Four volumes, worked separately
A closing looks like one job and is really four measurements that have nothing to do with each other. Doing them separately is what stops the April surprises.
| Volume | How it is worked out |
|---|---|
| Water pumped off | Surface area in square feet times the drop in feet, times 7.48 for gallons. |
| Water in the plumbing | Inside diameter in inches squared, times 0.0408, times the total feet of run. |
| Cover edge weighting | Perimeter divided by the spacing, rounded up. |
| Spring refill | The same gallons that came off, divided by the hose flow. |
The first one is the one that surprises people. Six inches off a 512 square foot pool is 256 cubic feet, which is 1,915 gallons. At 30 GPM that is just over an hour of pumping, and it is a substantial quantity of water going somewhere. Where it goes is often regulated, so that is a question to settle before the pump starts rather than while it runs.
How far to lower, and why not further
The drop is set by what the equipment and the cover need, not by a general rule. Pools with a skimmer that would hold water and freeze are typically taken below the skimmer mouth. Pools with a skimmer plug or a freeze-protection arrangement may be left much higher. Some cover types want the water close to the deck to support them, and taking such a pool low defeats the cover.
Going lower than needed costs in three ways. It is water you paid for and warmed, and it comes back as cold makeup that has to be balanced again in spring. A vinyl liner spends the winter dry above the line, shrinks, hardens, and can pull out of its track when you refill. And on an in-ground shell, a lower water level is a smaller weight holding the structure down against whatever the groundwater is doing underneath it — the same mechanism that makes a full drain a professional job, just less of it. The water replacement calculator covers that risk properly for the cases where the level really does have to go down a long way.
Line volume, and what the number is for
Pipe holds more than people picture. Two inch Schedule 40 holds 0.174 gallons per foot, so a hundred feet of run to an equipment pad is 17 gallons standing in the plumbing. One and a half inch holds 0.106 gallons per foot. The formula is the inside diameter in inches squared times 0.0408, which comes from the area of the circle in square inches times twelve inches per foot divided by 231 cubic inches per gallon. The pipe volume calculator does the same arithmetic for any run in the house.
What the figure is good for is planning: how much water a blow-out has to push out, how much a line treatment has to reach, whether the runs are long enough that the pad end and the pool end will not behave the same. What it is emphatically not is a dose. If antifreeze is part of how you close, it is a product sold specifically for pool and spa plumbing, used at the quantity and in the manner its own label states. Automotive antifreeze is ethylene glycol, it is toxic, it tastes sweet to animals, and it has no place in a pool system. And whatever goes in the lines does not get mixed with anything else, ever.
Covers, bags, and the hazard nobody puts on the list
Water bags go on the deck around the cover edge, butted end to end, not on the cover itself over open water. Perimeter divided by spacing, rounded up, gives the count, and 96 feet at 4 foot spacing is 24 bags. Tighter spacing on the side the wind comes from is worth more than a uniform layout, because a cover fails when wind gets underneath one edge and lifts it, not evenly all round.
The part that belongs in every closing conversation and rarely appears: a winter cover is a drowning hazard in its own right. Rain and melt collect on top of a solid cover in quantities that are deep enough to drown a small child or a pet, and a standard winter cover is not a safety cover and will not support anyone who steps onto it. Keep the standing water pumped off, keep the perimeter secured, and treat a covered pool as an open pool where children are concerned. In spring, the same water on top of the cover is the thing to remove before anything else, because pulling a loaded cover tips all of it into the pool.
The refill at opening is the slow part. A garden hose at 8 GPM puts back 480 gallons an hour, so 1,915 gallons takes four hours and a deeper drawdown takes most of a day. Two hoses halve it. Whatever the volume, it arrives cold and untreated, which is why opening is a test-and-wait process rather than a same-afternoon one. Chemistry after a refill is worked from your own test results and the labels on your own products, one adjustment at a time, and if the water is genuinely out of hand it goes to someone who can see it. Sizing the heater that will pull the temperature back up is the heater sizing calculator, and the cover that keeps it there is on the heat loss page.
Questions people ask
How much water do I need to pump off to winterize a pool?
Surface area in square feet times the drop in feet gives cubic feet, and 7.48 gallons per cubic foot converts it. Six inches off a 16 by 32 pool, which is 512 square feet, is 256 cubic feet or about 1,915 gallons, and at 30 GPM that is roughly an hour of pumping. The right drop depends on the pool: some closings take the level below the skimmer mouth, others use a skimmer plug and leave the water high, and some cover types need the water near the deck to support them. Take the figure from what your equipment and cover require rather than from a general rule.
How many water bags do I need for a pool cover?
Perimeter divided by the spacing, rounded up. A 16 by 32 pool has a 96 foot perimeter, so at 4 foot spacing it is 24 bags and at 3 foot spacing it is 32. Bags sit on the deck, butted end to end, not on the cover over the water. Spacing matters most on the side the prevailing wind comes from, because covers fail by wind getting under one edge and lifting rather than by uniform load. Fill bags to about two thirds so the water has room to expand when it freezes, and never use anything solid and heavy on a deck edge you care about.
How much water is in my pool plumbing?
Inside diameter in inches squared times 0.0408 gives gallons per foot. Two inch Schedule 40 is 0.174 gallons per foot and one and a half inch is 0.106, so a hundred foot run to an equipment pad holds around 17 gallons in two inch pipe. Add up every line — each skimmer, the main drain, every return, the cleaner line — because they are separate runs and the total is usually larger than people guess. The figure is for planning how far a blow-out or a line treatment has to reach, not a quantity of anything to pour in.
Can I use regular antifreeze in pool lines?
No. Automotive antifreeze is ethylene glycol, which is poisonous, tastes sweet enough that animals drink it, and is not intended to end up in water people swim in. Products sold specifically for pool and spa plumbing exist for this purpose and carry their own instructions for how much and where. Whether antifreeze belongs in your closing at all depends on how the pool is plumbed and how cold your winters get, and in many climates properly cleared lines are the method rather than antifreeze. Get that decision from someone who closes pools where you live, and take any quantity from the product label rather than from a calculation.
How long does it take to refill a pool in spring?
Divide the gallons by the hose flow. A garden hose commonly delivers 5 to 10 GPM depending on pressure, length and diameter, so 8 GPM is 480 gallons an hour. Putting back 2,000 gallons takes about four hours, and a pool that was taken down further, or drained for a liner replacement, can take a day or several. Two hoses on separate bibs roughly halve it if your supply can carry both without the pressure collapsing. If you are on a meter, this is also the moment to check what the fill water actually contains, because it sets the starting point for everything you do to the water afterwards.