How to Size a Sump Pump: GPH, Basin Size, and Discharge Needs

Choosing the right size sump pump isn’t just about picking the biggest or most powerful unit. If it’s oversized, your pump might short cycle and burn out early. If it’s too small, it won’t keep up during a storm—and you’ll end up with a flooded basement anyway.

As someone who’s been called to fix both mistakes more times than I’d like, here’s the no-nonsense guide to getting the sizing right the first time.

🔢 Step 1: Understand GPH and Head Height

GPH stands for gallons per hour. This is the main number you’ll see on pump packaging.

But here’s the catch: the GPH rating depends on how far the pump needs to lift the water—called the head height. Most labels give the GPH at 10 feet of lift.

To estimate your head height:

• Measure from the bottom of the sump pit to where the discharge pipe exits your home.
• Include any vertical rise plus a little extra for fittings and elbows.

Example: If your pit is 2 feet deep and your discharge exits 7 feet up, your total head height is about 9 feet.

Pro Tip: Choose a pump rated for at least 20-25% more GPH than you think you’ll need, to account for clogs, aging, and heavy storms.

📝 Step 2: Estimate Your Water Inflow Rate

Every home and yard is different. If you have heavy clay soil or poor grading, your pump might need to move a lot more water.

Here’s a basic test:

1. During a rainstorm, let the pump run.
2. When it shuts off, unplug it.
3. Let the pit fill for exactly 1 minute.
4. Measure the height of the water rise (in inches).
5. Multiply that by the basin’s diameter to estimate gallons per minute.

GPM = (π × radius² × height) / 231

Then multiply your GPM by 60 to get your target GPH. Add a safety buffer.

🏠 Step 3: Choose the Right Basin Size

Your sump pit (or basin) needs to match the size and style of your pump.

For pedestal pumps:

• A narrow, deep pit is okay (about 12″ wide)
• Float switch has room to move vertically

For submersible pumps:

• Pit should be at least 18″ wide and 22–24″ deep
• Allows float to move freely and prevents short cycling

Too small of a pit = more frequent cycles = shorter pump life.

🚧 Step 4: Match Your Discharge Line

The discharge pipe size affects how fast the pump can move water. Most sump pumps use a 1.5″ diameter PVC pipe.

Make sure:

• The pump’s outlet matches your pipe
• The pipe has a check valve installed
• You slope the pipe downward (1/8″ per foot minimum)

If you reduce the pipe size or have lots of elbows, your pump has to work harder—and that lowers its effective GPH.

🔧 Quick Reference Chart

Home Size / Water Volume	Recommended GPH @ 10 ft	Basin Size	Pump Type
Light duty (rare storms)	1,800–2,400 GPH	12″ x 18″	Pedestal or Sub
Medium duty (occasional flooding)	3,000–4,000 GPH	18″ x 22″	Submersible
High water table / finished space	4,200–5,500+ GPH	24″ x 24″+	Submersible

❓ FAQ: Sump Pump Sizing Edition

Q: Can I buy a bigger pump just to be safe?
A: Bigger isn’t always better. Oversized pumps short cycle, wear out faster, and may flood your neighbor’s yard if you’re not careful.

Q: How do I know how much water my pump moves now?
A: Use the bucket test: time how long it takes to empty a known amount of water (like 5 gallons), then calculate GPM.

Q: Should I size for the worst-case scenario?
A: No, size for your average storm and add 20–30% buffer. Backup systems can handle overflow in extreme conditions.

Q: What if my discharge pipe is smaller than the pump outlet?
A: That restricts flow. Always match the pump’s recommended discharge size or use a reducer only with caution.

Sizing your sump pump correctly means fewer surprises and longer life. It’s all about balancing power, capacity, and efficiency.

If you’d like help measuring your setup or double-checking your numbers, I’m happy to stop by and make sure you’re protected for the next storm.