What size sump pump do I need?

For most residential basements (800-1200 sq ft), a 1/3 HP pump with 2,000-3,000 GPH capacity is sufficient. If you have high water inflow (>10 GPM), severe flooding risk, or need to pump more than 15 feet vertically, upgrade to 1/2 HP with 4,000+ GPH capacity.

How do I calculate GPH for a sump pump?

Estimate water inflow rate (GPM or gallons per minute entering the pit), then multiply by 60 to get GPH. For safety, choose a pump rated at 2-3x your calculated inflow rate. For example, if water enters at 5 GPM (300 GPH), use a pump rated for 600-900 GPH minimum.

What is discharge head (vertical lift)?

Discharge head is the vertical distance the pump must push water from the pit to the discharge point (usually outside ground level). Add 1 foot of head for every 10 feet of horizontal run due to friction loss. For example: 10 ft vertical + 20 ft horizontal = 10 + 2 = 12 feet total head.

Do I need a battery backup sump pump?

Yes, if you experience frequent power outages during storms (when you need the pump most!). A battery backup pump can run for 5-7 hours on a marine battery, pumping 1,000-2,000 GPH. Consider installing a backup if your basement has finished living space or valuable storage.

What's the difference between 1/3 HP and 1/2 HP pumps?

A 1/3 HP pump typically handles 2,000-3,500 GPH at 10 feet of head. A 1/2 HP pump handles 4,000-5,500 GPH. The 1/2 HP also pumps to greater heights (15-20 ft) and recovers faster during heavy inflow. Use 1/2 HP for larger basements (>1500 sq ft) or severe flooding areas.

💧 Sump Pump Sizing Calculator

Calculate GPH & Horsepower for Basement Drainage

Basement Area (sq ft)

Total basement square footage

Water Inflow Rate

Vertical Lift (feet)

Height from pit to discharge

Horizontal Run (feet)

Pipe length to discharge point

💧 Sump Pump Sizing Fundamentals

A properly sized sump pump prevents basement flooding by removing groundwater faster than it enters. The key factors are capacity (GPH/GPM) and total head (vertical + horizontal equivalent).

Pump Horsepower & Capacity Guide

Horsepower	Typical GPH (at 10 ft)	Max Head	Best For
1/4 HP	1,800-2,500	15 ft	Small basements, light water
1/3 HP	2,500-3,500	18 ft	Most residential basements
1/2 HP	4,000-5,500	20 ft	Large basements, high inflow
3/4 HP	6,000-8,000	25 ft	Commercial, severe flooding

Calculating Total Head

Total Head = Vertical Lift + (Horizontal Run / 10)

Example: 10 ft vertical + 30 ft horizontal = 10 + 3 = 13 feet total head

Pump performance decreases as head increases. A pump rated at 3,000 GPH at 10 ft might only deliver 2,000 GPH at 15 ft.

Inflow Rate Estimation

Light (2-5 GPM): Minimal seepage, occasional use
Medium (5-10 GPM): Moderate groundwater, typical residential
High (10-20 GPM): High water table, heavy rain area
Severe (20+ GPM): Flooding risk, near river/lake

Rule of thumb: Choose a pump with 2-3x your estimated peak inflow rate for safety margin.

💡 From Tom Harris, Foundation Waterproofing Specialist (18 years):

"The #1 mistake homeowners make is buying a pump based on price alone. A $150 1/4 HP pump might work for 6 months, then during a big storm, it can't keep up and your basement floods. I always recommend oversizing to 1/2 HP for any basement with finished living space. Also, ALWAYS install a battery backup—90% of pump failures happen during power outages, which is exactly when you need it most!"

⚠️ Common Mistakes

1. Undersizing for the Lift: A pump rated at 3,000 GPH assumes a certain head (usually 10 ft). If you need to pump 20 feet vertically, that same pump might only deliver 1,500 GPH. Always check the pump curve chart!

2. Forgetting Check Valve: Without a check valve, water flows back into the pit after the pump stops, causing it to cycle constantly and burn out.

3. Not Testing: Pour 5 gallons of water into the pit monthly to ensure the float switch works. A stuck switch = a flooded basement when you're on vacation.

Tom Harris

Foundation Waterproofing Specialist, 18 years experience

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