The "best" diameter depends on the pump's inlet/outlet size and the distance the water needs to travel.
This is the most critical part of the plumbing. A pump is much better at "pushing" water than "pulling" it.
The Golden Rule: Never use a suction pipe smaller than the pump's intake port.
The "Upsize" Strategy: If your suction line is longer than 15–20 feet, it is best practice to use a pipe one size larger than the intake port (e.g., if the pump has a 1-inch inlet, use a 1.25-inch pipe). This reduces vacuum resistance and prevents cavitation (harmful air bubbles forming inside the pump).
The discharge pipe diameter determines how much pressure is lost as the water travels to its destination.
Standard Match: For short runs (under 50 feet), matching the pipe diameter to the pump's discharge port size is usually sufficient.
Long Distance Adjustment: For every 100 feet of pipe, "friction loss" increases. To maintain pressure over long distances, you should increase the pipe diameter.
If you know your pump's GPM (Gallons Per Minute) rating, use this table as a general guide for PVC or Poly pipe:
Water rubbing against the inside of the pipe creates resistance.
Velocity: Ideally, water should move at a speed of 5 feet per second or less.
Fittings: Every 90-degree elbow or T-joint adds "equivalent length" to your pipe, increasing the workload on the pump. Use "long-sweep" elbows where possible.
Narrow Pipe: High velocity, low pressure at the end (due to friction).
Wide Pipe: Low velocity, higher maintained pressure at the end.
Pro-Tip: If you are using a Fujian-style self-priming pump for domestic use, they almost always come with 1-inch (25mm) ports. Using 1.25-inch pipe for the suction line and 1-inch for the main delivery line is the most common high-performance setup.
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