What size water heater would you need for your walk in tub ?

Q: What size water heater do I need for a walk-in tub?

Most walk-in tubs need a 50 to 75 gallon tank. Sizing depends on the tub's occupied fill volume, your incoming water temperature, and the heater's thermostat setting. A tank only delivers about 70% of its rated volume at full temperature, so a 50-gallon tub usually needs more than a 50-gallon heater.

Q: Can I use a 40-gallon water heater with a walk-in tub?

Only with small compact tubs (under roughly 45 gallons occupied), a gas tank, or by raising the thermostat to 140°F with an ASSE 1070 mixing valve installed. For most standard walk-in tubs a 40-gallon tank finishes the bath lukewarm.

Q: Do tankless water heaters work with walk-in tubs?

Only top-output 199,000 BTU condensing gas units. At a typical 65°F temperature rise they produce about 5.8 GPM of hot water, so a fast-fill faucet slows to match the heater. Mid-size and electric tankless units are not recommended as the primary heater for a walk-in tub.

Q: Why does my walk-in tub run out of hot water?

Because a tank water heater only delivers about 70% of its rated capacity before incoming cold water pulls the outlet temperature down. Fix it by upsizing the tank, switching to gas, raising the thermostat with a mixing valve, or adding a second tank in series.

Step 1

Choose your walk-in tub

Brand Model

Step 2

Your home's hot water setup

Incoming cold water (pick your region)

Water heater type

Heater thermostat setting

Current tank size (optional — for a pass/fail check)

Bath temperature target (102°F is the comfort standard)

102°F

Your result

— GAL TANK

Hot water this bath needs—

Usable hot water from recommended tank—

Select a tub above to see your verdict

—

Gal to fill (occupied)

—

Of it must be hot

—

Est. fill time

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?

Get a factory-direct walk-in tub

Mobility Bathworks builds and ships direct — no dealer markup, no 3-hour in-home sales pitch. Triple gel-coat marine-grade fiberglass on a stainless steel frame, 18 GPM fast-fill, 110-second Hurry Drain™, hydro + air massage, heated seat & backrest, and 13 models fitting bathers 80–400 lbs including wheelchair-transfer doors. Veteran owned. Texas & California facilities.

Every quote includes a free water heater capacity check — tub, plumbing, and heater sized together so bath one is a hot one.

Prefer to talk? 512-522-2284 · b2b@mobilitybathworks.com · MobilityBathworks.com

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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: Mobility Bathworks capacities are factory specifications. Competitor capacities come from each manufacturer's published spec sheets and dealer listings where available; models marked EST are estimated from tub dimensions and comparable shells. Brand names are the property of their respective owners and appear for comparison only — Mobility Bathworks is not affiliated with the other brands listed.

Affiliate disclosure: water heater links go to Amazon. As an Amazon Associate, Mobility Bathworks 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.

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