When you actually need to drain and refill a pool

Draining a pool sounds like a big, drastic step, and a full drain genuinely is one. But most pool owners who land on this calculator don’t need to empty the whole thing. A partial drain and refill, swapping out a measured portion of the water and replacing it with fresh water, solves the vast majority of real-world cases. Knowing the difference between a partial drain and a full drain, and which one your situation actually calls for, is the first decision this calculator helps you make.

Partial drain and refill: the math behind it

A partial drain works on simple proportional math. You’re not trying to remove every trace of the elevated chemical, you’re diluting it down by replacing a calculated fraction of the pool’s total volume with fresh water that doesn’t carry the same problem.

Drain percentage = 1 minus (target reading divided by current reading)

Gallons to drain = pool volume multiplied by that percentage

Take a CYA reading of 100 ppm that needs to come down to 40 ppm in a 18,000 gallon pool. The math: 1 minus (40 / 100) equals 0.6, so 18,000 times 0.6 equals 10,800 gallons to drain and refill, 60% of the pool. Calcium hardness uses a slightly adjusted version of this formula since your fill water usually carries some calcium of its own, which the calculator above accounts for automatically.

Full drain and refill: what it actually involves

A full drain removes essentially all of the water rather than a calculated fraction. This is a bigger undertaking, both in terms of time and the physical risks involved for an inground pool. People reach for a full drain for reasons that have nothing to do with chemistry numbers at all.

Common reasons for a full drain

• Acid washing a stained or scaled concrete surface, which requires the pool to be empty
• Replastering or resurfacing, since the new finish needs a completely dry, prepared shell
• Vinyl liner replacement, which obviously can’t happen with water still in the pool
• Severe algae or biological contamination where the problem has gone past what a shock treatment and partial drain can fix
• A complete reset after years of neglect, where multiple chemistry parameters are so far out of range that partial dilution would need to remove 80% or more of the volume anyway, making a full drain simpler

For any of these, you’re not calculating a target percentage, you’re draining the whole pool, then refilling and balancing from scratch the same way you would for a brand new pool fill.

Safety first: what you need to know before draining an inground pool

This is the part of the process most pool owners never think about until something goes wrong. Pool water isn’t just there to swim in, it’s also weight that holds the shell down against pressure from groundwater underneath and around it. Remove that water and, in the wrong conditions, the ground itself can push an empty or partially empty inground shell upward out of position. This is called pool floating, and it’s a catastrophic, expensive failure to repair, typically $10,000 to $30,000 or more.

Above-ground pools don’t carry this groundwater risk since there’s no surrounding soil pressure to worry about, but the liner itself needs care. Never leave a vinyl liner pool sitting empty for more than a few hours in hot weather, since the liner can shrink permanently out of shape without water pressing it into place.

Where does the drained water actually go?

This trips people up more than expected. You can’t just run a hose to the storm drain and call it done in most US municipalities, since chlorinated water harms local waterways and storm systems typically drain straight into them untreated. The two accepted routes are connecting to your sanitary sewer cleanout, or spreading the water across a large area of landscaping after letting chlorine levels drop naturally for 5 to 7 days by pausing all chlorine additions before you drain. Check your specific city’s water discharge ordinance before draining any significant volume, since rules and acceptable methods do vary by location.

What a drain and refill actually costs, broken down

That range covers water cost alone, reflecting the spread in US rates from roughly $5 per 1,000 gallons in lower-cost areas to $18 or more per 1,000 gallons in expensive coastal markets. On top of water, factor in drain labor if you’re hiring a service ($150 to $350 typically) and refill chemistry afterward ($60 to $150 for a proper restart kit), both of which the calculator above adds in for your specific numbers.

Equipment you’ll actually use during a drain

For DIY drains, a submersible utility pump is the standard tool. A 3/4 HP pump moves 4,000 to 6,000 gallons per hour, draining a 20,000 gallon pool in roughly 3 to 5 hours. Rental runs $40 to $80 per day at most equipment rental shops, or you can buy one outright for $150 to $400 if you expect to need it again down the road. Run the discharge hose to wherever your local rules permit (sewer cleanout or landscaping), never toward a neighbor’s property or a storm drain.

For refilling, a standard garden hose at typical residential water pressure delivers around 8 gallons per minute, meaning a 20,000 gallon refill takes about 42 hours of continuous running. Running two hoses simultaneously roughly doubles your speed. If you need the pool back in service faster, a water truck delivery service can fill the same pool in a few hours, though at a significantly higher cost per gallon.

Protecting your pump, salt cell, and heater during a drain

This is the single most overlooked step in the entire process, and skipping it is the most expensive mistake a first-time drainer makes. Your circulation equipment is designed to run with water flowing through it, not sitting idle while empty. Before you start draining, switch off the pump, the salt chlorine generator, and the heater, and leave them off until the water level is back above the skimmer line during refill.

• Pump: running dry burns out the motor seal within minutes, sometimes seconds. A new pump motor runs $300 to $800 installed, an entirely avoidable cost.
• Salt cell: the electrode plates need water flow to stay cool. Running dry warps and permanently damages the cell, a $400 to $900 replacement.
• Heater: gas and electric heaters both rely on water flow to dissipate heat from the heat exchanger. Running one dry while it’s firing can crack the heat exchanger, a repair that often costs more than a new heater.

If you’re hiring a professional drain service, confirm they know to shut down your specific equipment setup before they start, particularly if you have an automation system that runs the pump on a schedule. A scheduled pump start while the pool sits empty defeats the whole point of shutting it off manually.

The refill: chemistry order that actually works

Fresh water straight from the tap or a truck needs balancing before anyone swims in it, and the sequence you add chemicals in genuinely matters. Get the order wrong and you waste product or create reactions that work against you.

1. Test the fresh water first. Tap water chemistry shifts by season and by municipality, so never assume it’s neutral.
2. Total alkalinity, 80 to 120 ppm. This stabilizes everything that comes after, particularly pH.
3. pH, 7.4 to 7.6. Adjust only once alkalinity is settled.
4. Calcium hardness if your fill water tested under 200 ppm, which is common with soft municipal water.
5. Shock the pool to establish a clean chlorine baseline.
6. CYA stabilizer, always last. Give it 24 to 48 hours to fully dissolve before retesting, and never add it the same day as shock.

Watch for local water restrictions before you start

Drought-prone regions across the US, particularly the Southwest, frequently impose temporary or seasonal restrictions on hose filling for pools, sometimes limiting it to specific days of the week, sometimes banning it outright during the worst of a drought. These rules change with conditions and aren’t always well publicized, so it’s worth a quick check with your local water utility before you commit to a hose refill plan, especially if your drain involves a large percentage of pool volume. If hose filling is restricted or banned in your area, a water truck delivery or a private well becomes your practical refill option instead.

What about an attached spa?

Many inground pools have a spa built into the same structure, sharing the same plumbing, filter, and circulation system as the main pool. If that’s your setup, the spa drains and refills along with the pool automatically, you don’t need to calculate it separately or treat it as a second project. If your spa instead has its own dedicated plumbing and equipment pad, separate from the main pool, it needs its own drain and refill calculation done independently, since its water chemistry and volume are completely separate from the pool’s.

How often does a pool actually need a drain and refill?

For pools running trichlor tablets as the main sanitizer, expect a CYA-driven partial drain roughly every 2 to 3 seasons, since trichlor steadily adds stabilizer that nothing removes except dilution. Switch to liquid chlorine or cal-hypo as your routine sanitizer and that need drops off significantly, since neither product adds CYA at all. Calcium-driven partial drains tend to repeat every 3 to 5 years in hard water regions like the Southwest. Full drains for resurfacing or major shell work are typically a once-per-decade event on a well-maintained concrete pool, less often for fiberglass and vinyl.

Frequently asked questions

How do I know if I need a partial drain or a full drain?

If a specific chemistry reading, like CYA, calcium, or TDS, has climbed too high, a partial drain calculated to your exact target gets you there with less water and lower cost. A full drain is for physical work on the shell, like acid washing or replastering, or for chemistry so far gone that a partial drain would need to remove most of the pool’s volume anyway.

How long does a full drain and refill take from start to finish?

Draining a 20,000 gallon pool with a 3/4 HP submersible pump takes roughly 3 to 5 hours. Refilling at a standard garden hose’s 8 gallons per minute takes about 42 hours for the same volume. Including chemistry settling time afterward, plan on the pool being out of swimmable service for 2 to 4 days total.

Where am I legally allowed to drain pool water?

Most cities prohibit draining chlorinated water into storm drains since it harms local waterways. Accepted methods are typically your sanitary sewer cleanout, or spreading the water over landscaping after stopping chlorine additions for 5 to 7 days beforehand to let residual chlorine dissipate naturally. Check your specific municipality’s discharge ordinance before draining.

Is it dangerous to fully drain an inground pool?

It carries real risk under the wrong conditions. Groundwater pressure can push an empty inground shell upward out of position, a failure called pool floating that costs $10,000 to $30,000 or more to repair. Avoid full drains after heavy rain, in low-lying yards, or without confirming your hydrostatic relief valve is functional. Partial drains under roughly 50% of volume carry far less risk.

What’s the cheapest way to refill a pool after draining?

A standard garden hose connected to municipal water is the lowest-cost refill method, though it’s the slowest, taking around 42 hours for 20,000 gallons. Water trucks fill much faster but charge significantly more per gallon. If you have a private well, refilling costs only electricity for the well pump, with no water utility charge at all.

Do I need to drain my whole pool if just one reading is high?

No. A partial drain calculated specifically to your current and target reading removes only what’s needed to bring that one number into range. Draining the entire pool when one parameter is elevated wastes water, money, and time compared to the targeted partial drain this calculator works out for you.

Do I need to turn off my pump before draining the pool?

Yes, always. Turn off the pump, salt cell, and heater before draining and keep them off until the water level is back above the skimmer during refill. Running a pump dry burns out the motor seal within minutes, and a dry salt cell warps and fails permanently. Both are unplanned repairs of $300 to $1,500 that one switch flip avoids entirely.

Can I fill my pool with a hose if my area has water restrictions?

It depends on your specific local restriction. Some areas limit hose filling to certain days or hours rather than banning it outright, in which case you can still refill, just on a schedule. Other areas ban pool filling with a hose entirely during drought conditions, in which case a water truck delivery or a private well becomes your only refill option. Check your local water utility’s current notices before starting.