What Size P-Trap For Bathtub?

When it comes to keeping your bathtub drainage system working properly, having the correctly sized P-trap is crucial. Essentially, a P-trap is the curved section of pipe underneath the bathtub drain that holds water and prevents sewer gases from entering your home. But if you don’t have the right size installed, it can lead to a host of problems like clogs, leaks and foul odors.

How to determine what size P-trap you need for your bathtub and ensure a proper fit. This is an important plumbing task that, done right, will keep your tub draining smoothly for years to come.

What Size P-Trap For Bathtub?

Choose a P-trap that has an inner diameter of not less than 1-1/2 inches. Ensure the water seal is no less than 2 inches but not more than 4 inches. Your trap can be made of various materials such as cast brass, cast iron, copper designed for DWV, galvanized durham, ABS, or PVC. Stick to these guidelines for the best performance.

My Recommendations For the Right P-Trap Size For Bathtub


For Acrylic Bathtubs

Flat P-Trap 1 1/2″ P Trap E000


For Acrylic Bathtubs

Low Profile p Trap 1 1/2″


For Freestanding Tub

Low Profile 1 1/2 P Trap, Flexible, Flat P Trap


Flexible Freestanding Tub

Low Profile Flat 1 1/2 P Trap


For Freestanding Tub

Low Profile 1 1/2 P Trap, Flat P Trap


For Freestanding Tub

LEKZIETU Bathtub Shower Drain Pipe, Low Profile Flat 1 1/2 P Trap

Typical Bathtub Drain Pipe Sizes

Most standard tubs have drain pipes that measure either 1 1/2 or 1 1/4 inches in diameter. Here’s a quick look at the typical sizes:

Drain SizeDescriptionCommonly Found InDrainage Capacity
1 1/2 inchMost common size for modern bathtub drain pipes, offers excellent water flowNewer homes, remodel projectsExcellent
1 1/4 inchSmaller size often seen in older homesOlder homesSuitable
2 inchLess common but used for larger or garden style tubsLarger/garden style tubsMaximum

So when choosing a new P-trap, you generally want it to match up with one of these measurements for your existing tub drain pipe size.

Measuring Your Existing Bathtub Drain Pipe

Before picking out a new P-trap, it’s wise to double check the size of your current bathtub drain plumbing. Here’s a simple way to measure it:

You’ll need a measuring tape. Then, remove the old P-trap and any washers or nuts still attached to the drain pipe. This will expose the end of the straight vertical drain pipe coming down from the tub.

Wrap your measuring tape around the full circumference of the drain pipe. Make sure your tape is tight and level. Then check the measurement. It should give you the diameter of the pipe in inches.

This measurement will tell you whether you have a standard 1 1/2, 1 1/4 or 2 inch tub drain. Knowing the size for sure before shopping for a new P-trap helps ensure the perfect fit.

P-Trap Size Options for Common Bathtub Drain Sizes

Okay, now that you know your bathtub’s drain pipe diameter, it’s time to get the right sized P-trap. Here are some great options to look for based on the most common tub drain sizes:

For 1 1/2 inch tub drains:

Type of P-TrapDescriptionConnection SizeMaterialAssembly Required
Slip-Joint PVCSlip-joint traps make a solid, leak-free fit.1 1/2″PVCNo
ABS PlasticLightweight traps that snap together simply.1 1/2″ABSNo
Metal P-Trap KitChrome finished look, includes 1 1/2″ slip nuts and washers.1 1/2″MetalMay Require

For 1 1/4 inch tub drains:

Type of P-TrapMaterialSizeSpecial Features
1 1/4″ PVC P-TrapPVC1 1/4″Solvent-welded joints for secure connection
Compact Design TrapsVarious1 1/4″Space-saving design, fits under tubs in tight areas
Clip-On P-TrapsVarious1 1/4″Clips directly onto drain pipe for fast connection

For 2 inch tub drains:

Type of TrapFeaturesDiameterUsability
2″ PVC P-TrapSturdy, maximum flow capacity2″Suitable for larger tubs
2″ Slip Joint TrapConvenient slip-joint design2″Easily adjustable for 2″ drain pipes
Large Drainage TrapsOversized, designed for heavy drainage2″Ideal for high flow conditions

Pick plastic, metal or whatever finish matches your bathroom decor. And don’t be afraid to ask a plumbing pro for advice on finding just the right P-trap size and type for your bathtub drain pipe. It’s worth getting this fitted properly.

How to Install a New P-Trap Under a Bathtub?

Once you’ve got the ideal new P-trap size for your bathtub, it’s time for installation. Here are some key steps:

  1. Remove old trap – Unscrew the slip nuts to detach the old trap and drain pipe section from the tub and wall stub out.
  2. Clean surfaces – Use a wire brush and rag to clear away old plumber’s putty and debris.
  3. Dry fit parts – Test fit the new trap pieces together and onto the tub and wall stub out prior to permanent assembly.
  4. Apply putty – Roll plumber’s putty and apply a ring around the drain opening in the tub.
  5. Attach new trap – Slide new slip nuts and washers onto trap, apply pipe dope to connections if needed, then screw trap onto drain pipe stub outs.
  6. Adjust position – Set the trap into ideal alignment under the tub and tighten all slip nuts.
  7. Check for leaks – With the trap installed, run water into the tub and look for any dripping connections. Tighten if needed.

Take your time when installing the new P-trap and ensure watertight slip-joint connections. Doing this tub drain job correctly avoids problems down the road.

Why Proper P-Trap Sizing Matters for Bathtubs?

You might wonder why it’s worth the effort to ensure your bathtub has a properly sized P-trap. Here are some of the top reasons getting this right makes a difference:

Preventing Drain Clogs and Backups

A P-trap that’s too narrow or undersized for your drain pipe won’t allow wastewater to flow freely. This causes backups and slow drainage. A properly sized trap maintains unobstructed flow.

Ensuring Proper Water Flow and Drainage

A trap that matches your drain diameter keeps drainage velocity high. This whisks water away quickly and prevents standing water in the tub.

Stopping Sewer Gas and Odor Entry

The right sized trap forms a tight seal and holds a deep column of water to block sewer gas from entering through the drain. Prevent gross odors from wafting up from your pipes.

Avoiding Leaks Between Tub and Drain

The ideal fitting P-trap aligns perfectly with the drain opening. This prevents water from seeping down and causing rot or mold under the tub.

Saving on Plumbing Bills

Taking the time upfront to get the right P-trap size avoids the need for expensive plumbing repairs down the road if you have drain problems.

Protecting Health

Proper drainage helps keep black mold and bacteria from accumulating in the moist tub environment. Critical for health, especially families with children.

Increasing Home Value

Functional plumbing boosts home value when it comes time to sell. Preventing major tub drain issues later makes your bathroom more appealing.

Bathtub P-Trap Sizing and Plumbing Codes

In addition to general best practices, you also need to consider plumbing codes when choosing a new P-trap for your bathtub drain. Here are some key legal and safety issues to keep in mind:

International Plumbing Code Standards

The IPC includes specifications for tub drainage piping and P-trap sizes. Follow minimum diameters and limit directional changes.

Local Plumbing Code Requirements

Your city or county building codes may include additional legally binding plumbing standards. Research these before doing work.

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