Boat Drain Plug Size Chart 2026: Official Maker Sizes

Short answer: the standard boat drain plug size is 1 inch. That is the tube-style transom drain that T-H Marine calls the standard drain opening, and it takes a 1 in expansion plug (T-handle, snap or twist). The next size up is 1-1/4 inch. Boats with a threaded bronze, stainless or chrome garboard fitting use a different system, sized by pipe thread: 1/2 in NPT is the size every maker here lists, and Perko also makes 3/4 in and 1 in NPT versions. Stern-drive boats may have a 7/8 in stainless threaded drain. Livewells and baitwells take small plugs from 3/8 to 3/4 in. Measure the inside of the tube, or identify the thread, before you buy.

The sizes and part numbers below come straight from the makers' own catalogs: the Attwood catalog (2023 edition, page 35) and Attwood catalog page 32, the Seachoice 2026 catalog (page 37), Perko's garboard drain plug catalog page and product page, and T-H Marine Supplies. Thread measurements come from Swagelok's Thread and End Connection Identification Guide (MS-13-77). The transport rules come from Minnesota Statutes section 84D.10 and the Minnesota DNR. All were read on November 4, 2026. We did not test any plugs; every number here is as published.

Boat drain plug size chart

Start here. Find the drain type that matches what you see on your boat, then read across for the size, how that size is measured, and real part numbers you can look up.

Sources: Attwood catalog pages 32 and 35; Seachoice 2026 catalog page 37; Perko catalog page 106 and perko.com; T-H Marine Supplies product listings; measured thread diameters from the Swagelok MS-13-77 thread guide. All read Nov 4, 2026. Part numbers are copied exactly as each maker lists them; packaging suffixes differ by pack size.

The second column is the size you shop by. The third column is the part people get wrong. An expansion plug is sized by the inside of the tube it sits in. A garboard plug is sized by a pipe thread name, and that name is smaller than anything you can measure on the plug. Both are covered in detail below.

Which kind of drain plug do you have?

Before measuring anything, identify the system. The four main types look different from the outside of the transom.

• Drain tube with an expansion plug. A smooth metal tube runs through the transom. The plug has a flexible body and a handle. Turning the handle clockwise makes the body expand against the tube. Attwood describes its T-handle this way: "A clockwise turn of the T-handle expands the plug to lock in place." Snap-handle versions add a lever that flips down to lock.
• Threaded garboard drain. A round bronze, stainless or chrome flange is screwed to the outside of the hull (Perko: "mounts on outside surface"). A metal plug threads into it. Attwood says its garboard drain "provides bilge drainage when hauling out" and uses "machined, tapered pipe threads" for a watertight fit.
• Screw-in plastic plug. A plastic receiver with a rectangular flange is screwed to the transom, and a plastic plug with an O-ring threads into it. Seachoice and T-H Marine both list this type for a 1 in hole.
• Livewell or baitwell drain. A small thru-hull inside a well, closed by a small snap plug or push-in stopper.

Some boats have more than one kind. A fishing boat can have a 1 in transom drain tube, a separate livewell drain, and a splashwell drain, each with its own plug size. Check each opening on its own.

1-inch and 1-1/4-inch expansion drain plugs

All three makers sell this type simply as a "drain plug." Attwood, Seachoice and T-H Marine all sell it in 1 in, and Seachoice and T-H Marine also sell 1-1/4 in. T-H Marine says its 1 in snap plug is "compatible with standard 1-inch drain openings." Attwood's T-handle and snap-handle plugs are listed only "for 1" diameter drains."

Sources: Attwood 2023 catalog page 35; Seachoice 2026 catalog page 37 and seachoice.com; T-H Marine Supplies listings. Read Nov 4, 2026.

The style is a matter of handling more than fit: every plug in this table is listed by the drain diameter it fits, and each locks by tightening inside the tube (Attwood: "tightens the plug in its tube"). Seachoice describes its twist-turn plugs as "fully adjustable, will not leak or jar loose," and its snap-lock plugs as "cam-action," which "will not jar loose." To remove its snap plug, T-H Marine says to lift the handle and turn counterclockwise; Attwood says to turn its T-handle counterclockwise to release.

Chain is a small but useful detail. Attwood lists a brass snap-handle plug with a 5 in chain (7524B1) in OEM packaging, which keeps the plug tied to the boat. Attwood's single-pack versions in the table are listed without chain.

Drain tubes for expansion plugs

The plug only seals as well as the tube it sits in. If a tube is cracked, corroded or out of round, a new plug will not fix the leak. Attwood and Seachoice both sell replacement tubes, and the size you need is set by two numbers: the inside diameter (which must match the plug) and the length (which depends on how thick the transom is).

Sources: Attwood 2023 catalog page 35 (OEM part numbers); Seachoice 2026 catalog page 37. Read Nov 4, 2026.

Attwood adds one rule worth following: "Use aluminum drain tubes with aluminum-handle drain plugs; use brass tubes with brass-handle plugs and stainless steel plug handles." In other words, match the tube metal to the plug, not just the size. Seachoice's splashwell drain tube 50-19131 fits a 1-1/4 in opening and is closed with one of its 1 in drain plugs (Seachoice names 18801, 18821, 18891 or 18901), so read the plug note for any tube you buy.

Threaded garboard drain plug sizes (1/2, 3/4 and 1 inch NPT)

Garboard drains use a pipe thread, so they are sized by the National Pipe Taper (NPT) name. Every maker in this guide lists a 1/2 in garboard drain: Perko, Attwood and T-H Marine call it 1/2 in NPT, and Seachoice says it "fits 1/2 in pipe." Perko also makes 3/4 in and 1 in NPT versions.

Sources: Perko catalog page 106 and perko.com (Fig. 0714); Attwood catalog page 32; Seachoice 2026 catalog page 37; T-H Marine Supplies listings. A dash means the maker does not state that detail. Read Nov 4, 2026.

All four makers sell the 1/2 in plug on its own, without the flange: Perko's spare plug 0742DP1, Attwood's 9842-3 pair, Seachoice's 50-18760 and 50-18761, and T-H Marine's BE-PL-54836-DP. If the flange is sound, buying just the plug saves you from re-bedding the flange.

The flange outside diameter matters only if you replace the whole fitting. Several listings give 2 in for the 1/2 in size. T-H Marine says its chrome version "replaces older bronze fittings with universal hole pattern" and "fits 1" hole." Perko's larger 1 in NPT flange is 3-1/8 in across and uses four #10 screws, so it will not drop into a 1/2 in fitting's screw holes. Plug heads differ too: Perko's 1 in NPT plug takes a 1/2 x 1/2 in square wrench, and T-H Marine's chrome plug has a square top for a wrench or socket. If you're picking a socket for one, our socket size chart for metric and standard lists exact inch-to-millimeter conversions.

The stainless 7/8 in drain is its own design. Attwood describes it as a "threaded removable plug" with a retention system that "keeps plug from getting lost," and a "large 7/8" (27mm) hole for servicing stern drives." Seachoice lists a similar 316 stainless 7/8 in plug (50-18661). These makers don't give an NPT size for it, so don't assume it mates with a pipe-thread fitting.

Why a 1/2-inch NPT plug doesn't measure 1/2 inch

This catches nearly everyone who measures a garboard plug with a ruler. The "1/2 in" in 1/2 in NPT is a nominal pipe size, not a thread diameter. Swagelok's thread guide lists the nominal male thread diameter of 1/2 in NPT as 0.832 in (21.14 mm), with 14 threads per inch. That is about 1.66 times the name.

Source: Swagelok Thread and End Connection Identification Guide (MS-13-77), NPT rows of the tapered-thread identification tables. Read Nov 4, 2026. Swagelok notes that measured diameters may not match the table exactly because threads have tolerances.

So if your garboard plug measures about 0.83 in across the threads, it is most likely a 1/2 in NPT plug, even though the reading is closer to 13/16 in. A plug near 1.04 in is 3/4 in NPT, and one near 1.31 in is 1 in NPT. Counting threads separates the sizes further: 1/2 and 3/4 in NPT have 14 threads per inch, while 1 in and 1-1/4 in have 11.5. Thread pitch has its own near-miss traps, which our guide to wheel stud thread pitch covers in detail.

Two cautions from Swagelok apply here. First, NPT threads are tapered, and "measured thread diameters may not match table measurements exactly" because all threads have tolerances. Measure near the large end of the threads and treat the table as a guide. Second, 1/2 and 3/4 in ISO 7/1 (BSPT) and NPT threads "can be difficult to identify because they are very close in design." Perko, Attwood and T-H Marine list their garboard threads as NPT, but on an imported hull the thread could be the other standard.

Screw-in plastic drain plugs

Plastic receiver-and-plug kits are common on small boats and on fish wells, coolers and tanks. They are sized by the hole the receiver mounts in, and the common size is 1 in.

• T-H Marine DP-1 (black) and DP-2 (white), packaged as DP-1-DP and DP-2-DP: polypropylene, with an O-ring. T-H Marine says it "mounts in a 1" diameter hole with 2 (two) number 8 countersunk stainless steel screws," has a mounting flange of "approximately 1 1/2" x 2"," and a groove on the back of the flange to hold sealant. The plug is retained, so it will not get lost.
• Seachoice 50-18781: black nylon screw-type plug with an O-ring and fine threads, mounting in a 1 in diameter hole with two #8 screws on a 1-1/2 x 2 in flange.
• Seachoice 50-18641: black nylon drain plug for a 1 in hole, mounted with #10 screws. Seachoice sells a plug-and-gasket accessory, 50-18651.
• Attwood drain plug and receiver kit: molded black plastic with a threaded receiver, a molded retaining strap and an elastomeric gasket, listed as a "3/4" dia. one piece plug" (17209-1 OEM, 17211-3 retail).

When you replace one of these, match the hole and the screw pattern. Moving from a #10-screw design to a two-#8 design can leave old screw holes in the transom that need to be filled and sealed.

Livewell and baitwell plug sizes

Livewell plugs are much smaller than transom plugs and are sized by the inside diameter of the thru-hull fitting. The makers list these sizes:

Sources: Attwood 2023 catalog page 35; Seachoice 2026 catalog page 37; T-H Marine Supplies listings. Read Nov 4, 2026.

Attwood's livewell plugs are "cam-action neoprene plugs" that "snap shut to form stopper for livewells / baitwells," and Attwood warns they are "not for use with chemicals or fuels." Don't use one as a stopper anywhere fuel or cleaning chemicals can reach it. T-H Marine says its push-in PVC plugs are a complement to its overflow drain tubes, with double seal rings and a pull tab or ring.

Seachoice links one of its splashwell drain tubes to a baitwell plug: tube 50-19121 "fits 1" opening" and is closed with its 3/4 in baitwell plug (50-18961). A tube's outside fit and the plug that closes it are different sizes, so read the plug note on the tube itself.

How to measure a boat drain for a new plug

A caliper makes this easy, but a good ruler works for expansion drains. Work from the outside of the transom with the boat on its trailer and the plug removed.

1. Identify the type. Smooth tube means expansion plug. Threads inside a flange mean a garboard or screw-in plug. Use the list above.
2. For a drain tube, measure the inside diameter. Measure across the opening in two directions. If both read about 1 in, you need a 1 in plug. About 1-1/4 in means a 1-1/4 in plug. If the two readings differ noticeably, the tube may be distorted, and the tube itself may need replacing.
3. For a threaded plug, measure the plug, not the hole. Caliper the outside of the threads near the large end and compare with the NPT table: about 0.83 in is 1/2 in NPT, about 1.04 in is 3/4 in NPT, about 1.31 in is 1 in NPT.
4. Count threads per inch with a thread gauge if you have one: 14 for 1/2 and 3/4 in NPT, 11.5 for 1 in NPT.
5. For a livewell, measure the inside of the thru-hull and match it to the 3/8, 7/16, 1/2, 5/8 or 3/4 in sizes in the livewell table.
6. Write it down. Note the type, size and the part number you bought, and keep a spare plug aboard.

If the original plug is still there and has a part number or brand on it, that is the fastest route. A boat builder's parts diagram can also name the drain fitting; our guide on how to read an exploded parts diagram explains how to go from a callout to an orderable part number.

Worked examples

Example 1: aluminum fishing boat with a brass drain tube

The tube inside diameter measures 1 in in both directions. Any 1 in expansion plug in the expansion-plug table is listed for that size: for example Attwood 7526A7 (brass T-handle), Seachoice 50-18801 (brass twist-turn) or T-H Marine BE-PL-54842-DP (brass snap). The tube is brass, so a brass- or stainless-handle plug follows Attwood's matching rule.

Example 2: older fiberglass inboard with a bronze garboard flange

The plug calipers at about 0.83 in across the threads and has 14 threads per inch. That is 1/2 in NPT. The flange is sound, so only a plug is needed: Perko spare plug 0742DP1, Attwood 9842-3 (pair), Seachoice 50-18761 or T-H Marine BE-PL-54836-DP are each listed for the 1/2 in garboard size (Seachoice describes its fitting as fitting 1/2 in pipe). For saltwater use, Attwood lists stainless replacement plugs (9844PD2) labeled for saltwater.

Example 3: livewell drain on a bass boat

The livewell thru-hull measures 5/8 in inside. Attwood 7534A3, Seachoice 50-18951 or T-H Marine BE-PL-54854-DP are each listed for 5/8 in.

Example 4: plug measures about 1.04 inches

A threaded T-handle plug that calipers near 1.04 in with 14 threads per inch is 3/4 in NPT. Perko lists 3/4 in NPT T-handle garboard plugs (Fig. 0266 and Fig. 0363) with spare plug 0370DP0PLB. This is not a 1 in expansion plug, even though the number looks close to 1.

Material: brass, stainless steel, chrome or plastic

All three expansion-plug makers sell brass and stainless versions of the same 1 in plug, so material is a choice rather than a fit issue. What the makers themselves say:

• Attwood: match the tube to the plug, with brass tubes for brass-handle and stainless-handle plugs and aluminum tubes for aluminum-handle plugs.
• Attwood lists brass replacement garboard plugs and separate stainless steel ones labeled "Saltwater Use." Its 7/8 in threaded drain is "high polished #316 stainless steel" that "reduces corrosion."
• T-H Marine's chrome-plated brass garboard fitting is "much more attractive than bronze and will not turn green," and is listed as "Salt Water & UV Resistant."
• T-H Marine says its brass snap plug uses "durable rubber components and corrosion resistant brass."

If an expansion plug no longer grips firmly after tightening, check the tube and replace the plug rather than forcing it.

Drain plug rules: launch and transport

The most common drain plug problem isn't the size; it's a missing plug. Discover Boating's 10 steps for launching a boat puts the plug first: "In a staging area, check that the drain plug is installed and the key is in the ignition."

Leaving the water has its own rules in some states. Minnesota is a clear example. Under Minnesota Statutes 84D.10, subdivision 4, "When leaving a water of the state, a person must drain water-related equipment holding water and live wells and bilges by removing the drain plug before transporting the water-related equipment." The same section says drain plugs, bailers and valves "must be removed or opened while transporting water-related equipment." The Minnesota DNR lists the penalty as a $100 civil penalty or misdemeanor, and says the rules have applied since July 1, 2012.

The Minnesota DNR's watercraft inspection manual adds two practical points: inspectors remind boaters whose plugs are out to install them before launching, and pontoon cylinders, which hold air rather than water, don't need their plugs removed for transport unless they are damaged and contain water. Rules differ by state, so check the rules where you boat. Either way, the habit is the same: plug out when you leave the ramp, plug in before you launch.

Troubleshooting a leaking or loose drain plug

• Expansion plug leaks or spins. Tighten it clockwise as the maker directs, then flip down the snap lever if it has one. If it still won't grip, the plug body may be worn, or the plug may be the wrong size for the tube. Re-measure the tube.
• New plug won't go into the tube. You may have a 1-1/4 in plug for a 1 in tube, or the tube may be dented. Check the inside diameter again.
• Garboard plug won't thread in. Check the size and the thread count. A 3/4 in NPT plug won't go into a 1/2 in NPT flange, and an ISO/BSPT thread can look very close to NPT at 1/2 and 3/4 in.
• Seeps around the flange, not the plug. That points to the fitting's bedding or screws, not the plug. T-H Marine's DP-1 design has a groove for sealant for that reason.
• Can't find the old part number. Makers sometimes renumber or repackage parts, and the same plug can carry different suffixes for single, bulk and display packs (Attwood lists the same brass T-handle plug as 7526A1 in its OEM column and 7526A7 in its part-number column, for example). Our guide to superseded part numbers explains how to follow a number that has changed.

Attwood, Seachoice and T-H Marine list their plugs by drain size and thread, not by boat brand, so those are the things to match. Our explainer on OEM vs OES vs aftermarket parts covers what those labels do and don't tell you.

Frequently asked questions

What size is a standard boat drain plug?

For the common tube-style transom drain, 1 inch. Attwood makes its T-handle and snap-handle drain plugs only for 1 in diameter drains, and T-H Marine describes 1 in as the standard drain opening. Larger tubes take 1-1/4 in plugs. Threaded garboard drains are a different system; 1/2 in NPT is the garboard size every maker in this guide lists.

Are boat drain plugs universal?

No. There are at least four systems: expansion plugs sized to the drain tube (1 in or 1-1/4 in), threaded garboard plugs sized by pipe thread (1/2, 3/4 or 1 in NPT), 7/8 in stainless threaded drains, and small livewell plugs (3/8 to 3/4 in). Within one system, plugs from Attwood, Seachoice and T-H Marine are listed for the same drain sizes.

How do I know if my drain plug is 1 inch or 1-1/4 inch?

Measure the inside diameter of the drain tube with a caliper or ruler. About 1 in takes a 1 in expansion plug and about 1-1/4 in takes a 1-1/4 in plug. Seachoice and T-H Marine both list 1-1/4 in versions; Attwood's T-handle and snap plugs are 1 in only.

Why does my 1/2 inch garboard plug measure more than 1/2 inch?

Pipe threads are named by a nominal pipe size, not by the thread diameter. Swagelok's thread guide lists the male thread diameter of 1/2 in NPT as 0.832 in (21.14 mm) with 14 threads per inch. A 3/4 in NPT plug measures about 1.043 in and a 1 in NPT plug about 1.305 in.

What size wrench fits a garboard drain plug?

It depends on the plug. Perko's 1 in NPT garboard plug (Fig. 0124) uses a 1/2 x 1/2 in square wrench, sold as part 012400099A. Attwood says its 1/2 in NPT garboard plug nut removes with a wrench or pliers, and T-H Marine's chrome plug has a square top for a wrench or socket. Check the plug you have.

Can I use a livewell plug in my transom drain?

Not unless the maker lists it for that size and use. Livewell plugs are sized for small thru-hulls (Attwood lists 7/16, 1/2 and 5/8 in I.D.), while the tube-style transom drains in this guide are 1 in or 1-1/4 in. Attwood also notes its livewell plugs are not for use with chemicals or fuels.

Do I have to remove my boat drain plug when trailering?

In some states, yes. Minnesota law requires boaters to drain bilges and live wells by removing the drain plug before leaving the access and to keep drain plugs out or open while transporting. The Minnesota DNR lists a $100 civil penalty or misdemeanor. Check your own state's rules, and put the plug back in before you launch.

Should I get a brass or stainless steel drain plug?

Both are listed by all three expansion-plug makers. Attwood says to use brass drain tubes with brass-handle and stainless-handle plugs, and aluminum tubes with aluminum-handle plugs. For saltwater, Attwood sells stainless replacement garboard plugs labeled for saltwater use and a #316 stainless threaded drain.

Sources

• Attwood catalog 2023, page 35: T-Handle Drain Plugs, Snap-Handle Drain Plugs, Livewell/Bailer Drain Plugs, Drain Tubes (quick guide PDF). Read Nov 4, 2026.
• Attwood catalog, page 32: Garboard Drain Plugs, Stainless Steel Garboard Drain Plugs, Drain Plug and Receiver Kit. Read Nov 4, 2026.
• Seachoice 2026 Marine Products catalog, page 37 (Plumbing) and Seachoice 50-18801 product page. Read Nov 4, 2026.
• Perko catalog page 106, Garboard Drain Plugs (Figs. 0266, 0363, 0714, 0124) and Perko Garboard Drain Plug Fig. 0714 product page. Read Nov 4, 2026.
• T-H Marine Supplies, Drain Plugs collection and 1" Snap Drain Plug (BE-PL-54842-DP). Read Nov 4, 2026.
• Swagelok, Thread and End Connection Identification Guide (MS-13-77). Read Nov 4, 2026.
• Minnesota Statutes section 84D.10, subdivision 4. Read Nov 4, 2026.
• Minnesota DNR, Minnesota invasive species laws. Read Nov 4, 2026.
• Minnesota DNR, Watercraft Inspection Manual 2026. Read Nov 4, 2026.
• Discover Boating (NMMA), How to Launch a Boat. Read Nov 4, 2026.