Water Heater Size Calculator

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Water Heater Size Calculator

Determine the ideal capacity for your home's hot water needs

Storage Tank (Gallons) Tankless (GPM)
Number of showers running at the same time.
Dishwashers or laundry running during peak use.

Your Recommended Capacity:

How to Size Your Water Heater Correctly

Choosing the right water heater size is critical for home comfort and energy efficiency. An undersized unit leads to cold showers, while an oversized unit wastes money on energy and the initial purchase price.

Sizing Storage Tank Water Heaters (FHR)

For traditional tank systems, the most important metric isn't the total volume of the tank, but the First Hour Rating (FHR). The FHR is the number of gallons a water heater can supply in its first hour of operation (starting with a full tank).

Example Calculation: A household with 3 people might have a peak hour usage consisting of:

  • 3 Showers (approx. 10-15 gallons each) = 45 gallons
  • 1 Load of laundry (approx. 20 gallons) = 20 gallons
  • Total Required FHR: 65 Gallons

Sizing Tankless Water Heaters (GPM)

Tankless heaters provide hot water on demand and are sized based on Gallons Per Minute (GPM) flow rate rather than storage capacity. You must calculate the maximum number of fixtures you intend to run simultaneously.

Fixture Type Average GPM
Standard Shower2.5 GPM
Low-flow Shower1.5 – 2.0 GPM
Kitchen Sink1.5 GPM
Dishwasher1.0 – 2.0 GPM

Key Factors to Consider

  1. Inlet Water Temperature: If you live in a cold climate, your heater needs to work harder to raise the temperature of the incoming ground water. You may need a higher GPM tankless unit or a higher FHR tank unit.
  2. Fuel Source: Electric water heaters generally have a lower recovery rate than natural gas or propane models.
  3. High-Flow Fixtures: If you have a luxury "rainfall" showerhead or a massive soaking tub, your demand will be significantly higher than average.
function toggleInputs() { var type = document.getElementById('heaterType').value; var resultArea = document.getElementById('resultArea'); resultArea.style.display = 'none'; } function calculateWaterHeater() { var type = document.getElementById('heaterType').value; var residents = parseInt(document.getElementById('householdSize').value) || 0; var showers = parseInt(document.getElementById('peakShowers').value) || 0; var appliances = parseInt(document.getElementById('peakAppliances').value) || 0; var outputDiv = document.getElementById('calcOutput'); var descDiv = document.getElementById('calcDescription'); var resultArea = document.getElementById('resultArea'); if (residents <= 0) { alert("Please enter a valid number of residents."); return; } if (type === 'tank') { // Calculation for FHR (First Hour Rating) // Avg shower: 15 gal, Avg person activity: 12 gal base, Avg appliance: 15 gal var recommendedFHR = (residents * 12) + (showers * 20) + (appliances * 15); // Map FHR to common tank sizes var tankSize = ""; if (recommendedFHR <= 40) tankSize = "30-40 Gallon Tank"; else if (recommendedFHR <= 60) tankSize = "40-50 Gallon Tank"; else if (recommendedFHR <= 85) tankSize = "50-65 Gallon Tank"; else tankSize = "80+ Gallon Tank or Dual Units"; outputDiv.innerHTML = tankSize; descDiv.innerHTML = "Based on your peak hour demand, you need a unit with a First Hour Rating (FHR) of at least " + recommendedFHR + " gallons."; } else { // Calculation for Tankless GPM // Shower: 2.5 GPM, Appliance: 2.0 GPM var totalGPM = (showers * 2.5) + (appliances * 2.0); // Round to 1 decimal totalGPM = Math.round(totalGPM * 10) / 10; outputDiv.innerHTML = totalGPM + " GPM (Flow Rate)"; descDiv.innerHTML = "To handle your simultaneous peak usage, your tankless water heater must provide " + totalGPM + " gallons per minute. Note: In colder climates, look for units rated higher to handle the temperature rise."; } resultArea.style.display = 'block'; resultArea.scrollIntoView({ behavior: 'smooth', block: 'nearest' }); }

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