Under most sinks, the hot water line is on the left and the cold water line is on the right — this is the standard plumbing convention in nearly all U.S. homes, whether it’s a bathroom sink or a kitchen setup. The easiest way to confirm which side is hot water on your specific sink is to trace each line back to its source: the line connected to your water heater is the hot line, and the line connected directly to the main water supply is the cold line. You can also just run the water and feel each line — the hot water pipe will warm up within a few seconds.
If you find hot water in what should be the cold water line (or the reverse), that usually means the supply lines were crossed during a past repair or installation — a common but fixable mistake. For a full picture of your plumbing’s hot and cold water lines, or if your sink’s hot and cold sides seem swapped, the plumbers at The 5 Star Plumbing can inspect and correct the connections.