Sump pump sizing Last reviewed June 2026
Sump Pump Installation Guide: Pipe Size, Check Valves & More
By the WhatSize editorial team 4 min read Standards-reviewed
Most sump pumps use 1.5" discharge pipe. The sump basin should be at least 18" wide and 24" deep. Always install a check valve to prevent backflow and ensure your pump primes properly.
Proper Installation Is as Important as Proper Sizing
Even a perfectly sized sump pump will fail if installed incorrectly. The three most common installation mistakes are: undersized discharge piping (creates back pressure and reduces GPH), missing check valve (water flows back into the pit between cycles, causing short-cycling), and improperly sized sump basin (too small fills too quickly, causing excessive cycling).
Discharge pipe sizing is critical: a 1.5" pipe carries nearly 2.5x more water than a 1.25" pipe at the same pressure. If your pump calls for 1.5" discharge, don't reduce it. For long horizontal runs (50+ feet), consider stepping up to 2" pipe to reduce friction loss.
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Sump Pump Installation Specifications
- Discharge pipe size: 1.5" for most residential pumps (check your pump's specifications). For runs over 50 ft, use 2" pipe
- Check valve: Required by most building codes. Install within 12-18" of the pump discharge. Silent check valves ($20-40) reduce water hammer noise
- Sump basin: Minimum 18" diameter, 24" deep for 1/3 to 1/2 HP. Minimum 24" diameter for 3/4 HP
- Drainage: Discharge water at least 10 feet from the foundation. Never discharge into a sewer line (illegal in most areas)
Frequently asked questions
How do I size a sump pump installation?
A discharge pipe that's too small creates back pressure on the pump, reducing flow rate significantly. A 1/2 HP pump rated for 3,600 GPH at 10 ft head might only deliver 2,000 GPH through undersized 1.25" pipe. The pump also works harder and wears out faster.
Do I need a check valve on my sump pump?
Yes. Without a check valve, water in the discharge pipe flows back into the sump pit after each pump cycle. This causes short-cycling (pump turns on/off too frequently), reduces pumping efficiency, and wears out the pump faster.
How far from the house should I discharge sump pump water?
At least 10 feet from the foundation, preferably 15-20 feet. Discharging too close to the house allows water to seep back into the foundation and reappear in the sump pit — creating an endless cycle. Use downspout extensions or buried drain pipe to route water away.