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 Size||Description||Commonly Found In||Drainage Capacity|
|1 1/2 inch||Most common size for modern bathtub drain pipes, offers excellent water flow||Newer homes, remodel projects||Excellent|
|1 1/4 inch||Smaller size often seen in older homes||Older homes||Suitable|
|2 inch||Less common but used for larger or garden style tubs||Larger/garden style tubs||Maximum|
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-Trap||Description||Connection Size||Material||Assembly Required|
|Slip-Joint PVC||Slip-joint traps make a solid, leak-free fit.||1 1/2″||PVC||No|
|ABS Plastic||Lightweight traps that snap together simply.||1 1/2″||ABS||No|
|Metal P-Trap Kit||Chrome finished look, includes 1 1/2″ slip nuts and washers.||1 1/2″||Metal||May Require|
For 1 1/4 inch tub drains:
|Type of P-Trap||Material||Size||Special Features|
|1 1/4″ PVC P-Trap||PVC||1 1/4″||Solvent-welded joints for secure connection|
|Compact Design Traps||Various||1 1/4″||Space-saving design, fits under tubs in tight areas|
|Clip-On P-Traps||Various||1 1/4″||Clips directly onto drain pipe for fast connection|
For 2 inch tub drains:
|Type of Trap||Features||Diameter||Usability|
|2″ PVC P-Trap||Sturdy, maximum flow capacity||2″||Suitable for larger tubs|
|2″ Slip Joint Trap||Convenient slip-joint design||2″||Easily adjustable for 2″ drain pipes|
|Large Drainage Traps||Oversized, designed for heavy drainage||2″||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:
- Remove old trap – Unscrew the slip nuts to detach the old trap and drain pipe section from the tub and wall stub out.
- Clean surfaces – Use a wire brush and rag to clear away old plumber’s putty and debris.
- Dry fit parts – Test fit the new trap pieces together and onto the tub and wall stub out prior to permanent assembly.
- Apply putty – Roll plumber’s putty and apply a ring around the drain opening in the tub.
- 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.
- Adjust position – Set the trap into ideal alignment under the tub and tighten all slip nuts.
- 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.
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.
- Do Bathtub Drains Have Traps?
- Can You Change the Drain on a Bathtub?
- 6 Most Common Bathtub Drain Stopper Types
- Can Mice Come Up The Bathtub Drain?
- How Does a Bathtub Drain Work?
William J. Bullock is a licensed plumber with over 15 years of experience installing and repairing bathtubs. He runs his own plumbing company in Greenville and serves residential and commercial clients. William is dedicated to providing honest, transparent advice to help homeowners make informed decisions about their bathroom renovations.
He has established expertise in selecting bathtubs, planning custom installations, diagnosing issues, and completing repairs. William aims to share practical tips and reliable recommendations based on extensive hands-on work. When he isn’t on a job site, William enjoys spending time with his family and volunteering at local community events. He takes pride in delivering quality service and enjoys helping people upgrade their homes.