Peak Demand Inputs
Tank / First Hour Rating

Fixtures used during your home's single busiest hour.

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Tankless / On-Demand

Fixtures running at the exact same moment.

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Climate Settings
°F
Sizing Results
Tank Style
Required First Hour Rating
--
gallons / first hour
Recommended Tank Capacity
--
Tankless / On-Demand
Required Flow Rate
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gallons per minute
At a Temperature Rise of
-- °F
Key Terms

Key Terms Explained

First Hour Rating (FHR)
The number of gallons of hot water a tank heater can deliver in the first hour of use, starting with a full tank. It combines stored volume with burner recovery speed and is the key metric for sizing a tank unit.
Gallons Per Minute (GPM)
The flow rate a tankless heater must sustain to supply all active fixtures simultaneously. Each fixture adds its demand to the total GPM requirement the unit must meet at your temperature rise.
Temperature Rise
The difference between your incoming groundwater temperature and your desired hot water delivery temperature. A northern home with 40 degree groundwater targeting 120 degrees needs a rise of 80 degrees, which demands more output from any tankless unit.
Peak Hour Demand (PHD)
The total gallons consumed during the single busiest hour in your home. Showers, baths, dishwashers, and washing machines each consume a standard gallon amount. PHD equals your required FHR.
Groundwater Temperature
The temperature of water entering your home from the municipal supply or well. It varies by geography - colder in the north, warmer in the south - and directly determines how hard a water heater must work.
Recovery Rate
The speed at which a tank heater can reheat a depleted tank, measured in gallons per hour. A high recovery rate lets a smaller tank perform like a larger one by replenishing hot water faster.
Condensing vs. Non-Condensing
Condensing tankless heaters extract heat from exhaust gases for extra efficiency (EF above 0.90). Non-condensing units are less expensive but exhaust more heat. Condensing units produce acidic condensate that must be neutralized before draining.
Standby Heat Loss
The energy a tank heater wastes continuously reheating stored water to maintain temperature when no hot water is being used. Tankless heaters eliminate standby loss entirely, which is their primary efficiency advantage.
Complete Guide

The Complete Guide to Water Heater Sizing

A water heater that is too small means cold showers and morning arguments. One that is too large wastes energy 24 hours a day heating water nobody is using. This calculator gives you two parallel answers: the First Hour Rating your tank needs and the GPM your tankless unit must deliver, both tailored to your actual household habits and climate.

How to Use This Calculator

  • 1In the Tank panel, enter the fixtures used during your home's single busiest hour - typically a weekday morning with overlapping showers and appliances.
  • 2In the Tankless panel, enter fixtures running simultaneously at the same instant - the true simultaneous peak, not spread over an hour.
  • 3Select your incoming water temperature. Use Northern US (40 degrees F) for cold-climate states, Central US (55 degrees F) for the middle of the country, and Southern US (70 degrees F) for warm-climate states.
  • 4Set your target delivery temperature. 120 degrees F is the standard recommendation balancing scalding safety and Legionella prevention.
  • 5Read both output cards. The left card shows the FHR and recommended tank gallon capacity. The right card shows the required GPM and the temperature rise the tankless unit must achieve.

How the Tank Calculation Works

The tank calculation uses industry-standard gallon-per-use values: a shower consumes 20 gallons, a bath 20 gallons, a dishwasher cycle 6 gallons, and a warm-water washing machine load 14 gallons. Summing all peak-hour usage gives your Peak Hour Demand (PHD), which equals the First Hour Rating your tank heater must carry. The recommended tank size is then bucketed into standard production capacities: under 40 gallons PHD maps to a 30-40 gallon tank, 40-59 gallons maps to a 50 gallon unit, 60-80 maps to a 65 gallon unit, and above 80 gallons maps to an 80 gallon unit.

Note that FHR is not the same as tank capacity. A 50-gallon tank with a powerful burner can have an FHR of 60-70 gallons because it begins recovering while you are still drawing hot water. When comparing models, always compare FHR numbers, not just tank gallon labels.

How the Tankless Calculation Works

The tankless calculation assigns standard flow rates to each fixture type: a shower runs at 2.5 GPM, a sink faucet at 1.5 GPM, and an appliance (dishwasher, washing machine) at 2.0 GPM. The total of all simultaneous fixtures is your required peak flow rate - the GPM the tankless unit must sustain without dropping temperature.

The temperature rise shown is the delta between your incoming groundwater temperature and your target delivery temperature. Tankless manufacturers publish output charts showing how GPM drops as temperature rise increases. A unit rated at 8 GPM at a 35-degree rise may only deliver 4.5 GPM at an 80-degree rise. Always verify the unit's rated GPM at your specific temperature rise before purchasing.

Tank vs. Tankless: Which Is Right for Your Home?

Tank water heaters cost less upfront, are compatible with all fuel types (gas, electric, propane, heat pump), and require no minimum flow rate to activate. They are the right choice for households with variable usage patterns or where installation simplicity matters. Their downsides are standby heat loss and a finite supply of hot water before recovery time is needed.

Tankless heaters deliver endless hot water as long as demand stays within their GPM rating, eliminate standby loss, and last 5-10 years longer on average than tank units. Their downsides are higher purchase cost, potential need for gas line upsizing, and reduced effective GPM in cold climates. Households in northern states with high simultaneous demand often find the required tankless unit is large and expensive to install properly.

Tips for Accurate Sizing

For the tank panel, think honestly about your peak hour - not your average morning, but your worst morning (guests visiting, everyone showering before work, a dishwasher running). For the tankless panel, think about the exact moment with the most simultaneous fixtures: two showers running while someone washes dishes and a faucet is left on. These are different scenarios and explain why the two calculators can yield very different answers for the same household.

Frequently Asked Questions

First Hour Rating (FHR) is the number of gallons of hot water a tank-style heater can supply in one hour starting with a full tank. It combines the stored volume with the burner recovery rate. FHR matters because the tank needs to meet the demand of your busiest single hour - a morning routine with two showers, a bath, a dishwasher, and a load of laundry, for example. Matching FHR to your Peak Hour Demand prevents cold showers and ensures your tank can keep up with real-world use.
A tankless heater heats water from the incoming groundwater temperature up to your target delivery temperature. The difference is called Temperature Rise. In the northern US, groundwater can be as cold as 40 degrees Fahrenheit, requiring a rise of 80 degrees to reach 120 degrees. In the southern US, groundwater may already be 70 degrees, requiring only a 50-degree rise. A higher temperature rise demands more BTU output from the unit, which reduces the maximum GPM it can deliver simultaneously. That is why a unit rated at 7 GPM in Florida may only supply 4 GPM in Minnesota.
No. Oversizing a tank wastes energy through standby heat loss - the tank constantly reheats stored water to maintain temperature even when nobody is using it. A correctly sized tank is more efficient and cheaper to operate. That said, if your household has unpredictable usage spikes or you plan to add occupants, sizing up by one tier (for example from 50 to 65 gallons) can provide useful buffer without significant waste.
Yes, but only if the unit's GPM rating meets or exceeds your simultaneous peak flow. A standard shower uses about 2.5 GPM. Two simultaneous showers require at least 5 GPM from the tankless unit at your home's specific temperature rise. Many residential tankless units are rated for 6 to 10 GPM, but that rating is usually specified at a moderate temperature rise. In colder climates with 80 degrees or more of required rise, effective GPM can drop by 30 to 50 percent. Always check the manufacturer's output chart at your actual temperature rise before purchasing.