What a Full Bathtub Actually Weighs: Cast Iron vs Acrylic, From the Spec Sheets

The short answer

A standard 60-inch bathtub, filled to its rated capacity with an adult in it, lands somewhere around 730 to 760 pounds. That figure is roughly the same whether the tub is cast iron or acrylic, which is not what most people expect.

It surprises people because the comparison everybody makes is the empty one. Empty, a cast iron tub is four times heavier than an acrylic one. Full, the gap nearly closes, because the acrylic tub in this comparison holds 24 more gallons of water — and water is the heaviest thing in the room.

The manufacturers publish this, and almost nobody reads it

Kohler’s specification sheets carry four fields that matter here: product weight, water capacity, water depth, and a Minimum floor load given in pounds per square foot. That last one is unusual — it is the manufacturer stating what the floor under the tub needs to be rated for — and it rarely appears in any consumer comparison.

Here are two tubs of the same nominal size from the same manufacturer, one in each material.

The arithmetic, shown

Water weighs about 8.34 pounds per US gallon. That single conversion is what makes the empty-weight comparison misleading — and it can be checked against the spec sheets themselves. USGS gives water’s density as roughly 1 gram per millilitre, and Kohler lists each capacity in litres as well: 123.4 L for the Bellwether and 215.8 L for the Archer, which is about 272 lbs and 476 lbs of water. Warm bath water is very slightly lighter than cold, by well under 1%, which changes nothing here.

Cast iron Bellwether: 310 lbs of tub + (32.6 gal × 8.34) = 272 lbs of water = 582 lbs, before anybody gets in.

Acrylic Archer: 73 lbs of tub + (57 gal × 8.34) = 475 lbs of water = 548 lbs, before anybody gets in.

Add a 180-pound bather and the two land at roughly 762 lbs and 728 lbs. The cast iron tub — the one every article warns you about — ends up about 34 pounds heavier in use, a difference of under 5%, despite outweighing the acrylic by 237 pounds on the delivery truck.

The deeper tub is the reason. The Archer has a 15⅛-inch water depth against the Bellwether’s 9 inches, which is exactly what people buy a soaking tub for. Depth is comfort, and depth is water, and water is load.

What the code says, and what that does and does not mean

The IRC prescribes minimum uniformly distributed live loads for residential floors in Table R301.5: 40 psf for rooms other than sleeping rooms, and 30 psf for sleeping rooms. A bathroom falls in the 40 psf category.

Both Kohler figures above — 52 and 46 lbs/ft² — exceed that. That is not a scandal and it is not a reason to panic: it is why the manufacturer publishes the number at all. A filled bathtub is a concentrated load over a small area, which is a different engineering question from the uniform live load a floor is designed against, and design minimums are not the point at which anything fails.

What it does mean is that a filled tub is a legitimate design-stage question rather than a delivery-day surprise, and that the question applies to more tubs than people think. The honest position for a remodeler is the narrow one: we can tell you what the manufacturer publishes and what the code prescribes. Whether a specific floor — with its span, its joist size, its spacing, its age and whatever a previous owner did to it — satisfies that is a question for a structural engineer, and it is a cheap question to ask before a tub is ordered rather than after it is installed.

The other number on the same sheet: hot water

Both Kohler specification sheets carry the same note: “The hot water supply should be 70% of the capacity of the bath or greater.” It sits a few lines below the floor load, and it is the half of a deep-tub decision that tends to surface only after the tub is in.

Applied to these two tubs, the Bellwether’s 32.6 gallons asks for roughly 23 gallons of hot water. The Archer’s 57 gallons asks for roughly 40.

Forty gallons is the nameplate size of a lot of household water heaters. Upgrading to a deeper tub can quietly turn into a water-heater conversation, and it is a much cheaper one to have at the design stage than on the first evening someone runs a bath.

When it is worth asking

• Ground floor on a concrete slab: not a question. The slab does not care.
• Second storey, tub staying exactly where the old one was, similar capacity: generally not a question either. The floor has been carrying that load already.
• Second storey and the tub is getting deeper or bigger: worth asking. Going from a 32-gallon alcove tub to a 57-gallon soaker adds roughly 200 lbs of water in the same footprint, and nothing about the room looks different afterwards.
• Freestanding tub on a small footprint: worth asking. The same load concentrated on four feet rather than spread across a full-length base is a different case.
• Older house, long joist spans, or any prior structural alteration: worth asking regardless of material.
• You are being told "cast iron is too heavy for upstairs" with no reference to capacity: that is folk wisdom, not analysis. Ask what the filled weight is.

The numbers for your own tub

Major tub manufacturers publish weight and water capacity on their specification sheets, and the arithmetic takes a minute:

Filled weight = product weight + (water capacity in gallons × 8.34) + occupant weight.

Not every manufacturer publishes a minimum floor load. If the spec sheet gives one, that is the manufacturer telling you what they expect the structure to be rated for — take it to whoever is assessing the floor rather than converting it yourself. If it does not give one, the filled weight and the tub’s footprint are what an engineer will want.

And if a supplier cannot produce a spec sheet for a tub they are selling you, that is worth noticing on its own. The figures in this article came from two PDFs that took under a minute each to find.