When it’s time to lower or empty a pool for cleaning, winterizing, or repairs, most homeowners reach for the tool they already own. Using a sump pump to drain a pool is a common, practical, and inexpensive method — but it comes with real risks if you rush it. Drain too fast, discharge the water in the wrong place, or empty a pool completely without understanding the hydrostatic pressure underneath it, and you can damage your yard, violate local rules, or even pop your pool shell out of the ground. This guide covers how to do it right, from choosing the pump to picking a safe discharge point.
Can a Sump Pump Drain a Pool?
Yes. A submersible sump pump is well suited to the job. You drop it into the pool, connect a discharge hose, plug it in, and it moves water out steadily. A typical 1/3 to 1/2 horsepower submersible sump pump moves somewhere between 30 and 60 gallons per minute depending on the head height and hose, which will empty a residential pool over a matter of hours to a day.
The advantages are obvious: you probably already own a sump pump, it is submersible and self-contained, and it handles the volume of water a pool holds without complaint. A pool-specific cover pump or a rented trash pump works too, but a standard sump pump is the everyday go-to.
Choosing and Sizing the Pump
Match the pump to the job. Consider these factors:
- Flow rate (GPM): higher is faster. A 1/2 HP pump drains more quickly than a 1/4 HP model, which matters for a large pool.
- Head height: the vertical distance the pump must lift water reduces its effective flow. If you’re pushing water uphill to the street, expect slower drainage.
- Solids handling: for a dirty pool with leaves and sediment, a pump that tolerates some debris avoids clogging.
- Automatic vs. manual float: a manual pump keeps running until you unplug it; an automatic float switch shuts off at a set level. For draining, a manual pump gives you more control, but never leave it fully unattended.
For most residential pools a 1/3 to 1/2 HP submersible pump strikes the right balance of speed and gentleness.
Setting Up the Hose and Discharge
Connect a discharge hose rated for the pump’s outlet — a standard garden hose works but drains slowly; a wider 1.25 to 2 inch discharge hose moves water far faster. Run the hose to your chosen discharge point and secure it so it can’t whip around or fall back into the pool. Here is the part people get wrong most often: where the water goes matters enormously.
Safe Discharge Options
- A sanitary sewer cleanout (where local codes allow) is often the safest and most compliant route for chlorinated water.
- A landscaped area on your own property that can absorb the water — but only after the chlorine has dissipated.
- A gravel bed or dry well that lets water percolate slowly.
Where NOT to Discharge
- Storm drains: in many jurisdictions it is illegal to send chlorinated pool water into storm drains, which flow untreated to streams and lakes.
- A neighbor’s property or a shared drainage easement.
- Directly onto your foundation, septic drain field, or a slope that will erode.
Dechlorinate Before You Drain
Chlorine and other pool chemicals harm lawns, plants, and aquatic life. Before draining onto any landscaping or into any waterway-connected system, stop adding chlorine and let the level drop. Test the water and aim for a chlorine reading at or near zero — typically achieved by simply not chlorinating for several days, or by adding a neutralizer like sodium thiosulfate. Also check that the pH is close to neutral. Dechlorinated, pH-balanced water is far safer to discharge onto your yard and is often required by local rules.
The Critical Warning: Don’t Fully Drain Carelessly
This is the mistake that turns a chore into a disaster. An empty in-ground pool is vulnerable to hydrostatic pressure — the force of groundwater beneath and around the shell. Normally the weight of the pool water holds the shell in place against that pressure. Remove the water and, if the water table is high, the ground can literally push the pool shell upward, cracking or “floating” a fiberglass or vinyl pool out of the ground. Concrete pools can heave and crack too.
To avoid this:
- Only fully drain when truly necessary, and ideally during dry conditions when the water table is low.
- Check for a hydrostatic relief valve in the pool floor; some pools have one that must be opened before draining.
- For routine lowering — winterizing or cleaning — you usually only need to drop the level below the skimmer, not empty the pool entirely.
- When in doubt, consult a pool professional before a full drain, especially in areas with high groundwater.
Step-by-Step Draining Process
- Stop chlorinating several days ahead and confirm the water is dechlorinated and pH-balanced.
- Choose a legal, safe discharge point and confirm local rules.
- Place the submersible pump on the deepest part of the pool floor, ideally on a flat surface or a milk crate to keep it off silt.
- Connect and secure the discharge hose to the chosen outlet.
- Plug into a GFCI-protected outlet — water and electricity demand ground-fault protection, no exceptions.
- Monitor the process. Don’t leave it running unattended for long stretches; check the discharge and the water level periodically.
- Stop at the right level — below the skimmer for maintenance, or per your winterizing or repair needs — and never fully empty without addressing hydrostatic pressure.
How Long Draining Takes
Set realistic expectations before you start so you can plan the day. Drainage time depends on the pool volume, the pump’s flow rate, and the head height it must push against. A typical residential in-ground pool holds 15,000 to 30,000 gallons. At an effective 40 gallons per minute — realistic for a 1/2 HP sump pump after accounting for hose and lift losses — a 20,000-gallon pool takes roughly 8 hours to empty. A slower pump or a longer, uphill hose run can stretch that to a full day or more.
To estimate your own timeline, divide your pool’s gallons by the pump’s real-world GPM, then divide by 60 for hours. Remember that manufacturer-rated flow assumes zero lift and a short hose; actual flow is usually lower. Starting early in the day gives you daylight to monitor the process and stop at the right level rather than discovering an over-drained pool at night.
Draining vs. Lowering: Know the Difference
Most of the time you don’t need to fully empty the pool at all. Understanding the difference between lowering and draining saves work and reduces risk:
- Lowering below the skimmer is all that’s needed for routine winterizing, so you can blow out and plug the lines. This is low-risk and quick.
- Lowering a few inches handles heavy rain overflow or minor water-chemistry resets.
- Partial draining to replace a portion of water dilutes high total dissolved solids without emptying the shell.
- Full draining is only for major repairs, acid washing, or resurfacing — and carries the hydrostatic risk covered above.
Match the amount you remove to the actual task. Emptying more water than the job requires only increases the time, the discharge volume, and the risk to the pool structure. When a full drain truly is necessary, timing it for a dry season and confirming the hydrostatic relief valve is functional keeps the shell safe.
The Takeaway
A sump pump is a perfectly good tool for draining a pool, provided you respect a few non-negotiables. Size the pump for a reasonable flow rate, run a proper discharge hose to a legal spot that isn’t a storm drain, dechlorinate the water before it touches your lawn, and always plug into GFCI protection. Above all, understand hydrostatic pressure and avoid fully emptying an in-ground pool unless it is genuinely necessary and done carefully. Follow those rules and you’ll drain your pool cleanly, safely, and without the costly surprises that catch careless homeowners off guard.
