28 mm inner diameter at both ends, compatible with water bottles that have a 28 mm outer thread and standard water bottles. Use our filter coupling to connect the water filter to a cleaning water bottle, enabling quick backwashing of the filter. Also compatible with a threaded water bag to build a filtration system, so you can easily obtain ample cleaning water and keep your hands free. Our 2-piece water filter adapter is supplied with 10 spare gaskets that fit tightly with the couplings, improving sealing performance, reducing water leakage, and enhancing water filter efficiency. Practical applications - Attach to a cleaning water bottle for rapid backwashing of the water filter. - Build a filtration system with a threaded water bag to obtain large volumes of cleaning water, enabling hands-free operation. - Use with 28 mm outer-thread bottles or standard water bottles for versatile compatibility and setup.