Basin Parameters
Define your sump pit size and the rate at which groundwater enters during a storm.
Basin Parameters
in
in/min
Discharge Plumbing
ft
ft
ea
ea
Results
Required Pump Capacity
--
Gallons Per Hour (GPH)
Target Flow Rate
--
GPM
Total Dynamic Head
--
feet of head
Min. Recommended HP
--
horsepower
High Head Warning: Your Total Dynamic Head exceeds 25 feet. A standard residential sump pump may lose significant flow capacity at this head. Consider a high-head or commercial-grade effluent pump rated for your calculated TDH. Check the pump's performance curve at your specific head value before purchasing.
Key Terms Explained
Total Dynamic Head (TDH)
The total pressure resistance a pump must overcome, including vertical lift, pipe friction, and fitting losses. It is the correct metric for selecting a pump, not vertical lift alone.
Static Head / Vertical Lift
The straight-line vertical distance from the pump discharge port to the highest point in the discharge pipe. This is the largest single component of TDH in most residential installs.
GPH vs. GPM
Gallons Per Hour (GPH) and Gallons Per Minute (GPM) both measure flow rate. GPH = GPM x 60. Pump performance curves are typically published in GPH at various head values.
Basin / Sump Pit
The cylindrical underground collection pit that receives groundwater and foundation drainage. Standard residential pits are 18 or 24 inches in diameter and 24 to 36 inches deep.
Check Valve
A one-way valve in the discharge pipe that prevents water from back-flowing into the basin when the pump stops. Every sump system needs at least one. It adds roughly 9 feet of equivalent pipe friction.
Friction Loss
The head pressure lost to turbulence and drag as water moves through pipes, elbows, and valves. Narrower pipe, longer runs, and more fittings all increase friction loss.
Float Switch
A buoyant sensor that activates the pump motor when water rises to a set level and shuts it off when the basin drains to a lower level. Tethered, vertical, and electronic types are the most common.
Short-Cycling
When an oversized pump empties the basin so quickly that it restarts within seconds of shutting off, wearing out the motor and float switch prematurely. Proper sizing prevents this.
The Complete Guide to Sump Pump Sizing
Choosing the wrong sump pump is one of the most common and expensive basement waterproofing mistakes. Too small, and the pump cannot keep up during a heavy rain event, flooding your basement. Too large, and short-cycling destroys the motor in a few seasons. This guide explains exactly how to use the calculator above to get the right pump the first time.
How to Use This Calculator
- Enter your basin diameter in inches. Check the pit liner for a stamped diameter, or measure across the top opening. Standard sizes are 18 and 24 inches.
- Estimate your water rise rate - how fast the water level climbs in inches per minute during a typical heavy storm. If you have observed your current pump cycling, that gives you a real data point. A common starting value for a moderate water table is 1 to 2 inches per minute.
- Measure the vertical lift from the pump discharge outlet to the point where the pipe exits above grade or turns horizontal toward your discharge point. This is static head.
- Add your horizontal run - the total length of horizontal pipe from where it exits the foundation to its discharge point (a downspout, a dry well, or a daylight exit).
- Select your pipe diameter, count your elbows, and confirm your check valve count. All fields update results instantly as you type.
How the Math Works
The calculator first figures out how many gallons your basin holds per inch of water height, using the formula for the volume of a cylinder divided by 231 cubic inches per gallon. It then multiplies that by your rise rate to get an inflow in GPM. A 50% safety margin is applied (the pump must drain the pit, not merely match inflow), giving a target GPM that is converted to GPH.
For TDH, the calculator converts each fitting into an equivalent pipe length (2.5 feet per 90-degree elbow, 9 feet per check valve), adds the horizontal run, then applies a friction coefficient based on pipe diameter to compute friction head. That friction head is added to the static (vertical) lift to produce TDH. If TDH exceeds 25 feet, the result turns red and a warning appears, because most residential pumps lose flow significantly at high head values.
Finally, the recommended horsepower is mapped from the computed GPH and TDH: a 1/3 HP for low-demand applications, 1/2 HP for moderate head or flow, and 3/4 HP when either metric is high. Always cross-reference the specific pump's published performance curve to confirm rated GPH at your calculated TDH before purchasing.
Frequently Asked Questions
Vertical lift (static head) is only part of the resistance your pump must overcome. Water also loses energy to friction as it travels through pipes, fittings, elbows, and check valves. Total Dynamic Head adds all of these friction losses to the static head, giving you the true pressure load the pump must work against at its rated flow. A pump selected using only vertical lift will underperform in real installation and may fail to keep up during a heavy storm when you need it most.
No. Oversizing a sump pump causes it to cycle on and off too rapidly, a condition called short-cycling. Because the pump empties the basin faster than water can refill it, the motor starts, runs for only a few seconds, shuts off, and immediately starts again. This constant starting draws a large inrush of current each time, dramatically shortening motor life and burning out the float switch faster. Size the pump to handle your calculated GPH with the 50% safety margin already built in, and you will get longer run cycles and a much longer pump lifespan.
A 1/3 HP pump is appropriate for most average homes with modest basement depth and short discharge runs, typically handling up to 2,000 to 3,000 GPH at low head pressures. A 1/2 HP pump is rated for higher flows and taller lifts, making it the right choice when your vertical lift exceeds 10 feet, your discharge run is long, or your basin collects water quickly during heavy rain. The 1/2 HP motor can maintain its rated GPH against more resistance without bogging down.
A narrower pipe forces the same volume of water through a smaller opening, increasing velocity and dramatically raising friction loss. Upgrading from 1.25-inch to 1.5-inch pipe can reduce friction head by 30 to 50 percent on a typical run, which lowers the TDH your pump must overcome and lets it operate at a more efficient point on its performance curve. For high-flow applications or runs over 20 feet, 2-inch pipe is often the right choice to keep friction losses manageable and protect pump motor life.
A check valve is a one-way valve installed in the discharge pipe that prevents water from flowing back down into the basin when the pump shuts off. Without one, every pump cycle would need to re-lift all the water sitting in the vertical discharge pipe. Check valves are essential, but they add friction resistance equivalent to roughly 9 feet of straight pipe. Most installations need exactly one check valve. If you have two (for example, one at the pump and one at the foundation wall), account for both in your TDH calculation.
This calculator provides estimates based on standard engineering heuristics for residential sump pump sizing. Actual pump performance varies by manufacturer and model. Always consult a licensed plumber or waterproofing contractor for high-TDH installations, commercial applications, or if your basement has unusual drainage conditions. This tool is not affiliated with any pump manufacturer.