Water Heater Size Calculator
Use our free Water heater size Calculator for quick, accurate results. Get personalized estimates with clear explanations.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Peak Hour Demand = (Showers × 12) + (Dishwasher × 6) + (Laundry × 7) + (People × 2)
Calculate your peak hour hot water demand by adding up all hot water uses during your busiest hour. Each shower uses about 12 gallons, a dishwasher cycle 6 gallons, and a laundry load 7 gallons. Add 2 gallons per person for hand washing and miscellaneous use. Your water heater's First Hour Rating (FHR) should match or exceed this demand. Tankless sizing uses GPM (flow rate) and temperature rise.
Worked Examples
Example 1: Family of 4 — Morning Rush
Problem:4 people, 3 showers during peak hour, 1 dishwasher load, no laundry during peak. 60 deg F temp rise.
Solution:Peak demand:\n Showers: 3 × 12 = 36 gallons\n Dishwasher: 1 × 6 = 6 gallons\n Hand washing: 4 × 2 = 8 gallons\n Total: 50 gallons\nRecommended tank: 50 gallons\nTankless: 7.5 GPM needed
Result:50 gallon tank | 7.5 GPM tankless | FHR: 50 gal
Example 2: Couple — Low Usage
Problem:2 people, 1 shower during peak, no dishwasher, no laundry. 50 deg F temp rise.
Solution:Peak demand:\n Showers: 1 × 12 = 12 gallons\n Hand washing: 2 × 2 = 4 gallons\n Total: 16 gallons\nRecommended tank: 30 gallons\nTankless: 2.5 GPM needed
Result:30 gallon tank | 2.5 GPM tankless | FHR: 16 gal
Frequently Asked Questions
What size water heater do I need for my family?
General guidelines by household size: 1-2 people: 30-40 gallons, 3-4 people: 40-50 gallons, 5+ people: 50-80 gallons. However, usage patterns matter more than household size. A family of 4 where everyone showers at different times may only need 40 gallons, while one where everyone showers within the same hour needs 50-65 gallons. Focus on first hour rating (FHR) — it should match or exceed your peak hour demand.
Should I get a tank or tankless water heater?
Tank water heaters: lower upfront cost ($400-1,500 installed), simpler maintenance, work during power outages (gas models), but higher energy costs and limited hot water supply. Tankless water heaters: higher upfront cost ($1,000-3,500 installed), 20-30% energy savings, unlimited hot water supply, 20+ year lifespan vs 10-15 for tanks, but may require gas line or electrical upgrades. Tankless is better for small households or high-demand situations.
What is the right temperature for a water heater?
The DOE recommends setting your water heater to 120 degrees F. This prevents scalding (especially important with children), saves 3-5% on energy costs compared to 140 degrees F, and reduces mineral buildup. However, 140 degrees F better kills bacteria (including Legionella) and is preferred if you have a dishwasher without a booster heater. For each 10 degrees F reduction, you save about 3-5% on water heating costs.
How do I calculate temperature rise for a tankless water heater?
Temperature rise = desired output temperature minus incoming water temperature. Average incoming water temperatures by region: Northern US: 35-45 degrees F, Central US: 50-60 degrees F, Southern US: 60-75 degrees F. If you want 120 degrees F output and your incoming water is 50 degrees F, the temperature rise is 70 degrees F. Higher temperature rise requirements need a more powerful tankless unit. Check your local groundwater temperature for accurate calculations.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy