Do stone bathtubs keep water warm?
Last updated October 2026.
Quick answer
- Short answer
- Yes, once warmed; a cold tub takes heat from the first fill
- Heat a marble tub stores
- about 460 kJ per °C, vs about 70 kJ for acrylic
- First fill in a cold marble tub
- can lose up to about 10°F (6°C)
- Same fill in acrylic
- loses about 2°F (1°C)
- The fix
- rinse the tub with hot water first, or fill a few degrees hotter
Yes, once the stone is warm. A solid stone tub weighs over 1,000 lb, so a cold one draws heat from the first fill, up to about 10°F. Rinse it with hot water first and that mass works the other way: the warm stone holds a large store of heat and gives it back, so the bath stays warm for a long soak.
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Why does a cold stone tub cool the first fill?
Heat moves from the hot water into the cooler tub until they are close in temperature. A light acrylic tub stores very little heat, so it barely changes the water. A solid marble tub stores about 460 kJ for every degree Celsius it warms (its weight times marble's specific heat of about 880 J/kg·°C), around 40% as much as the water in it. If the whole tub warmed to the water's temperature, a 104°F bath poured into a 68°F tub would settle near 94°F. In practice the inside surface warms first, so the real loss is smaller, but it is noticeable.
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How do stone, acrylic and cast iron compare?
| Tub | Heat stored per °C | Conductivity (W/m·K) | First fill into a 68°F tub |
|---|---|---|---|
| Solid marble (67″ tub, ~1,150 lb) | about 460 kJ/°C | 2.1–2.9 | up to about 10°F (6°C) |
| Acrylic (~100 lb) | about 70 kJ/°C | about 0.2 | about 2°F (1°C) |
| Cast iron (~300 lb) | about 60 kJ/°C | about 50–80 | about 2°F (1°C), then cools fastest |
Worst case: about 73 gallons at 104°F into a tub at 68°F, if the whole tub reached the water's temperature. Material data from Engineering ToolBox (sources below); tub weights from our weight guide.
Does a warm stone tub keep a bath hot for longer?
Yes. Once warmed, the stone is a store of heat sitting around the water: as the water cools from the surface, the stone gives heat back. The catch is the first fill, so a minute of pre-warming matters more than the material.
Does a marble bathtub feel cold?
At first, yes. Marble conducts heat about ten times better than acrylic, so bare skin on a room-temperature stone rim feels cool. Once hot water has been in the tub for a few minutes the stone warms through and the effect goes.
Which bathtub material holds heat best?
Acrylic loses least heat to the tub itself on the first fill because it is light and insulating. Solid stone stores the most heat once warm, roughly six times as much as an acrylic tub, so a pre-warmed stone tub keeps a long bath warm well. Cast iron is in between: heavy, but it stores less heat than stone and conducts it quickly.
How do I keep a stone bath hot?
- Run hot water into the empty tub for a minute or two, then drain it.
- Or fill 2–4°F hotter than you like and let the tub take the difference.
- Keep the bathroom warm and the door closed; a cold room cools the water surface fastest.
- Choose a deeper tub: more water for the same surface area loses heat more slowly.
See also bathtub sizes for soaking depths and stone resin vs solid stone.