The one pool calculation with no chemistry in it
Some things dissolved in pool water are not consumed and are not removed by filtration. They accumulate. Whatever the measure is, once it is above where you want it, the only mechanism that lowers it is replacing water containing it with water that contains less.
The mixing arithmetic is exact. Replace a fraction f of the volume with fill water at level Cf, and the new level is Cnew = Cold(1 - f) + Cf x f. Solve for f and you get f = (Cold - Ctarget) / (Cold - Cfill). Going from 100 to 40 with fill water at zero needs 60 percent replaced. With fill water at 10 it needs (100 - 40) / (100 - 10), which is 67 percent, because you are diluting with something that is already partway there.
Notice that the units never appear. This works for anything dissolved and conserved, in any unit, which is why the fields here just say "your units". It also means the page does not need to know, and does not want to know, what you are measuring.
This is not a dosing calculator, and the difference matters
Everything above is water arithmetic. It answers how much water to swap, not what to put in the pool. Working out a quantity of any pool chemical to add is a different problem, it depends on the specific product's concentration and on the full tested balance of the water, and a wrong answer harms swimmers and equipment. That calculation does not appear anywhere on this site and it should not appear on any web form.
If you are adjusting water chemistry at all, four things hold regardless of what you are adjusting. Never mix pool chemicals with each other, in a container, in a scoop, or in the same spot in the water — some combinations react violently or release chlorine gas. Always add chemical to water, never water to chemical. Test before you adjust anything. And make one adjustment at a time, then let the pump circulate it fully before you test again or add anything else, because a second dose added on top of an unmeasured first is how people end up chasing numbers for a week. The quantity itself comes from the label of the product you are actually holding, or from a pool professional looking at your test results.
Draining is the risky part, not the arithmetic
Two hazards deserve to be stated flatly before anyone opens a valve.
The first is suction entrapment. A main drain is a suction point, and a cover that is missing, cracked or loose can hold a person against it with enough force that they cannot get free. Children have been killed this way, including by hair and by limb entrapment. If a drain cover is not intact and correctly fastened, the pool is out of use until it is replaced with a correct compliant cover. This is not a maintenance item to schedule.
The second is hydrostatic uplift. An in-ground pool shell is held down mostly by the weight of the water in it. Take the water out while the ground around it is wet and the pressure underneath can lift the shell — a fibreglass or vinyl pool can float out of the hole, and a concrete shell can crack, lift or shift. Pools have hydrostatic relief provisions for exactly this reason, and whether yours works is not something to find out empirically. Anything approaching a full drain belongs to someone who can assess the groundwater and the relief before the level drops. Staged partial exchanges avoid the whole question by never letting the level get low, and that is why this page defaults to them.
Where the water goes afterwards is also frequently regulated. Pool water carries dissolved solids and treatment residuals, and discharge to a storm drain, a creek or a neighbouring property may be restricted or prohibited where you live. The rules differ from town to town, so the answer comes from your local authority rather than from a page.
Staged exchanges, and why they use more water
Replacing a third of the volume leaves two thirds of whatever you are diluting. Do it again and you are at four ninths. Each stage multiplies what remains by (1 - p), so after n stages the level is Cfill + (Cold - Cfill)(1 - p)^n. Three stages of 33 percent take 100 down to about 30, which is more than one 60 percent exchange achieves, but it uses 99 percent of the pool volume in water to get there instead of 60 percent.
That is the trade: staged exchanges cost more water and more time, and they keep the pool safely full throughout. On a 20,000 gallon pool the extra water is a few thousand gallons. Against the possibility of lifting a shell out of the ground, it is not a close decision. Refill volume and time is also worth planning — a garden hose at 8 GPM takes 25 hours to put 12,000 gallons back, which is the constraint people notice least and complain about most. The winterizing and opening calculator handles the same fill arithmetic for the seasonal case.
Questions people ask
How do I work out what percentage of the pool to drain?
Take the current tested level, the target, and the level in your fill water, then compute (current minus target) divided by (current minus fill). That is the fraction of the volume to replace. Multiply by the pool gallons for the volume. The units cancel, so it works for anything dissolved and conserved without the calculation needing to know what it is. The one input people skip is the fill water, which is rarely zero for hardness or dissolved solids and can make the required fraction considerably larger than expected. Test the tap before planning the job.
Is it safe to drain my pool completely?
Not without someone assessing the groundwater first, and on many in-ground pools not at all. Water in the pool is most of what holds the shell down. With the pool empty and the surrounding ground wet, hydrostatic pressure underneath can float a fibreglass or vinyl shell out of the hole or crack and lift a concrete one, and that is a structural failure rather than an inconvenience. Pools are built with hydrostatic relief arrangements for this reason and they can be blocked or non-functional without anyone noticing. Above-ground pools do not share this problem but have their own, since an empty wall is far less stable than a full one. When a level needs lowering, staged partial exchanges avoid the question entirely.
Why does the calculator need the level in my fill water?
Because dilution can only take you toward the level of what you dilute with, never past it. If your fill water carries the same measure at 10 and your target is 40, you need to replace 67 percent rather than 60, and if the fill water is at 45 you cannot reach 40 by dilution at all no matter how much you swap. Well water in particular can be high in hardness and dissolved solids. Testing the fill source takes one sample and it changes the plan more often than people expect.
Does backwashing count toward the water replacement?
Yes, and it is the reason some pools drift down on their own. Every backwash sends pool water to waste and gets replaced by fresh fill, which is exactly the exchange this page calculates, just in small increments spread over the season. If you backwash a sand filter regularly you are running a slow continuous dilution. Count that volume if you are trying to reach a target deliberately: a filter that dumps 200 gallons per backwash, done weekly, is 800 gallons a month of exchange happening whether you planned it or not.
Can I use this to work out how much of a chemical to add?
No. This page deliberately handles only the case where the answer is water, because that is the case a form can get right. Working out a quantity of a pool chemical depends on the concentration of the specific product, on the complete tested balance of the water at that moment, and on interactions between things already in the pool, and getting it wrong affects the people who swim in it. Take dosing from the label of the product in your hand or from a pool professional who has seen your test results. Test first, adjust one thing, circulate, then test again.