Choosing the correct size under-sink water heater depends on how the sink is used, how long hot water is typically needed, and how much cabinet space is available.
Under-sizing can lead to running out of hot water quickly. Over-sizing can waste cabinet space and increase cost unnecessarily.
If you’re new to these systems, start with What Is an Under-Sink Water Heater and How Does It Work?
Common Under-Sink Tank Sizes
Most mini-tank under-sink water heaters are available in:
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2.5 gallons
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4 gallons
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6 to 7 gallons
Tankless units are sized differently (by flow rate and power), but for mini-tank systems, gallons are the main sizing factor.
If you’re comparing system types, see Tank vs Tankless Under-Sink Water Heaters: Which Is Better?
Bathroom Sink Sizing
Bathroom sinks typically use small amounts of hot water for:
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Handwashing
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Shaving
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Quick rinsing
In many homes, a 2.5-gallon unit is sufficient.
For a detailed breakdown, read Is a 2.5-Gallon Under-Sink Water Heater Enough?
Kitchen Sink Sizing
Kitchen sinks generally require more hot water due to:
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Dish rinsing
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Filling pots
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Cleaning tasks
A 4-gallon unit is often a practical starting point for light to moderate kitchen use.
For more guidance, see Is a 4-Gallon Under-Sink Water Heater Enough?
For heavier use or longer runs, 6–7 gallon models may be more appropriate.
Consider Recovery Time
Mini-tank heaters reheat automatically after hot water is used. Smaller tanks recover quickly but run out sooner. Larger tanks last longer but may take more time to fully reheat.
If you frequently use hot water for extended periods at that sink, consider moving up one size rather than selecting the smallest available unit.
Cabinet Space Matters
Under-sink heaters take up physical space inside the cabinet. Before choosing a size:
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Measure cabinet height, width, and depth
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Account for plumbing connections
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Leave clearance for servicing
For placement guidance, review Where Should an Under-Sink Water Heater Be Installed?
Electrical Considerations
Larger tanks and tankless units may require more electrical capacity.
Before selecting a unit, read Electrical Requirements for Under-Sink Water Heaters
You may also need to confirm Do Under-Sink Water Heaters Need a Dedicated Circuit?
Quick Sizing Guidelines
Use this as a simple starting point:
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Light bathroom use → 2.5 gallons
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Light kitchen use → 4 gallons
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Frequent or extended kitchen use → 6–7 gallons
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Continuous demand with strong electrical supply → Consider tankless
Choosing the right size improves convenience and avoids frustration with limited hot water.