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Inline Flow Heater Calculator
Inline Flow Heater Calculator — Watts Needed for Flowing Water
Inline or flow-through heaters are used in aquarium plumbing when you need to heat water as it moves through a pipe — for example, heating ATO top-off water to tank temperature before it enters the system, pre-warming water during water changes, or supplementing heating on a high-flow return line. Unlike tank heaters which heat a static body of water, inline heaters must continuously transfer heat to moving water. The wattage required depends on the temperature increase needed and the flow rate in gallons per hour. This calculator uses the specific heat capacity of seawater (1.0564 kJ/kg·°C at 35‰) or freshwater (4.186 kJ/kg·°C) to find the exact power requirement. Note that inline heater efficiency rarely exceeds 85%, so select a heater rated at least 15–20% higher than the calculated value.
How to Use This Calculator
1
Enter the incoming water temperature — the temperature of your tap, RODI unit output, or make-up water.
2
Enter the target temperature — for ATO or water changes, this should match your display tank temperature.
3
Enter the flow rate in GPH — check your pump's specs or measure with a bucket and timer.
4
Select seawater or freshwater — seawater has a lower specific heat capacity.
5
Choose an inline heater rated at the recommended wattage or higher to account for efficiency losses.
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Frequently Asked Questions
What is an inline heater used for?
Inline heaters heat water flowing through a pipe rather than heating a static tank. Common uses: (1) Pre-heating ATO top-off water to tank temperature to prevent sudden temperature drops; (2) Warming make-up saltwater during water changes; (3) Supplemental heating on high-flow return lines.
Why does flow rate affect required wattage?
More water flowing past the heater per hour means more mass needs to be heated per unit of time, requiring more power. A heater that handles 20 GPH at a given temperature rise will be overwhelmed at 100 GPH.
Can I use a standard aquarium heater inline?
Standard submersible heaters are not designed for inline use — they require being submerged in still or slow-moving water and can overheat and fail if water flow is too high or if they run dry. Use heaters specifically rated for inline/flow-through installation.
What brands make inline aquarium heaters?
Hydor, Fluval, Eheim, and Cobalt Aquatics make popular inline models. For automated systems, titanium heaters with external controllers (ITC-306A etc.) are a cost-effective alternative.
