Bathtub Materials Compared: Acrylic, Cast Iron, Stone Resin & Fiberglass

Four materials, four different trade-offs

Nearly every bathtub sold today is made from one of four materials: acrylic, cast iron, fiberglass, or a stone-and-resin composite. Each solves the "hold water and support a body" problem differently, and that difference shows up in weight, cost, how long the tub lasts, and even how warm the water stays while you soak.

This comparison is deliberately about the material itself, not the tub's shape — for the freestanding-vs-built-in shape decision, which is a separate question, see our freestanding vs. built-in tub comparison.

Quick comparison

The factors that actually decide this for most bathtub buyers.

Acrylic: the balanced default

This Old House describes acrylic tubs as made from a lightweight, flexible plastic material that is heat-resistant and easy to maintain — a practical, no-drama choice that works for most households, including, per This Old House, homes with small children or elderly users where a lighter tub and simpler upkeep matter. Fixr prices acrylic installation at an average of $6,000.

Acrylic is also the most common material in a built-in alcove tub-shower combo, since it is light enough to handle easily and molds well into a one-piece or three-piece surround. If a rarely used acrylic or fiberglass alcove tub is the actual problem in your bathroom, a tub-to-shower conversion may be worth pricing before you replace it with another tub.

Cast iron: built to outlast the house around it

Cast iron tubs are made by pouring liquid iron into a mold, smoothing it, and finishing it with a porcelain enamel coating. This Old House calls it the most durable bathtub material available, scratch- and dent-resistant, and Fixr notes some cast iron tubs last nearly 100 years. That durability is exactly why a vintage cast iron tub is so often a refinishing candidate rather than a replacement one — see our bathtub refinishing vs. replacement guide for when restoring an old cast iron tub beats buying new.

The trade-off is weight: a cast iron tub is heavy enough that floor framing and the number of installers needed both become real considerations, and This Old House notes the porcelain enamel coating requires more ongoing upkeep than some other materials to avoid chipping. Fixr's installed-cost average for cast iron is $5,000 — notably not the most expensive material here, since a big share of any bathtub's installed cost is labor, not the tub itself.

Fiberglass: the budget-friendly workhorse

Fiberglass is, per This Old House, the most common material used in bathtub manufacturing — lightweight, strong, and simple to install and maintain. It is the material most likely to show up in an inexpensive alcove tub-shower combo, and Fixr prices its average installed cost at $6,500.

The catch is lifespan: This Old House puts fiberglass at roughly a 10-year working life, meaningfully shorter than the other three materials here. For a tub that gets heavy daily use in a family bathroom, that shorter lifespan is worth weighing against fiberglass's lower upfront cost.

Stone resin: the heat-retention and looks upgrade

Stone resin (also marketed as cultured marble or solid surface) tubs are a solid, non-porous composite material, and This Old House's spa-bathroom coverage is specific about the payoff: a limestone-resin composite tub "retains heat better than acrylic and avoids the weight of cast iron" — a real advantage if you actually soak rather than just rinse. Fixr's closest cost analogues, cultured marble and solid surface, price installation at $9,000 and $10,000 respectively, a real premium over acrylic or fiberglass.

Stone resin tubs are heavier than acrylic or fiberglass, though generally lighter than a full cast iron tub of the same size, and they show up most often in freestanding soaking tubs rather than built-in alcove installs — see our freestanding tub ideas for how designers place them.

Weight and installation: the factor that's easy to overlook

Every material comparison above assumes the tub gets installed correctly, and weight is a bigger part of that than most buyers expect. Fixr notes labor typically runs 20–50% of a bathtub's total installed cost, and heavier materials — cast iron especially, stone resin close behind — can require additional installers and, in some cases, floor reinforcement before the tub goes in.

This is worth raising with your contractor before you fall in love with a specific cast iron or stone tub: ask directly whether your existing floor framing can support it, or whether reinforcement needs to be part of the budget.

Which material should you choose?

Choose acrylic if you want the balanced, no-surprises default — light, affordable, easy to maintain, and well suited to a built-in tub-shower combo. Choose cast iron if you want a tub that will outlast the rest of your bathroom's finishes and you have (or can add) the floor support to carry it. Choose fiberglass if budget is the deciding factor and you are comfortable with a shorter working life. Choose stone resin if you actually soak regularly and want the best heat retention and the most luxurious look, and the price premium fits your budget.

A master bathroom retreat build is where we help you weigh material, shape, and budget together against your actual bathroom, rather than picking a tub in isolation.