"What size do I need" almost always means one of two things: will it fit past my pipes, and will it hold what I own? Cabinet width alone won't answer either. Here's how to translate your cabinet into a size that works.
Standard cabinets, non-standard interiors
Kitchen sink base cabinets are usually 24, 30, or 36 inches wide, but the usable interior is smaller after the frame, and the drain splits what's left into two zones. A 30-inch cabinet often gives you two open lanes of roughly 10 to 12 inches — so you're sizing for a lane, not for 30 inches.
Height is the number that fails people
Measure from the cabinet floor to the bottom of the P-trap curve. Under 8 inches and you need a low first tier with the second tier set above the pipe. Over 12 inches and you can run a full two-level setup on both sides. That single measurement determines more than width does.
Size to your tallest bottle
Pull out the tallest thing you keep under there — usually a spray bottle or a dish soap refill — and measure it. Your lower shelf clearance has to beat that number, or you'll be laying bottles on their sides within a week.
Why adjustable beats exact
Buying a fixed-size rack means betting that your cabinet matches a catalog. An expandable unit fits a range instead, and a height-adjustable shelf lets you re-set clearance when your bottles change. That's the case for the SinkFlex 2-Tier Pull-Out Under Sink Organizer ($33, 2-pack): it widens to your opening, the shelf raises and lowers around plumbing, and because you get two units you can size each side of the pipe separately — a narrow tall stack on the pipe side, a wider one on the open side.
When to size down instead
If the trap sits very low and both lanes are under 8 inches, don't force a floor unit. Move sponges, gloves, and bags to a no-drill door back hanger ($23) and keep the floor for tall bottles. Want the sizing decided for you? The 3-Piece Bundle ($55) covers shelf, baskets, and caddy in matching sizes.