited hot water. Tankless works best for households with moderate simultaneous demand.
How do I calculate BTU for a tankless water heater?
The formula is BTU = GPM x 500 x Temperature Rise. Temperature rise is the difference between incoming cold water (typically 50 degrees F) and desired hot water temperature (typically 120 degrees F), giving a 70 degree F rise. A 3 GPM unit needs about 105,000 BTU.
Which fuel type is cheapest for water heating?
Natural gas is typically the most cost-effective for water heating, with annual costs 30-50% lower than electric. Heat pump water heaters (electric) are very efficient but have higher upfront costs. Propane falls between gas and electric in operating cost. Solar is cheapest long-term but has the highest installation cost.