How to Size a Water Heater: Tank Size vs. First Hour Rating

When a homeowner runs out of hot water halfway through a morning shower, their first instinct is to buy a bigger tank. But a massive tank isn't always the right answer. The real secret to hot water sizing lies in a metric called the First Hour Rating (FHR).

Tank Capacity Isn't Everything

A water heater's gallon capacity (e.g., 40 or 50 gallons) tells you how much hot water sits in reserve. But it does not tell you how fast the heater can replenish that water once you start draining it. If you have four people taking showers back-to-back, a 50-gallon tank will run out of water entirely if the recovery speed is too slow.

The Golden Metric: First Hour Rating (FHR)

The First Hour Rating (FHR) is the number of gallons of hot water the heater can supply in one continuous hour, starting with a full tank. It is calculated by adding the Tank Capacity to the Recovery Rate (how many gallons the burner can heat in 60 minutes).

• Peak Demand: You need an FHR that matches your household's peak hour of use. A standard shower uses about 20 gallons. Running a dishwasher uses about 10 gallons. If three people shower while the dishwasher runs, your peak demand is 70 gallons. Your heater's FHR must be 70 or higher.