Pool antifreeze only — propylene glycol, usually pink. Never automotive antifreeze (ethylene glycol): it is toxic to pets and children and ends up in your pool in spring. Never pour antifreeze into the pump, filter or heater; those drain by their own plugs.
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The formula
gallons per foot = 1¼" 0.078 · 1½" 0.103 · 2" 0.171 · 2½" 0.244 full line volume = gallons per foot × length × number of lines
The familiar "one gallon per 10 feet" rule is just the 1.5-inch number rounded — a 1.5-inch line holds 0.103 gallons per foot, so ten feet is almost exactly a gallon. Two-inch pipe holds two-thirds more, which is why the same rule quietly under-doses larger plumbing.
A worked example
Three 1.5-inch lines averaging 25 feet hold about 7.7 gallons of water in total. After a proper blowout almost all of that is gone, so you're buying 3 gallons — one per line — to sit in the last low stretch each one keeps. Cost: around $30, against a cracked return line that means digging up a deck.
Where it actually goes
Blow the line, plug it at the pool end, then pour antifreeze into the open end at the equipment pad and reinstall that plug. The antifreeze sits in the horizontal low point — the exact spot where a blowout leaves a cup of water behind and where freeze damage starts. Skimmers get a Gizzmo or a partly-filled bottle instead, so expanding ice has somewhere to go.
About one gallon per 10 feet of 1.5-inch pipe, or one gallon per 6 feet of 2-inch pipe — that's the pipe's actual internal volume. Most backyard pools use a gallon per line as the practical dose after a blowout, because you're protecting the last stretch that still holds water, not filling the whole run.
Can I use antifreeze instead of blowing out the lines?
No, and this is the mistake that cracks plumbing. Antifreeze poured into a water-filled line doesn't push the water out — it mixes with it, dilutes itself, and the blend freezes at close to the temperature plain water does. Antifreeze is insurance on the water a blowout couldn't reach, not a replacement for the blowout.
Can I use car antifreeze in my pool lines?
Never. Automotive antifreeze is ethylene glycol — sweet-tasting, lethal to pets, wildlife and kids, and a serious problem the moment it enters your pool water in spring. Pool antifreeze is propylene glycol, non-toxic, and usually dyed pink so nobody confuses the two jugs on a garage shelf.
Does antifreeze go in the pump, filter or heater?
No. Lines only. Those components drain via their own plugs — antifreeze can attack pump seals and heater internals, and in a cartridge filter it just makes a mess you rinse out for an hour in April. Pull the drain plugs, store them in the pump basket so they're findable, and leave the equipment dry.
Do I need antifreeze if I live somewhere mild?
If your ground never freezes, no — a blowout and plugs are plenty. The case for antifreeze is the freeze-thaw belt: winters that swing above and below freezing repeatedly, where a slug of water in a low spot cycles over and over. That's where cracked returns happen.