Water Heater Sizing Calculator

Input the number of people living in your home as the primary sizing factor. Each person typically uses 10-15 gallons of hot water daily for showers, handwashing, and cooking. Larger households need proportionally bigger tanks or higher GPM tankless units to meet combined demand. Consider both permanent residents and frequent guests when calculating occupancy.

Enter the number of full bathrooms in your home. More bathrooms indicate higher potential for simultaneous hot water use during morning and evening peak times. Three bathrooms suggest possible scenarios where multiple showers run concurrently, requiring larger capacity or faster recovery. This factor helps determine peak hour demand rather than just total daily usage.

Choose between traditional tank (storage) or tankless (on-demand) water heaters. Tank heaters size by gallon capacity and recovery rate, while tankless units size by GPM flow rate and temperature rise capability. Each technology requires different sizing approaches and offers distinct advantages. Tank heaters cost less upfront but tankless provides endless hot water with higher efficiency ratings.

Select low, moderate, or high usage based on your household habits. Long showers, frequent laundry loads, and running dishwashers multiple times daily classify as high usage. Water-conscious families with quick showers and efficient appliances fall into low usage. This multiplier adjusts base calculations to reflect actual consumption patterns beyond simple occupant count.

The calculator provides tank capacity in gallons for storage heaters or required flow rate in GPM for tankless units. Results include first hour rating showing peak capacity, recovery rate indicating how fast the system reheats, and recommended energy type. Verify the FHR meets your household's peak hour demand calculated from simultaneous fixture usage during busy morning periods.

Undersized water heaters run out during peak morning demand when multiple family members shower back-to-back or when showers overlap with dishwashers and laundry. A 40-gallon tank serves 2 people adequately but leaves a family of 4 waiting for reheating between showers. Proper sizing ensures adequate hot water supply for your household's peak hour needs, eliminating frustrating temperature drops mid-shower.

Oversized tanks keep more water hot than needed, wasting energy around the clock through standby heat loss. An 80-gallon heater costs 15-20% more annually to operate than a properly sized 50-gallon unit for the same household. Right-sizing reduces utility bills and environmental impact while maintaining comfort. Every gallon of unnecessary capacity adds to your monthly energy costs year after year.

First hour rating indicates how many gallons of hot water the heater can supply in one hour starting with a full tank. This metric matters more than tank size alone because it accounts for recovery during use. Match FHR to your calculated peak hour demand for optimal performance. A 50-gallon tank with poor recovery may have lower FHR than a 40-gallon unit with excellent recovery rate.

Gas water heaters recover 40-50 gallons per hour compared to 10-15 gallons for electric units. If you consistently use hot water throughout the day rather than concentrated morning peaks, recovery rate becomes more important than static tank capacity. Households with staggered schedules benefit from faster recovery, potentially allowing smaller tank sizes with high recovery rates to outperform larger tanks with slow recovery.