How To Tell What Your Bathtub Is Made Of | Everything Explained

Are you planning a home improvement project in your bathroom? Did your contractor ask you if you have an acrylic or cast iron tub? How do you even find out what your bathtub is made of? The answer is not that difficult to find when you know what to look for.

In this post, I’ll explain why you might need to know what your bathtub is made of, how to tell what material it might be as well as some common questions around the topic.

Let’s dive in!

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Why You May Need To Know What Your Bathtub Is Made Of

If you are thinking about doing a bathroom remodel, but don’t want to spend too much, you might want to keep your sanitaryware. However, if you have a fibreglass tub that is more than 20 years old, it probably should be replaced. The fibreglass is going to continue to degrade, while an enamelled tub will likely last another five decades.

Do you have a third-floor flat and would have trouble getting an antique cast iron tub down the stairs? You can break a fibreglass tub down into smaller bits. Are you being charged by weight for your demolition debris? Tossing the cast iron will increase your budget.

Finally, if you have noticed cracks around the drain or edges of your tub, you could repair it. But, you will need a repair kit that works with fibreglass, acrylic, or porcelain. Attempting to use the wrong repair chemicals could actually damage the bath leading to a costly replacement.

bathtub installation

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How To Tell What Material Your Bathtub Is Made Of

Bathtubs fall into two similar categories: cast iron and steel or acrylic and fibreglass.

The easiest test is to tap and press on the side of your bath. An acrylic or fibreglass bath will sound hollow with a higher pitch. They may give a little when you press on the sides. A metal tub sounds deeper and is totally rigid against your knuckles.

Do you have a magnet? Some shower curtains have magnets in the bottom hem designed to keep the curtain out of your way. The magnet doesn’t need to be big. You can use one that is on the front of your refrigerator. If it clings to the tub, it’s either iron or steel.

Does your bath have panels that hide the side of it? Most panels can be removed when you take out a couple screws or nails. Now you can see the underside of the bath. Fibreglass looks like a web of strings. Acrylic will be smooth and the same colour as the surface. Iron has a pebbled texture and stainless steel appears metallic with a brushed finish.

If your vintage bath has claw feet, it is most likely cast iron and it may sit on some reinforced plates to protect the floor. 

An enamelled steel or iron tub could show signs of cracking throughout its surface, much like one of your grandmother’s teacups. Acrylic and fibreglass will crack in long lines and the cracks will go through the depth of the material.

An old fibreglass tub may show signs of fading around its edge, while other types of tubs are not as likely to show their age.

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How Do I Know If My Bathtub Is Steel Or Cast Iron?

Now that you know your tub is metal, which one is it?

A freestanding clawfoot tub is likely cast iron. An enamelled tub that is built-in and supported by a wooden structure hidden by panels is likely stainless steel. However, there are some modern slipper tubs formed out of steel, so you really do need to look a little deeper.

Remove the overflow drain cover on the side of the tub. Under the enamel or porcelain top coat, you will see either black and possibly rusty cast iron or a thin bright sheet of steel. Cast iron will be about a centimetre thick. Stainless steel is generally between two and four millimetres thick.

Why does it matter? A stainless steel tub weighs about 75 pounds. That ancient cast-iron beauty can weigh as much as 35 stone. How many people will you need to toss it in the skip?

Both stainless steel and cast iron are commonly coated in porcelain enamel. When properly cared for, enamel can last for hundreds of years. Minor chips can be repaired on either iron or steel.

How Do I Know If My Bathtub Is Steel Or Cast Iron?

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How Do I Tell If I Have A Fibreglass Or Acrylic Bathtub?

Is your bathtub white and made out of flexible plastic material? There is a good chance that it is fibreglass as acrylic is commonly manufactured in designer colours. Other indicators that it is fibreglass include:

  • Minor chips and cracks around the edges of the tub. Acrylic is less likely to show these kinds of wear and tear.
  • Stains and faded spots also indicate fibreglass. Acrylic is made out of layers of thin coloured plastics, so that it maintains its original colour.
  • The underside of the bathtub looks like a mat of long strings pressed into a mould. Acrylic does have fibreglass in its material, but it is hidden by the layers of acrylic.

Both fibreglass and acrylic bathtubs are lightweight and flexible. They are installed over a wooden support structure. They can be easily removed and tossed into a skip.

You can use a gel-coat repair kit on a fibreglass bathtub. These kits are designed for sealing the hull of fibreglass boats and can be colour-matched to your tub. They are inexpensive and a reasonable DIY project.

Minor scratches can be buffed out on an acrylic tub and sealed with waterproof acrylic paint. Major cracks should be referred to a professional for repair. A DIY repair will not likely be invisible and could lead to more damage when the repair begins to fail. However, if it is a serious crack, replacing the tub may be the right choice.

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Final Thought…

There you have it! Everything you need to know to understand how to tell what your bathtub is made of.

While it may not seem important to most, knowing what material your bathtub is made of can help you out during an installation, renovation or repair down the road. Using these quick tests and tricks can help you identify the material in no time.

So, give that bathtub a knock and see what material your bathtub is made of!


Michael R

Michael is a KBB designer from the UK. He's been designing and project managing new Kitchen, Bedroom and Bathroom installations for over eight years now, and before that, he was an electrician and part of a KBB fitting team. He created The Bathroom Blueprint in early 2020.