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There is something about a cast iron tub that just draws you in. Maybe it's the rustic charm or the history that comes with them. Whatever the reason, if you're considering adding one to your home (especially on an upper floor), you need to consider how much they weigh.
Cast iron tubs can weigh anywhere from 225 to more than 500 pounds unfilled, depending on the dimensions. Once filled with water and occupied by a bather, you can add up to around 500 pounds more.
Before settling on this durable, vintage-inspired material, you'll want to calculate the total weight that your bathroom floor can support. The construction and size of cast iron make it more complicated to install than other lightweight materials like acrylic, fiberglass, or steel.
You also have to consider the capacity of cast iron tubs, which are well-known for their particularly deep soaking areas. More water means more weight, and some bathroom floors aren't up to the task of supporting the load.
Standard cast iron tub dimensions and weights
How much a cast iron tub weighs depends on the dimensions and whether it's freestanding or alcove-style.
Freestanding and claw-footed cast iron tubs
Freestanding cast iron tubs don't require walls for support, presenting you far more options for size, placement, and design than you would with an alcove tub.
They tend to be the aesthetic focal point of the room, thanks to their striking appearance that exemplifies both timeless elegance and modern utility. The classic claw-footed cast iron tub is particularly popular.
If you're a fan of taking long soaks, you'll appreciate that many freestanding cast iron tubs are explicitly designed with lounging in mind. Single-slipper and double-slipper styles feature gently sloping raised ends so that you can lay back comfortably.
Like most freestanding bathtubs, you'll find that a standard cast iron tub ranges from 55-72" long and 27-32" wide.
55" cast iron tubs will weigh anywhere from 250 to 300 pounds, while a 72" tub can exceed 500 pounds.
This of course does not include the weight of the water or bather. We'll get to that in a second.
Alcove cast iron tubs
Unlike their freestanding cousins, alcove cast iron tubs are enclosed on three sides by water-safe walls. They're more common in smaller bathrooms, where floor space is a hot commodity.
Alcove tubs present fewer design features and are almost exclusively rectangular. The size of your alcove also limits your ability to upgrade the dimensions when swapping out older tubs.
That being said, they offer the same decades-long durability, impressive heat retention, and increased soaking depth of freestanding cast iron tubs.
Standard bathtub alcoves are 60" long, though they may be smaller or larger based on the overall size of the bathroom.
Because alcove cast iron bathtubs are unfinished on three sides, they're usually significantly lighter than similarly-sized freestanding tubs. You can expect a 60" tub to weigh 300-325 pounds.
Keep in mind that installing alcove cast iron tubs may come with additional costs. The significant weight makes the placement and fitting process more complicated than a plastic version.
Why is cast iron so heavy?
Iron is an incredibly brittle metal with low ductility, so it breaks easily on impact. To create a high-strength tub, cast iron must be very thick and covered in a protective coating of enamel.
The combined density of cast iron and enamel tip the scales at an average of 10.13 g/cm3 or 632 lbs/ft3. All of that weight quickly adds up if manufacturers want to craft a cast iron tub thick enough to withstand generations of use.
If you're concerned about your bathroom floor supporting the weight of a cast iron tub, you may need to compromise on the size.
Not only does a smaller bathtub weigh less, but it also limits the amount of water you can add, ensuring that the floor doesn't buckle under pressure.
Do I need additional floor supports for my cast iron tub?
Before you toss your dreams of taking long, luxurious soaks in an antique cast iron tub out the window, do some quick calculations to determine the load-bearing capacity of your bathroom.
These calculations are suitable for general estimates, but it's always safer to check with a professional, especially if you're planning to install an extra-large bathtub.
Calculating your bathroom floor capacity
According to the 2020 International Residential Code, home flooring must support at least 40 pounds per square foot. You can figure out whether or not your bathroom will support your tub by multiplying the dimensions of the bathroom by this number.
For example, if your bathroom is 6 feet by 7 feet, you would multiply 6 x 7 x 40 to get a floor weight capacity of 1680 pounds. Larger bathrooms should naturally be able to accommodate heavier cast iron tubs.
Remember, though, that this weight capacity has to accommodate all the objects in your bathroom, including the toilet, sink, and vanity.
Calculating the total weight of a cast iron tub
Next, you'll want to find how much your tub will weigh during use.
For simplicity, we'll use the 48" by 30" Norfolk Cast Iron Classic Clawfoot Tub as our bathtub of choice. It has an unfilled weight of 225 pounds and can hold 37 gallons of water.
Figure out the weight of the water by multiplying the capacity by 8.3 lbs (the weight of a gallon of water). In this case, 37 x 8.3 = 307.1 pounds.
Then, add that number to the unfilled weight of the tub (225 pounds) and the weight of the heaviest person who will use the tub (let's estimate 180 pounds).
That total comes to 712 pounds, which a fully-outfitted, 42-square foot bathroom should be able to support.
Options for increasing floor support in a bathroom
If you find that your bathroom can't reasonably handle the load, you have a few different options for increasing the weight capacity.
Before taking on any of these projects, you'll want to talk with a professional contractor to prevent damage to your home.
Adding floor joists
Underneath your home's flooring, you'll find long, evenly-spaced planks called joists. Most home support joists are 16" apart, but this number can range anywhere from 12" to 24", depending on the species and quality of the wood.
A contractor may be able to bolster your current joist system with additional planks underneath where the bathtub will sit.
Pouring polished concrete
Adding polished concrete to an existing floor is a little complicated, but a standard 4" thick concrete floor can hold up to 151.56 pounds per square inch, significantly increasing the range of cast iron tub sizes you can choose from.
The weight of a cast iron tub depends on its features, when it was made, and the dimensions of the tub. If you want to add one to your home, it's necessary to make careful calculations of the weight of the tub and the dimensions of your bathroom floor to make sure it will fit safely in your home.
As always, once you've done a quick calculation yourself, it's a good idea to get a professional opinion before attempting to install a cast iron tub yourself.