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Walk in Tub Water Heater Tool
💧AVORA BATH
Factory Direct
Walk-In Tub
Water Heater Sizing Calculator
An undersized water heater is the #1 complaint after a walk-in tub install — the bath goes lukewarm before it's full. Pick any brand and model below and get the tank size that actually works. Free, no signup.
Choose your walk-in tub
Your home's hot water setup
Your result
— GAL TANK
Select a tub above to see your verdict
Thinking tankless?
If your tank comes up short
Safety: any thermostat setting above 120°F requires a thermostatic mixing valve (ASSE 1070) at the tub to prevent scalding. Quality walk-in tubs ship with anti-scald valves — confirm before turning the heater up.
Show the math (how we size it)
1. Fill volume. We use the manufacturer's occupied capacity at full fill — the worst case, since a smaller bather displaces less water and needs more of it.
2. Hot water fraction. (bath temp − cold water) ÷ (heater setting − cold water). Example: (102 − 55) ÷ (120 − 55) = 72% of the fill must come from the heater. This is why the old "two-thirds hot" plumber's rule exists.
3. Usable tank output. A tank only delivers about 70% of its rated volume at temperature before incoming cold water drags the outlet temp down (75% for gas — the burner reheats during the fill). This is the step almost every dealer skips, and it's why "the tub is 50 gallons and my heater is 50 gallons" still ends in a cold bath.
4. Recommended tank = hot gallons needed ÷ usable output, rounded up to the next standard tank size.
Tankless: flow at temperature is the limit, not gallons. Available GPM ≈ BTU × 0.95 ÷ (500 × temperature rise). A 199,000 BTU unit at a 65°F rise makes ~5.8 GPM of hot water — a fast-fill faucet wants 2–3× that, so the fill slows to match the heater.
Cross-checked against municipal plumbing guidance and manufacturer install specs. Final sizing should always be confirmed by a licensed plumber.
Still shopping for the tub?
Shop factory direct
Avora Bath builds walk-in tubs factory direct — no dealer markup. Triple gel-coat fiberglass shell on a stainless steel frame, 18 GPM fast-fill faucet, 3-minute fast drain, and soaker / air / water / dual-jet options in custom colors and Eco-Stone finishes. Installation and financing available across North America.
Questions, custom colors, or installation help? Call or text 512-853-9690.
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Walk-in tub water heater FAQ
What size water heater do I need for a walk-in tub?
Most walk-in tubs need a 50–75 gallon tank. It depends on the tub's occupied fill volume, your incoming water temperature, and the thermostat setting — that's exactly what the calculator above works out. Remember: a tank only delivers ~70% of its rated volume at temperature.
Can I use a 40-gallon water heater with a walk-in tub?
Only with small compact tubs (under ~45 gal occupied), a gas tank, or by raising the thermostat to 140°F with an ASSE 1070 mixing valve installed. For most standard tubs, a 40-gallon tank finishes the bath lukewarm.
Do tankless water heaters work with walk-in tubs?
Only 199,000 BTU condensing gas units. At a typical 65°F rise they make ~5.8 GPM of hot water, so the fill slows to match the heater — and you're seated the whole time. Mid-size and electric tankless shouldn't be the primary heater for a walk-in tub.
Why does my walk-in tub run out of hot water?
A tank only delivers about 70% of its rated capacity before incoming cold water drags the outlet temperature down. Fixes: upsize the tank, switch to gas, raise the thermostat (with a mixing valve), or add a second tank in series. See "If your tank comes up short" above.
Data sources: Avora Bath prices and product links are pulled from our current catalog; fill-volume capacities are based on published specs for matching factory tub codes and models marked EST are estimated from dimensions. Competitor capacities come from each manufacturer's published spec sheets and dealer listings where available. Brand names are the property of their respective owners and appear for comparison only — Avora Bath is not affiliated with the other brands listed.
Affiliate disclosure: water heater links go to Amazon. As an Amazon Associate, Avora Bath earns from qualifying purchases at no extra cost to you.
This tool provides planning estimates. Actual performance varies with plumbing, water pressure, heater age, and simultaneous household use. Have final sizing confirmed by a licensed plumber.