Thread sizes explained (BSP)
What does BSP mean?
BSP stands for British Standard Pipe. This is the standard thread used for most plumbing components including water tank fittings in the UK. It ensures that fittings can be joined together through compatible connections.
When looking at water tank accessories, you will often see the outlet and inlet sizes referred to as 1/2″ BSP, 3/4″ BSP, 1″ BSP ect. These measurements refer to the thread size and not the actual measured outside diameter of the fitting.
Difference between BSP Male and BSP Female
Across our site and plumbing applications in general, you will see fittings describes as Male BSP and Female BSP.
- Male BSP (BSPM) has an external thread that screws into another fitting.
- Female BSP (BSPF) has an internal thread that accepts a male fitting.
As an example. a water tank with a 1″ BSP Female outlet will typically connect to a 1″ BSP Male fitting such as a tap or hose tail
Common BSP Sizes
| BSP Size |
Typical Applications |
| ½” BSP |
Small taps and hose connections. Standard garden hose size. |
| ¾” BSP |
Garden irrigation and general water transfer. Common for waste outlets on campervan water tanks. |
| 1″ BSP |
Standard water tank outlets for mid-size tanks. |
| 1¼” BSP |
Higher flow rate applications |
| 1½” BSP |
Large pumps and commercial installations. Common equivalent size for a campervan filler inlet. |
| 2″ BSP and larger |
High-flow tanks and industrial systems |
Parallel vs Tapered Fittings
Parallel thread fittings (BSPP) seal using a washer, whilst tapered fittings (BSPT) create a seal as the threads tighten together.
The majority of water tank manufacturers will use BSPP parallel threads and this is pretty universal across most water tanks. It is always worth checking whether the fitting is parallel (BSPP) or tapered (BSPT) before installation.
💡Using a tapered thread can cause irreparable damage to the threads so check before installing that the threads are compatible.
Internal vs External Diameter
ID (Internal Diameter) is the measurement across the idside of a fitting or thread, whilst OD (Outside Diameter) is the measurement across the outside of a pipe of threaded fittings,
💡BSP sizes refer to the thread standard rather than the actual outside diameter of the fitting.
Overflow
An overflow allows excess water to leave the tank once it reaches maximum capacity. This helps to prevent overfilling and reduces the risk of water damage from an excess of water.
An overflow is common on tanks that are filled automatically, for example on rainwater harvesting systems, mains-fed break tanks and livestock water storage tanks.
Most overflows are installed near the top of the tank and should direct any overflow to a suitable drainage point. Depending on the application, they may also be fitted with an insect screen or rodent guard to prevent pests from entering the tank.
💡An overflow only carries water when the tank is full and is not the same as a tank vent.
Inspection Lid
The inspection lid (also known as an access lid, inspection cap or access hatch) provides access into a water tank, for a number fo different reasons. This includes inspection, cleaning and maintenance through its life.
Depending on the tank, the lid can also be used to install fittings or access equipment that may be inside.
Examples of uses for the inspection cap;
- Cleaning and routine maintenance
- To visually inspect the internal condition
- Install or service a submersible pump
- Fit float stiches or level sensors
- Remove debris or sediment
Most water tanks we supply have a screw or hinged tank lid. Typically, the larger the tank, the larger the access hatch in order to make tank maintenance easier.
Types of lid;
The type of inspection lid can vary, with options that include;
- Vented lids – allows air to escape or enter the tank as the water level rises and falls.
- Plain lid – secure seal, ideal for potable water or tanks being transported.
- Internal lid – a lid that sits in an internal thread and protrudes into the tank.
- external lid – a lid that screws onto a neck and protrudes from the top of the tank.
- Lockable lids – a lid that can be locked
💡The inspection lid should remain closed when not in use to prevent dirt or debris entering the tank.
Nominal Capacity
Nominal capacity is a term to articulate the manufacturer’s stated storage volume of a water tank under normal conditions. For example, a 100 Litre water tank is designed to hold approx. 100 Litres of water. In practise, the amount of usable water varies on a number of variables including;
- location of the outlet
- the installation and location of other fittings such as a float valve
- whether space is left at the top for expansion or overflow.
There is also a manufacturing tolerance to consider. In most cases, this can be +/- 3%. For example, a tank that has a nominal capacity of 100 litres, could actual vary from 97 to 103 Litres due to the manufacturing variables.
💡A tanks nominal capacity can be used for naming the tank and comparison but the actual usable volume can vary slightly depending on how the tank is installed
Common Water Tank Terms
The below table details water tanks terms and other names to which they can be referred.
| Water Tank Term |
Also referred to as; |
| Inlet |
Fill point, fill connection, fill port. |
| Outlet |
Drain point, discharge outlet, outlet connection |
| Bulkhead fitting |
Tank connector, tank fitting, threaded tank fitting |
| Lid |
Inspection lid, inspection cap, access cover, access hatch |
| Vent |
Breather, vent fitting, air vent |
| BSP |
British Standard Pipe (thread) |
| ID |
Internal diameter, inside diameter |
| OD |
Outside diameter, external diameter |
| Seal |
Gasket, sealing washer, rubber seal |
| Hose tail |
Hose barb, hose connector, hose fitting |
| Tap |
Ball valve, gate valve, shut-off valve, isolation valve |
