Tankless Water Heater Size Calculator
Size your tankless water heater correctly. Enter total GPM demand and temperature rise to get required BTU for gas models or kW for electric units.
How Tankless Heater Sizing Works
Tankless water heaters heat water instantly as it flows through the unit, unlike tank heaters that store pre-heated water. Sizing requires calculating two values: peak flow rate in gallons per minute and temperature rise needed. Flow rate comes from adding simultaneous fixture usage. Temperature rise equals desired output temperature minus incoming cold water temperature.
The formula multiplies GPM by temperature rise by 500 to get required BTU per hour for gas units. For example, 5 GPM with a 65-degree rise requires 162,500 BTU (5 × 65 × 500). Electric units divide BTU by 3412 to convert to kilowatts, yielding 47.6 kW for this example. Few homes have electrical service capable of 47.6 kW, which is why whole-house electric tankless heaters are rare.
Gas tankless units range from 120,000-199,000 BTU for whole-house applications. These handle 4-8 GPM depending on temperature rise. Cold climates with 50-degree groundwater require higher capacity for the same GPM output compared to warm climates with 70-degree groundwater. Always calculate based on your local coldest groundwater temperature to avoid undersizing that leaves you with lukewarm water in winter.
Installation Requirements and Costs
Gas tankless heaters need proper venting and gas supply upgrades. Most require 3-4 inch PVC or stainless steel venting that exits through a sidewall or roof. Existing tank heater vents are often inadequate. Gas line sizing must deliver the higher BTU load without pressure drops. A 180,000 BTU unit requires 1-inch gas line if the run exceeds 25 feet. Undersized gas lines cause the unit to shut down or operate at reduced capacity.
Electric tankless installations demand electrical upgrades in most homes. A whole-house unit may require three separate 50-amp 240-volt circuits, totaling 150 amps of additional capacity. Many older homes have 100-150 amp main service, requiring complete panel and service upgrades costing $2,000-4,000 before the heater installation begins. Point-of-use electric units serving single fixtures need only one 30-50 amp circuit and make more sense for most retrofit applications.
Installation costs for gas tankless heaters run $2,500-4,500 including the unit, venting, gas line upgrades, and labor. Add another $500-1,500 for water softeners in hard water areas, essential to prevent scale buildup that destroys heat exchangers. Electric whole-house installations cost $1,500-3,000 for the unit plus electrical upgrades. Despite high upfront costs, energy savings of $100-150 yearly and unlimited hot water appeal to many homeowners, especially large families who regularly run tank heaters dry.
Optimizing Tankless Performance
Proper sizing prevents the most common complaint: inadequate hot water during cold weather. Tankless units can't raise water temperature beyond their rated capacity. A unit rated for 5 GPM at 70-degree rise will only produce 3.5 GPM at a 100-degree rise. Cold climates with 40-degree groundwater face larger temperature rises than warm climates with 70-degree water. Size generously or accept reduced simultaneous flow during winter months.
Water quality affects tankless heater longevity. Hard water deposits calcium and magnesium on heat exchangers, reducing efficiency and eventually causing failure. Annual descaling with vinegar extends life but doesn't eliminate the problem. Installing a water softener protects the investment in hard water areas. Softener costs pale compared to heat exchanger replacement at $400-800 for parts alone, plus labor.
Temperature modulation helps with small draws. Low flow rates below the unit's activation threshold (typically 0.4-0.6 GPM) don't trigger heating, producing cold water. This affects quick hand washing or slow sink fills. Some units include buffer tanks or recirculation systems that maintain hot water availability for small draws. Point-of-use tankless heaters at remote fixtures solve this better than whole-house units trying to serve low-flow distant fixtures through long pipe runs.
Frequently Asked Questions
How do I calculate total GPM needed?
Add the GPM rating of each fixture you might use simultaneously. A shower (2.5 GPM) plus a dishwasher (2 GPM) equals 4.5 GPM total. Size the heater for your realistic peak demand, not every fixture at once.
What groundwater temperature should I use?
This varies by region and season. Southern states average 60-75°F year-round. Northern states drop to 35-45°F in winter. Use your coldest expected temperature for sizing to ensure adequate hot water in worst-case conditions.
Is gas or electric tankless better?
Gas units provide more BTU capacity at lower operating cost, ideal for whole-house applications with high GPM demands. Electric units work well for point-of-use or small homes but require heavy electrical service (often 150+ amps) for whole-house use.
Can I install a tankless heater myself?
Gas units require gas line sizing, venting, and permits better left to professionals. Electric units need large dedicated circuits and often electrical panel upgrades. Both require plumbing skills. DIY installation may void warranties and violate code.
How much money do tankless heaters save?
Tankless units are 24-34% more energy efficient than tank heaters because they heat on demand without standby losses. Actual savings depend on usage patterns. High-use households save more, while low-use homes may not recoup the higher upfront cost.