at least 1.25 in.
2 in.
Two inches
For a bathroom sink, a standard size P-trap with a 1 1/4 inch diameter is typically used.
4 inch or 110mm NO less then 11/4" and normally 11/2" depending on the code your using Most installations are 11/4" INLET and 11/2" outlet used in conjunction with a reducing slip nut (11/4 -11/2) 4" is the MAIN House trap or in some cases a strorm trap depending on the area that is being drained
The standard size of a bathroom P-trap is typically 1 1/4 inches in diameter.
The standard size of a bathroom sink p-trap is typically 1 1/4 inches in diameter.
The recommended p trap size for a standard kitchen sink installation is typically 1 1/2 inches in diameter.
The standard size of a 2-inch P-trap used in plumbing installations is typically 2 inches in diameter.
I suspect that you have a "p" trap in either a floor drain, or possible the laundry tub or a sink somewhere in the house. If you look under the kitchen sink, or a lavatory in the bathroom you can see a p-trap, it's shaped sort of like the curved part of the letter p. The purpose of the p-trap is to trap, or prevent, sewer gasses in the sewer lines from backing up into the house. By design, p-traps catch and hold some water, which blocks the pipe and stops the gas from coming up through the drain. IF you have a floor drain, or a lavatory drain, either of which don't get used enough to keep the water in the trap from evaporating away, leaving the drain pipe open, then when a lot of water is poured or flushed, pumped out of a washer, then that flowing water will push sewer gasses back up through the drain with the dry p-trap. What you are smelling is sewer gas.
The same as any other P trap except it should empty into a properly trapped and vented fixture
P-trap may be cracked, nuts are loose on p-trap, or check for leak above p-trap that may be dripping down on p-trap.
Who invented the p trap