Is My Tub Cast Iron or Steel?

Cast Iron Tubs

While most cast iron and steel tubs are candidates for tub shower conversions, it is difficult for most people to distinguish between the two. Both tubs are metallic (i.e. magnets will stick to them) and both can have porcelain-like finishes, a good rule of thumb is that bathtubs installed prior to the mid 1960’s  are usually cast iron. Here are some examples.
cast iron bump-out tub
Cast iron bump-out tub

The “Bump-Out”, a post-war classic; the outer tub wall starts out narrow from each end. About 10”-12” in, the wall takes a 60° turn out into the room. The wall thickness on the bump-out expands to approximately 5” wide (thick) and is about 36” long. Cascading ripples at the bottom of the apron is also an indicative characteristic of the cast iron tubs.

View a converted bump-out tub example.

Birds-eye view of bump-out tub
Birds-eye view of bump-out tub.

The “Expanding Rail” – another classic shown to the right. The tub’s rail starts out narrow (usually at the rear of the tub) and gets wider as it goes towards the opposite end.

View a converted tub example.

tapering rail tub
expanding rail tub
Birds-eye view of expanding rail tub. Generally found at the faucet end.

The “Colorful Fifties” – from blue, to pink to green, cast iron bathtubs in 1950-60’s era joined the retro décor. Most non-white tubs, like the to the right, tend to be cast iron.

colorful fifties cast iron tub
colorful fifties cast iron tub

Steel Tubs

Raised Embossing on the front apron of the tub shown in Figure 1 is a distinguishing characteristic that the tub is steel.

Steel tub with raised embossment on front apron panel
Figure 1 – Steel tub with raised embossment on front apron/panel.

The “muffin top” apron feature seen in Figure 2 and illustrated in Figure 2b, is a style found in some steel tubs. Muffin tops can come with either a bowed or a straight apron (Figures 3 & 4 respectively).

muffin top steel tub
Figure 2 – “Muffin top” steel tub.
cross section of muffin top steel tub
Fig. 2 b – Cross section of a “muffin-top” steel tub.

A third, but not always a defining characteristic of steel tubs, is that their inside and outside walls are generally parallel; that is, until they reach the ends of the tub. This is shown by the red lines in Figures 3 & 4 birds-eye sketches.

steel tub with bowed outer wall
Figure 3 – Tub with bowed outer wall.
parallel tub wall steel tub
Figure 4 – Parallel tub walls.

And due to their lighter weight, the bathtubs found in mobile and manufactured homes are generally constructed of either steel or fiberglass.