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Identify the busiest hour of hot water use — typically morning showers plus dishwasher or laundry. Sum the gallons used by each fixture during that hour based on flow rate and duration.
Peak demand (gal) = Σ (Fixture GPM × Duration × Simultaneity factor)FHR is the gallons of hot water a tank delivers in the first hour starting full. It equals tank capacity times recovery factor plus tank volume at usable temperature.
FHR = Tank volume × (0.7) + Recovery rate × 60 minSelect a tank whose FHR meets or exceeds peak hour demand. Recovery rate depends on burner BTU input and temperature rise from incoming cold water.
Recovery (GPH) = BTU input × Efficiency / (8.33 × ΔT)Updated: July 2026
Two showers at 2.5 GPM × 10 min = 50 gallons peak demand in the morning hour. A 50-gallon gas tank with 67 GPH recovery and ~70 FHR handles this with margin.
One shower plus kitchen use peaks at ~25 gallons in an hour. A 30–40 gallon tank with adequate recovery is sufficient — oversizing to 50 gallons adds unnecessary standby heat loss.
Five people with three bathrooms and laundry during peak hour may need 70–80 gallons demand. An 80-gallon tank or a tankless unit rated for 8+ GPM is appropriate.
Two adults who shower simultaneously need more FHR than four people who stagger showers over two hours. Map your actual peak hour fixture use, not just headcount.
A 40-gallon gas heater with 40 GPH recovery outperforms a 50-gallon electric with 22 GPH recovery during peak demand. Compare FHR, not just gallon capacity.
A water heater that is too small runs out of hot water during peak use; one that is too large wastes energy on standby losses. This calculator estimates the ideal tank size in gallons from household size, fixture demand, and peak simultaneous usage patterns.