Short definition
An overflow tube is the vertical pipe rising from the flush valve in a toilet tank, with its top about 1 inch below the tank rim. It does two jobs: prevents the tank from overfilling (water entering above the tube top spills harmlessly into the bowl) and receives the refill tube from the fill valve, which routes a portion of refill water into the bowl after each flush to recharge the trap.
What it is
The overflow tube is integral to the flush-valve assembly — they’re sold and replaced as one part. The tube extends straight up from the flush-valve seat to a height set by the manufacturer. A small flexible refill tube from the fill valve clips onto the top of the overflow tube via a metal or plastic clip; refill water passes from the fill valve, through the refill tube, into the overflow tube, and down to the bowl.
UPC backflow rules require the refill tube to enter the overflow via that clip — above the water line — not be submerged. Submerged refill tubes can siphon contaminated tank water back into the supply.
Why it matters to a homeowner
The overflow tube is the calibration reference for tank water level. The standard target is 1 inch below the top — most overflow tubes have a marker line at the manufacturer’s recommended fill level. When you adjust the float, you’re setting the water level relative to that line.
Other things that involve the tube:
- Toilet refilling but bowl water not recharging. The refill tube has fallen out of the overflow tube, or it was never clipped above the water line. Reposition with a clip.
- Cracked overflow tube (rare). Tank water bypasses the bowl on refill; flush is weak. Replace the whole flush valve.
Common variants
- Overflow tube (this entry, in-tank) vs. commercial flushometer overflow. Different mechanism.
- Overflow tube vs. refill tube. Refill tube is the small flexible hose from the fill valve. Overflow tube is the rigid vertical pipe in the flush-valve assembly. Both work together.