Tankless Water Heater Size Calculator
Use this calculator to determine the appropriate size (GPM and BTU/hour) for a tankless water heater based on your household's hot water demands and local climate.
Understanding Tankless Water Heater Sizing
Choosing the right tankless water heater size is crucial for ensuring you have a continuous supply of hot water without running out or experiencing temperature fluctuations. Unlike traditional tank heaters that store a limited amount of hot water, tankless units heat water on demand. Their capacity is measured by how much hot water they can produce per minute (Gallons Per Minute – GPM) at a specific temperature rise.
Key Factors for Sizing:
- Temperature Rise (°F): This is the difference between your incoming cold water temperature and your desired hot water temperature. Colder climates or seasons will require a higher temperature rise, meaning the heater needs to work harder and thus produce less GPM, or require a higher BTU rating.
- Flow Rate (GPM): This is the total amount of hot water, in gallons per minute, that your household needs at any given time. It's determined by summing the GPM of all hot water fixtures you anticipate using simultaneously (e.g., a shower, a sink, and a dishwasher all running at once).
- BTU/hour Rating: British Thermal Units per hour (BTU/hour) is a measure of the heater's power. A higher BTU rating means the heater can produce more hot water or achieve a higher temperature rise. The general formula for calculating required BTU/hour is:
GPM × Temperature Rise × 500.
How to Use This Calculator:
Our calculator simplifies the sizing process by asking for your specific household conditions:
- Incoming Water Temperature: This is a critical input. If you're unsure, consider the average ground water temperature in your region during the coldest months. For example, in northern states, it might be 40-50°F, while in southern states, it could be 60-70°F.
- Desired Hot Water Temperature: Most people prefer their hot water between 105-120°F for showering and general use.
- Simultaneous Demands: Think about your peak hot water usage. Do you often run a shower and a dishwasher at the same time? Or perhaps two showers? Input the number of fixtures you expect to use concurrently. We provide typical GPM values for common fixtures:
- Shower: ~2.5 GPM
- Faucet/Sink: ~1.0 – 1.5 GPM
- Dishwasher: ~1.0 – 2.0 GPM
- Washing Machine: ~1.5 – 2.5 GPM
- Other Fixtures GPM: If you have a large soaking tub, a high-flow showerhead, or other specific appliances, you can add their GPM here.
Interpreting Your Results:
The calculator will provide you with three key metrics:
- Calculated Temperature Rise: This shows how many degrees your tankless heater needs to raise the water temperature.
- Required Flow Rate (GPM): This is the total GPM your household needs at the calculated temperature rise. When shopping for a tankless water heater, look for models that can deliver at least this GPM at your specific temperature rise. Manufacturers often provide charts showing GPM output at various temperature rises.
- Required BTU/hour: This indicates the minimum power output your tankless heater should have. Match this with the BTU rating of potential units.
Example Scenario:
Let's say your incoming water temperature is 50°F, and you desire 110°F hot water. You anticipate one shower (2.5 GPM) and one kitchen faucet (1.5 GPM) running simultaneously.
- Temperature Rise: 110°F – 50°F = 60°F
- Total GPM: 2.5 GPM (shower) + 1.5 GPM (faucet) = 4.0 GPM
- Required BTU/hour: 4.0 GPM × 60°F × 500 = 120,000 BTU/hour
In this scenario, you would look for a tankless water heater that can provide at least 4.0 GPM at a 60°F temperature rise, with a BTU rating of at least 120,000 BTU/hour.
Always consult with a qualified plumber or HVAC professional to confirm your sizing requirements and for proper installation.
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function calculateTanklessSize() {
var incomingWaterTemp = parseFloat(document.getElementById('incomingWaterTemp').value);
var desiredWaterTemp = parseFloat(document.getElementById('desiredWaterTemp').value);
var simultaneousShowers = parseFloat(document.getElementById('simultaneousShowers').value);
var simultaneousFaucets = parseFloat(document.getElementById('simultaneousFaucets').value);
var dishwasherUsage = document.getElementById('dishwasherUsage').checked;
var washingMachineUsage = document.getElementById('washingMachineUsage').checked;
var otherFixturesGPM = parseFloat(document.getElementById('otherFixturesGPM').value);
var resultDiv = document.getElementById('tanklessResult');
resultDiv.innerHTML = "; // Clear previous results
// Validate inputs
if (isNaN(incomingWaterTemp) || incomingWaterTemp 80) {
resultDiv.innerHTML = 'Please enter a valid Incoming Water Temperature (30-80°F).';
return;
}
if (isNaN(desiredWaterTemp) || desiredWaterTemp 140) {
resultDiv.innerHTML = 'Please enter a valid Desired Hot Water Temperature (90-140°F).';
return;
}
if (desiredWaterTemp <= incomingWaterTemp) {
resultDiv.innerHTML = 'Desired Hot Water Temperature must be higher than Incoming Water Temperature.';
return;
}
if (isNaN(simultaneousShowers) || simultaneousShowers < 0) {
simultaneousShowers = 0;
}
if (isNaN(simultaneousFaucets) || simultaneousFaucets < 0) {
simultaneousFaucets = 0;
}
if (isNaN(otherFixturesGPM) || otherFixturesGPM < 0) {
otherFixturesGPM = 0;
}
// Define GPM constants for common fixtures
var SHOWER_GPM = 2.5;
var FAUCET_GPM = 1.5;
var DISHWASHER_GPM = 1.5;
var WASHING_MACHINE_GPM = 2.0;
var BTU_FACTOR = 500; // Approx. (8.34 lbs/gal * 60 min/hr)
// Calculate Temperature Rise
var temperatureRise = desiredWaterTemp – incomingWaterTemp;
// Calculate Total GPM
var totalGPM = (simultaneousShowers * SHOWER_GPM) + (simultaneousFaucets * FAUCET_GPM);
if (dishwasherUsage) {
totalGPM += DISHWASHER_GPM;
}
if (washingMachineUsage) {
totalGPM += WASHING_MACHINE_GPM;
}
totalGPM += otherFixturesGPM;
// Calculate Required BTU/hour
var requiredBTU = totalGPM * temperatureRise * BTU_FACTOR;
// Display results
var resultsHTML = '
Your Tankless Water Heater Sizing Recommendations:
';
resultsHTML += '
Calculated Temperature Rise: ' + temperatureRise.toFixed(1) + '°F';
resultsHTML += '
Required Flow Rate (GPM) at ' + temperatureRise.toFixed(1) + '°F Rise: ' + totalGPM.toFixed(2) + ' GPM';
resultsHTML += '
Required BTU/hour: ' + requiredBTU.toLocaleString(undefined, {maximumFractionDigits: 0}) + ' BTU/hour';
resultsHTML += '
Look for a tankless water heater that meets or exceeds these GPM and BTU/hour specifications for your specific temperature rise.';
resultDiv.innerHTML = resultsHTML;
}