How Much CYA Do You Lose Draining 6 Inches From a 20,000-Gallon Pool?

You tested your pool water, the cyanuric acid reading came back sky-high, and someone online suggested draining a few inches to bring it down. Now you're wondering exactly how much CYA you'd actually lose - because draining 6 inches of water out of a 20,000-gallon pool sounds like a lot of effort for a number you're not sure about.

Here's the direct answer: draining 6 inches from a 20,000-gallon pool removes between roughly 940 and 1,920 gallons depending on your pool's surface area - that's approximately 5% to 10% of your total volume. Because cyanuric acid doesn't evaporate or degrade, it leaves only with the water you remove. So a 6-inch drain drops your CYA by the same 5% to 10%. If you were sitting at 80 ppm, you'd land somewhere between 72 and 76 ppm afterward. This article walks through the exact math, shows you a worked example, explains when a partial drain makes sense, and covers how much stabilizer you'd need to add back.

Why CYA Behaves Differently From Other Chemicals

Chlorine burns off in sunlight, gets consumed fighting algae and bacteria, and fluctuates every day. Cyanuric acid does none of that. It is chemically stable under UV light, it is not consumed in the chlorination process, and it does not evaporate off the surface. The only thing that removes CYA from a pool is physically removing water. That makes it a "one-way accumulator" if you use trichlor tabs or dichlor as your main sanitizer, since both of those carry bound CYA that releases every time the tablet dissolves. Understanding that behavior is what makes the partial-drain math so predictable: the percentage of water you remove equals the percentage of CYA you lose, every time.

If you've ever noticed that your pool loses chlorine faster than expected even when it's covered, elevated CYA is often the hidden reason - it shifts the chlorine equilibrium and reduces the amount of free active chlorine available.

The Math: How Much Water Does a 6-Inch Drain Actually Remove?

The volume of water removed in a partial drain is straightforward: multiply the pool's surface area in square feet by the drain depth in feet, then multiply by 7.48 (gallons per cubic foot).

• Formula: Gallons removed = Surface area (sq ft) x Drain depth (ft) x 7.48
• For a 16x32 pool (512 sq ft): 512 x 0.5 x 7.48 = 1,914 gallons
• For a roughly oval 15x30 pool (roughly 354 sq ft): 354 x 0.5 x 7.48 = 1,324 gallons
• For a 20-foot round pool (314 sq ft): 314 x 0.5 x 7.48 = 1,174 gallons

Notice that two pools can both hold 20,000 gallons and still have very different surface areas. A deep, narrow pool drains far fewer gallons per inch than a wide, shallow one. That matters because it changes how much CYA each 6-inch drain actually removes.

Worked Example: CYA Drop After a 6-Inch Drain

Say your pool is a 16x32 inground with an average depth of 5 feet, holding right around 20,000 gallons. Your CYA tests at 90 ppm - well above the 30-50 ppm ideal for an unstabilized chlorine pool. You drain 6 inches.

Using the formula above, you removed about 1,914 gallons. Your remaining water volume is 20,000 - 1,914 = 18,086 gallons. The fraction of original water still in the pool is 18,086 / 20,000 = 0.904, or 90.4%. Your new CYA level: 90 ppm x 0.904 = 81.4 ppm. You drained 6 inches, did a bunch of work, and dropped CYA by less than 9 ppm. That's the honest reality of small partial drains.

To get from 90 ppm down to the recommended 30-50 ppm range, you'd need to replace roughly 45% to 67% of your pool's water - closer to 9,000 to 13,400 gallons removed and refilled, not a single 6-inch pull. The pool drain calculator breakdown from Founders Pool Service illustrates this well if you want to run different starting CYA scenarios before you commit to a bigger drain.

When a 6-Inch Drain Is Actually the Right Move

A 6-inch drain is not useless, it just has a narrow job. Here are the situations where it genuinely makes sense:

• CYA just slightly elevated: If you're at 55 ppm and want to get back to 50 ppm for a salt pool, a small drain-and-refill is perfectly appropriate.
• Lowering water level before a storm: You need to drop the level anyway, so you get a small chemistry bonus at the same time.
• Correcting after an accidental over-dose: You added too much stabilizer during startup and want to undo a chunk of it quickly.
• Combined with other chemistry corrections: You're also draining to address high calcium hardness or TDS, and the CYA reduction is a secondary benefit.

If your CYA is above 80 ppm and you're relying on chlorine as your sanitizer, a 6-inch drain alone won't fix the problem. You need a more aggressive drain-and-refill plan, or you need to look at enzymatic CYA reducer products. Pool Shark H2O's rundown of CYA reducer options is worth reading if you're trying to avoid a major drain, and Pool Chem Tracker's step-by-step drain-and-refill guide is the right reference if you do decide to go the dilution route on a larger scale.

How Much Stabilizer Do You Need to Add Back?

After any drain-and-refill, you're adding fresh water that contains zero CYA. That dilutes your remaining stabilizer further. To calculate the top-up dose, figure out the ppm difference and treat it like a fresh dose for the volume of water you added.

Granular cyanuric acid raises CYA by roughly 10 ppm per 1.3 oz per 1,000 gallons of water. Here's a quick reference for a 20,000-gallon pool:

Why These Numbers Matter for Your Budget

A 6-inch drain on a 16x32 pool only drops CYA by about 9 ppm, so you'd need to add back only about 13 oz of stabilizer to restore your original level - a minimal cost. But if you're doing a 50% drain-and-refill to genuinely solve a high CYA problem, you're adding back roughly 3 to 5 lbs of granular stabilizer on top of all the other chemistry you'll need to rebalance. Factor that into your decision before you open the drain valve.

Common Mistakes With Partial Drains and CYA

Pool owners make a few predictable errors here. The first is expecting a 6-inch drain to make a meaningful dent in severely elevated CYA (above 100 ppm). It won't - you're removing less than 10% of the problem. The second mistake is not testing CYA before adding stabilizer back, assuming the drain wiped out more than it did. Always retest first. The third is confusing "draining for winterization" with "draining to fix chemistry." If you're closing the pool for winter anyway, a partial drain to lower the water level is not the same calculation as a purposeful drain-and-refill to dilute CYA. For a closer look at how chemistry fits into a full seasonal shutdown, the guide on closing a pool without draining completely walks through what you can balance before the cover goes on. And when you're ready to put the pool to bed for the season, our Pool Closer - All-in-one Pool Winter Closing Kit bundles the chemicals you need so you're not making three separate trips to the store to balance everything before closing day.

Frequently Asked Questions

How much CYA do you lose draining 6 inches from a 20,000-gallon pool?

A 6-inch drain from a 20,000-gallon pool removes roughly 1,000 to 1,900 gallons depending on the pool's surface area - that's about 5% to 10% of the total volume. CYA drops by that same percentage. If your CYA was 80 ppm before the drain, expect it to land between 72 and 76 ppm afterward. The loss is exactly proportional to the fraction of water removed.

How many gallons does a 6-inch drain remove from a 20,000-gallon pool?

It depends on your pool's surface area, not just its total volume. A 16x32-foot pool (512 sq ft of surface) loses about 1,914 gallons in 6 inches. A 20-foot-round pool loses about 1,174 gallons. Use the formula: gallons removed = surface area (sq ft) x 0.5 x 7.48.

How much CYA do I need to add back after a partial drain?

After a 6-inch drain on a 20,000-gallon 16x32 pool, you'd need to add back roughly 13 oz of granular cyanuric acid to restore a 9 ppm drop. Always retest your CYA after refilling fresh water before dosing, since the dilution from the new water also factors in. Plan on about 1.3 oz of stabilizer per 1,000 gallons per 10 ppm of CYA increase you need.

How many inches do I need to drain a 20,000-gallon pool to cut CYA in half?

You cannot cut CYA in half with a 6-inch drain. To drop from 100 ppm to 50 ppm, you need to remove and replace 50% of your water - roughly 10,000 gallons, or well over a foot of water in most pools. That typically means draining, refilling, and sometimes repeating the process, not a single shallow pull.

Does cyanuric acid evaporate or break down on its own?

No. Cyanuric acid does not evaporate, degrade under sunlight, or get consumed by chlorine. The only ways to reduce CYA are dilution through drain-and-refill, an enzymatic CYA reducer product, or a full drain. This is why CYA climbs steadily over multiple seasons in pools that rely on trichlor tabs or dichlor as the primary sanitizer.

The bottom line: a 6-inch partial drain on a 20,000-gallon pool removes 5% to 10% of your CYA - useful for small corrections, but nowhere near enough to fix a seriously elevated reading. Know the math before you open the valve so there are no surprises when you test the water after refilling.

If you're unsure how to approach your specific situation, we're glad to help you think it through. AquaDoc carries chemistry products for every stage of pool care, from stabilizer top-ups to full closing kits, so you can find what you need and get back to a pool that actually works the way it should.