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Water Heater Sizing Calculator - First Hour Rating & Tank Size

Calculate Hot Water Demand, Recovery Rate, and Compare Energy Costs by Fuel Type

Free water heater sizing calculator for homeowners, plumbers, and HVAC contractors. Enter your household size, fixture count, and peak usage pattern to calculate first-hour rating (FHR), required recovery rate, and recommended tank size. Compare energy costs across gas, electric, propane, and heat pump water heaters. Supports both tank and tankless sizing with GPM demand calculations for on-demand systems.

Pro Tip: The most common water heater sizing mistake is looking only at tank capacity. A 50-gallon tank with a low recovery rate runs out of hot water faster than a 40-gallon tank with a high recovery rate. What matters is the first-hour rating, which combines stored volume with recovery capacity. A gas water heater with a 40,000 BTU burner recovers roughly 40 gallons per hour, while a standard 4,500-watt electric element recovers only about 21 gallons per hour. That recovery gap is why a 50-gallon electric tank often runs out before a 40-gallon gas tank.

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Read the complete water heater sizing guide

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How It Works

  1. Enter Household Profile

    Input the number of occupants, bathrooms, and typical usage pattern (low, average, high). The calculator uses DOE usage tables to estimate daily hot water demand in gallons.

  2. Specify Fixture Details

    Add your specific fixtures: showerheads with flow rate, bathtub volume, dishwasher and clothes washer hot water usage. Overriding defaults with actual fixture specs improves accuracy significantly.

  3. Define Peak Demand Period

    Select your peak-hour scenario: everyone showering in the morning, back-to-back baths, or running the dishwasher during showers. The calculator determines the worst-case first-hour demand to ensure the heater can handle the peak.

  4. Select Fuel Type

    Choose from natural gas, propane, electric resistance, or heat pump. Enter your local fuel price. The calculator shows the estimated annual operating cost and recovery rate for each fuel type.

  5. Review Sizing Recommendation

    See the recommended tank size, minimum first-hour rating, and recovery rate needed to meet your demand profile. For tankless systems, see the required GPM flow rate and minimum BTU input needed for your groundwater temperature.

Built For

  • Homeowners replacing a water heater who need to know what size to buy for their household
  • Plumbers sizing water heaters for new construction to meet peak demand without oversizing
  • HVAC contractors comparing gas, electric, and heat pump water heater options for customers
  • Property managers standardizing water heater sizes across apartment units based on occupancy
  • Builders selecting water heaters for spec homes that will pass plumbing inspection and satisfy buyers
  • Energy auditors recommending water heater upgrades based on fuel cost comparisons and recovery rate analysis
  • Homeowners considering tankless conversion who need to verify their peak GPM demand is achievable

Features & Capabilities

First-Hour Rating Calculation

Calculates the required first-hour rating based on your peak usage scenario. FHR combines the stored hot water volume with the recovery capacity during the first hour, which is the number that should match between your demand and the water heater spec sheet.

Recovery Rate Analysis

Shows the recovery rate in GPH for different fuel types and BTU inputs. A 40,000 BTU gas burner recovers approximately 40 GPH with a 70-degree rise. A 4,500W electric element recovers about 21 GPH. This explains why gas heaters use smaller tanks for the same household.

Tankless GPM Sizing

For tankless water heaters, calculates the required flow rate in GPM based on simultaneous fixture use, and the BTU input needed to achieve the desired temperature rise at your groundwater temperature. Cold-climate homes with 40-degree groundwater need more BTUs than warm-climate homes at 70 degrees.

Energy Cost Comparison

Side-by-side annual operating cost comparison across gas, propane, electric, and heat pump water heaters using your local energy prices. Includes the effect of energy factor (EF) or uniform energy factor (UEF) on actual operating cost.

DOE Sizing Tables

Uses Department of Energy guidelines for estimating hot water demand by household size and usage pattern. Defaults are based on national averages but can be overridden with actual fixture data for more accurate results.

Temperature Rise Calculator

Enter your groundwater temperature and desired delivery temperature to calculate the temperature rise needed. This is critical for tankless sizing because BTU output determines GPM at a given temperature rise.

Frequently Asked Questions

A family of four with average usage typically needs a first-hour rating of 60-80 gallons. For gas water heaters, this usually means a 40-50 gallon tank with a 40,000+ BTU burner. For electric, a 50-65 gallon tank is common because the recovery rate is slower. If your morning routine involves back-to-back showers plus the dishwasher, size toward the high end. If your usage is spread throughout the day, a smaller unit may work. The calculator accounts for your specific fixture layout and usage pattern.
First-hour rating (FHR) is the number of gallons of hot water the heater can deliver in the first hour of use, starting with a full tank at setpoint temperature. It combines storage capacity with recovery rate. A 50-gallon tank with a slow recovery might have an FHR of 60 gallons, while a 40-gallon tank with a powerful burner might have an FHR of 70 gallons. When shopping for a water heater, match the FHR on the EnergyGuide label to your calculated peak-hour demand.
Tankless sizing depends on two numbers: the maximum simultaneous flow rate in GPM and the temperature rise needed. A typical shower uses 2-2.5 GPM. Running two showers simultaneously requires 4-5 GPM capacity. The temperature rise is the difference between your incoming groundwater temperature and the desired delivery temperature (usually 120 degrees F). In northern states with 40-degree groundwater, you need an 80-degree rise. Most residential gas tankless units deliver 5-8 GPM at a 35-degree rise but only 3-4 GPM at an 80-degree rise. This is why cold-climate tankless installations sometimes disappoint.
Heat pump water heaters use about one-third the electricity of conventional electric units, saving $200-400 per year in most climates. The upfront cost premium is $500-1,000 over a standard electric unit. With a 3-5 year payback, they make financial sense in most situations where the unit is installed in a space with adequate air volume (at least 700 cubic feet of surrounding air). They do cool the surrounding air by 2-5 degrees, which is a benefit in summer but a liability in winter in conditioned spaces.
Run a cold water tap for two minutes and measure the temperature with a kitchen thermometer. Groundwater temperature correlates roughly with annual average air temperature. In the northern US, expect 40-55 degrees F. In the south, 60-75 degrees F. This number matters most for tankless sizing because it determines the temperature rise the unit must achieve. You can also check your local water utility reports, which sometimes publish average supply water temperatures.
Disclaimer: This calculator provides water heater sizing estimates for planning purposes. Actual hot water demand varies with fixture types, usage patterns, and household behavior. Water heater installation must comply with local plumbing and building codes. Consult a licensed plumber for final sizing, installation, and code compliance. ToolGrit is not responsible for equipment selection or installation outcomes.

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