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A barrel connector must match both the hole in the barrel and the pipe, hose, or fitting you want to attach. Those two measurements are not always the same.
For most rain barrels, the connector is a bulkhead fitting, tank adapter, or threaded fitting that passes through the barrel wall and creates a watertight connection. To choose the right size, identify the existing fitting if there is one, measure the opening carefully, and confirm the thread or hose size on the part that will connect to it.
What Is a Barrel Connector?
A barrel connector is a fitting that lets water enter or leave a rain barrel without leaking around the opening.
Common examples include:
- A spigot connection near the bottom of a rain barrel
- An overflow fitting near the top
- A hose connection between two barrels
- A pipe connection leading to a pump or irrigation line
- A fitting used to connect several barrels together
Many systems use a bulkhead fitting. A bulkhead fitting passes through a hole in the barrel wall. A gasket seals against the barrel, while a nut holds the fitting in place.
The size printed on a bulkhead fitting usually describes the pipe connection. It does not necessarily describe the diameter of the hole you need to drill.
That difference is one of the main reasons barrel connector sizing can be confusing.
First Decide Which Size You Are Trying to Find
Before measuring anything, identify what you need to replace or connect.
You may need to determine:
- The size of an existing barrel fitting
- The diameter of an existing hole
- The pipe thread size on the fitting
- The hose size that connects to the fitting
- The correct hole size for a new bulkhead fitting
These measurements are related, but they are not interchangeable.
For example, a fitting sold for a certain nominal pipe size may require a noticeably larger hole through the barrel because the body of the fitting must pass through that hole.
How to Measure an Existing Barrel Connector
If the barrel already has a connector, measuring the connector itself is usually better than measuring only the hole.
1. Identify the Type of Connection
Look at the side of the fitting where your hose or pipe connects.
Common connections include:
- Female pipe threads
- Male pipe threads
- Garden hose threads
- Hose barbs
- Slip-fit pipe sockets
- Compression fittings
Do not assume two fittings are compatible just because their openings look about the same size.
A garden hose thread, for example, is different from common pipe threads.
2. Measure the Threaded Section
If possible, use calipers to measure the outside diameter of male threads.
For a female fitting, measure the opening and identify the thread type rather than relying only on its diameter.
A ruler can give you a rough measurement, but calipers are much easier to use when fittings are close in size.
Pipe fittings are often identified by a nominal size. Nominal means the name of the pipe size does not necessarily equal the exact diameter you measure with a ruler.
Because of this, you should compare your measurement with a thread-size chart for the specific thread standard used by your fitting.
3. Check the Thread Pattern
Diameter alone is not enough.
Threads also have a specific spacing and shape. Two connectors with similar diameters may not screw together correctly.
If you are replacing an existing fitting, take the old fitting with you when choosing the replacement whenever possible. It gives you a much more reliable reference than a measurement written on a piece of paper.
How to Measure a Hole in a Rain Barrel
If the connector is missing and only the hole remains, measure the hole itself.
Use calipers if possible and measure straight across the widest part of the circular opening.
If the hole is slightly rough or uneven, take several measurements in different directions.
Record the measurement in both:
- Inches
- Millimeters
Millimeters can make it easier to compare fittings when the difference between two possible sizes is small.
Keep in mind that the hole diameter normally corresponds to the outside body of the bulkhead fitting, not its nominal pipe size.
For this reason, you should not buy a connector simply because its listed pipe size appears to match the hole.
Instead, check the manufacturer's required hole diameter for the exact fitting you plan to use.
Barrel Hole Size and Connector Size Are Different
This is the most important sizing rule.
A bulkhead fitting has several parts:
- The pipe connection
- The threaded body that passes through the barrel
- The gasket
- The retaining nut or flange
The barrel hole must be large enough for the fitting body to pass through it.
That means a connector with a certain pipe connection normally needs a larger hole than the named connection size might suggest.
Do not guess the drilling size from the fitting name.
Always use the hole-size specification supplied for that particular bulkhead fitting.
Measure the Pipe or Hose You Need to Connect
The barrel fitting also needs to match the rest of your water system.
If you are connecting a hose, determine whether the hose uses:
- Garden hose threads
- A hose barb
- A quick-connect fitting
- Another adapter
If you are connecting rigid pipe, identify:
- Pipe material
- Nominal pipe size
- Threaded or slip connection
- Male or female connection
PVC, polyethylene tubing, garden hose, and threaded metal fittings can all use different connection systems.
Adapters are available for many combinations, but using fewer adapters usually makes a rainwater system simpler and easier to maintain.
Choosing a Connector for a New Barrel
If you are adding a connection to a barrel that does not already have a hole, start with the water use rather than the drill size.
For a Garden Hose or Spigot
Choose a fitting that works with the spigot or hose connection you plan to use.
Also think about flow. A very small outlet may work for filling a watering can but can restrict water if you are trying to feed a larger irrigation line.
For Connecting Two Barrels
The connecting line should be large enough for water to move between the barrels at the rate your system needs.
Small connections can work, but they may take longer to equalize the water level between barrels.
Place connected-barrel fittings carefully so both barrels can fill and drain as intended.
For an Overflow
An overflow fitting must handle water entering the barrel during heavy rainfall.
A small overflow can become a bottleneck even if it works during light rain.
The overflow also needs to discharge water somewhere that will not damage the foundation, create erosion, or send water toward a neighboring property.
For a Pump
Match the barrel outlet to the pump's inlet requirements.
Explore practical steps for installing a bulkhead fitting in a rain barrel to verify airflow and overflow provisions around the storage vessel.
Using a connector that is much smaller than the pump inlet can restrict flow and contribute to poor pump performance.
Pump selection also depends on flow rate and head height. Head height is the vertical distance, along with system resistance, that affects how hard a pump must work to move water.
Check the Barrel Wall Before Installing a Fitting
Connector size is only part of the job.
The barrel wall must also provide a suitable sealing surface.
Look for:
- A flat area
- Enough room for the gasket
- Enough room inside for the retaining nut
- Clearance from molded ribs
- Clearance from the barrel bottom
- Enough space to tighten the fitting
Curved or heavily textured areas can be harder to seal.
Avoid placing a fitting so low that the nut or gasket cannot sit properly against the inside of the barrel.
Gasket Position Matters
Most bulkhead fittings use a gasket to create the seal.
Install the gasket according to the fitting manufacturer's directions. Different fitting designs may place the gasket differently.
Do not assume that adding extra sealant will correct a badly sized hole or an uneven mounting surface.
A hole that is too large can prevent the gasket from sealing properly.
A hole that is too small may damage the fitting or barrel if you force the connector into place.
What If the Existing Hole Is Too Large?
You may still be able to use the barrel, but the solution depends on how oversized or damaged the opening is.
Possible approaches include:
- Using a properly sized larger bulkhead fitting
- Using a fitting designed for the existing opening
- Relocating the connection to a sound section of the barrel and closing the old hole with a suitable repair method
Do not depend on large amounts of sealant to compensate for a badly oversized or cracked opening.
If the plastic around the hole is split, brittle, or badly distorted, a connector may not create a dependable seal.
What If the Existing Hole Is Too Small?
If you have confirmed the required hole size for the new fitting, you may be able to enlarge the hole carefully.
A hole saw is commonly used for new openings in plastic rain barrels.
Before drilling:
- Empty the barrel
- Make sure you can reach the inside of the fitting location
- Confirm the exact required hole diameter
- Check that no plumbing or other fittings are behind the drilling area
- Support the barrel so it cannot move
Do not enlarge the opening until you have the fitting specification in hand. Removing a little more plastic is easy. Making an oversized hole smaller is much harder.
Do Not Size a Connector From the Nut
The outside nut or flange of a bulkhead fitting is usually much larger than both the pipe connection and the required barrel hole.
Measuring the nut can therefore give you the wrong size.
Measure or identify the part that actually matters:
- Pipe connection size when matching plumbing
- Thread size when replacing a threaded part
- Fitting body diameter when checking an opening
- Manufacturer's required hole diameter before drilling
Consider Flow as Well as Fit
A connector can physically fit your barrel and still be too small for the job.
Water flow is affected by the smallest restriction in the system.
A small barrel outlet may slow:
- Gravity-fed irrigation
- Water transfer between barrels
- Pump supply
- Tank draining
- Overflow discharge
Gravity systems are especially sensitive because a rain barrel does not create much pressure.
Water pressure from a barrel generally decreases as the water level drops.
For garden watering, a larger unrestricted outlet can sometimes be more useful than trying to force water through several small adapters.
Avoid Unnecessary Reducers
It is common to find a large tank fitting reduced through several adapters until it reaches a small hose.
Sometimes that is necessary. But every extra fitting adds another possible place for:
- Leaks
- Clogs
- Flow restriction
- Freeze damage
- Difficult maintenance
When planning a new connection, try to keep the plumbing simple.
Choose a barrel connector that is reasonably close to the size of the line you actually need.
Check Material Compatibility
Barrel connectors are made from different plastics and metals.
Make sure the fitting is suitable for:
- The barrel material
- The water use
- Outdoor exposure
- Any pipe or hose attached to it
Be especially careful when connecting rigid plumbing to a thin plastic barrel. Heavy pipe can put stress on the barrel wall.
Support long or heavy pipe runs separately rather than making the barrel fitting carry their weight.
Think About Freezing Conditions
Water trapped inside fittings, valves, and short pipe sections can freeze and expand.
In freezing climates, connector placement should allow the system to be drained or protected according to the overall winter setup.
Do not assume a fitting is protected from freezing just because the barrel itself has been emptied. Low points in valves and plumbing may still hold water.
A Simple Way to Choose the Right Size
For most rain barrel projects, use this order:
- Decide what the opening will do.
- Determine the hose or pipe connection you need.
- Choose a fitting that matches that connection.
- Check the fitting's required hole diameter.
- Confirm the barrel has a flat, accessible mounting area.
- Drill only after you have verified the exact fitting dimensions.
If you are replacing an existing connector, remove it when practical and identify the thread and fitting type directly.
This approach is more reliable than trying to choose a connector from the barrel hole alone.
Frequently Asked Questions
How do I know what size connector my rain barrel has?
Identify the connection type first, then measure the threaded or hose connection. If possible, remove the fitting and compare it directly with replacement fittings. Remember that the nominal pipe size may not equal the diameter you measure.
Is the barrel hole the same size as the connector?
Usually not. The hole must fit the body of the bulkhead fitting, while the advertised connector size normally describes the pipe connection. Check the required hole diameter for the exact fitting.
Can I measure a barrel connector with a ruler?
Yes, but a ruler gives only a rough measurement. Calipers are better when you need to distinguish between similar fitting sizes or measure a barrel opening accurately.
Can I use any bulkhead fitting in a rain barrel?
No. The fitting needs to match the hole size, barrel wall, pipe or hose connection, and intended water use. You also need enough space inside and outside the barrel to install the gasket and retaining nut.
Should I use a bigger connector for better water flow?
A larger connector can reduce restriction, but bigger is not automatically better. Size the outlet for the flow your irrigation line, pump, overflow, or barrel connection actually needs.
How do I know what hole saw size to use?
Use the hole diameter specified by the manufacturer of the bulkhead fitting. Do not select a hole saw based only on the fitting's nominal pipe size.
Can I install a fitting in a curved part of the barrel?
Sometimes, but a flat area usually seals more reliably. Strong curves, ribs, seams, or textured surfaces can prevent the gasket from sitting evenly.
