Pool Drain and Refill Calculator
Several pool readings only go up. Cyanuric acid, salt and calcium hardness all behave that way, and the only way down is to drain water and refill. The share to replace is (current − target) ÷ (current − fill water). Going from 100 ppm to 50 ppm on 0 ppm tap water means swapping exactly half. The fill water is where this quietly goes wrong: in a hard-water area whose supply already reads 250 ppm, a pool being pulled toward 200 ppm cannot get there at all — replace every drop and it still lands on 250.
Your numbers
Share to drain
60%
Water to drain
30,000L
Reading after half a change
50ppm
What it means
Drain and replace 60% of the water. Changing half of it only reaches 50 ppm — dilution pays out strictly in proportion to what you removed.
Formula
Share to drain = (Reading now − Reading wanted) ÷ (Reading now − Fill water reading) × 100, Water to drain = Litres × Share to drain ÷ 100
The formula assumes the water mixes evenly. Pumping only off the shallow end mixes less than that and drops the reading less than predicted, so keep the pump running while you drain. Where the water table is high, emptying a lot at once can float the shell, so replacing a third at a time is the safer way to do it.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Litres (L)Capacity in litres; one litre is 1,000 cm³. | 50,000 | 0 and up |
| Reading now (ppm)The current reading you want down — cyanuric acid, salt or calcium hardness. | 100 | 0 ~ 10,000 |
| Reading wanted (ppm)The reading you want; set below the fill water it cannot be reached by draining. | 40 | 0 ~ 10,000 |
| Fill water reading (ppm)What the replacement water already carries — the floor that dilution cannot pass. | 0 | 0 ~ 10,000 |
Step by step
Quick reference table
Results when only Reading now (ppm) changes and everything else stays put.
| Reading now (ppm) | Share to drain (%) | Water to drain (L) | Reading after half a change (ppm) |
|---|---|---|---|
| 50 | 20 | 10,000 | 25 |
| 75 | 46.7 | 23,333 | 37.5 |
| 100 | 60 | 30,000 | 50 |
| 150 | 73.3 | 36,667 | 75 |
| 200 | 80 | 40,000 | 100 |
What each result means
| Result | At default values |
|---|---|
| Share to drain (%)The share of water to drain and replace; dilution pays out only in that proportion. | 60 |
| Water to drain (L)That share expressed as litres — replace it in stages rather than all at once. | 30,000 |
| Reading after half a change (ppm)What is left after changing half — why "drain a bit" rarely gets you there. | 50 |
Common mistakes
The formula assumes the water mixes evenly. Pumping only off the shallow end mixes less than that and drops the reading less than predicted, so keep the pump running while you drain. Where the water table is high, emptying a lot at once can float the shell, so replacing a third at a time is the safer way to do it.
Glossary
- Litres
- Capacity in litres; one litre is 1,000 cm³.
- Reading now
- The current reading you want down — cyanuric acid, salt or calcium hardness.
- Reading wanted
- The reading you want; set below the fill water it cannot be reached by draining.
- Fill water reading
- What the replacement water already carries — the floor that dilution cannot pass.
- Share to drain
- The share of water to drain and replace; dilution pays out only in that proportion.
- Water to drain
- That share expressed as litres — replace it in stages rather than all at once.
- Reading after half a change
- What is left after changing half — why "drain a bit" rarely gets you there.
Frequently asked questions
Q. How is Pool Drain and Refill Calculator calculated?
Share to drain = (Reading now − Reading wanted) ÷ (Reading now − Fill water reading) × 100, Water to drain = Litres × Share to drain ÷ 100 — Several pool readings only go up. Cyanuric acid, salt and calcium hardness all behave that way, and the only way down is to drain water and refill. The share to replace is (current − target) ÷ (current − fill water). Going from 100 ppm to 50 ppm on 0 ppm tap water means swapping exactly half. The fill water is where this quietly goes wrong: in a hard-water area whose supply already reads 250 ppm, a pool being pulled toward 200 ppm cannot get there at all — replace every drop and it still lands on 250.
Q. Can you walk through an example?
With Litres 50,000L, Reading now 100ppm, Reading wanted 40ppm, Fill water reading 0ppm, the answer is Share to drain 60%.
Q. What do I need to enter?
Enter Litres, Reading now, Reading wanted, Fill water reading. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Reading now (ppm) moves the answer to Reading now (ppm) 50 → Share to drain (%) 20 and Reading now (ppm) 200 → Share to drain (%) 80. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
The formula assumes the water mixes evenly. Pumping only off the shallow end mixes less than that and drops the reading less than predicted, so keep the pump running while you drain. Where the water table is high, emptying a lot at once can float the shell, so replacing a third at a time is the safer way to do it.
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