Chlorinate evenly.

The dose is arithmetic. The guessing is the problem.

For a typical 15,000 gallon pool sitting at 0 ppm, it takes about 47 fluid ounces of 10 percent liquid chlorine to reach 3 ppm. Not a "glug." Not "half the jug, roughly." Forty-seven ounces. Pool chlorine is one of the few home maintenance jobs where the exact right answer is three multiplications away, and yet most owners dose by feel all summer and then wonder why the water swings between green and swimsuit-bleaching.

The math needs three inputs: how many gallons you have, how far you need to raise the chlorine level, and which product you are pouring. The calculator above handles all of it, including the conversion into gallons of liquid, pounds of granular, or a count of 3 inch tablets. This page covers the parts a calculator cannot do for you: finding your true volume, choosing between the four common products, and figuring out what went wrong when the test strip says zero the morning after you dosed.

The one number that runs everything

Every chemical decision you will ever make for this pool divides by its volume. Get the gallons wrong by 30 percent and every dose you pour for the next decade is wrong by 30 percent. It is worth twenty minutes with a tape measure to nail it once.

Rectangular pools are easy: length times width times average depth times 7.5. The 7.5 is gallons per cubic foot (7.48, rounded the way the trade rounds it). Average depth is the shallow end plus the deep end, divided by two. A 16 by 32 pool that runs 3 feet to 7 feet averages 5 feet deep and holds 19,200 gallons. Round pools square the diameter, multiply by average depth, then by 5.9. Freeform and kidney shapes resist clean formulas, so measure the longest length and widest width, run the rectangle math, and subtract 15 percent. Close enough for chlorine.

Above-ground pool sizes and what they actually hold

Above-ground owners have it easier, because the pools come in standard diameters and nearly all of them run 48 inches of water inside 52 inch walls. If you know the size printed on the box, you know the gallons. The table below also shows the liquid chlorine needed to raise each size by 3 ppm, which is the full dose for a freshly filled or fully depleted pool.

Two things jump out of that table. First, even the big 33 footer needs less than a gallon of liquid to fully chlorinate from zero, so buying chlorine by the case is about convenience, not necessity. Second, the common 24 foot pool holds nearly 14,000 gallons. People routinely guess 8,000 and then cannot understand why their "full dose" barely moves the needle.

Liquid, tablets, granular: pick one lane

Four products dominate the residential market, and they are not interchangeable. Each carries a different concentration of available chlorine and, more importantly, each leaves something different behind in the water. The concentration decides the dose. The leftovers decide what your water chemistry looks like in August.

If you want one recommendation: liquid for the routine, cal-hypo for the weekly shock. It is the boring answer and the right one. Liquid chlorine adds nothing that accumulates, which means your water in September behaves like your water in June. The tablet-only routine feels effortless right up until the stabilizer it quietly deposits crosses 100 ppm and your chlorine stops working no matter how much you add. More on that failure below, because it is the single most common one.

Tablets and floaters, done right

Trichlor tablets are genuinely useful in two places: a floating dispenser while you are away for a week, and an inline feeder plumbed after the filter. A standard 3 inch tablet weighs 8 ounces and dissolves over roughly 3 to 7 days depending on water temperature and how far open the floater vents are. One tablet per 5,000 gallons is the working rule, so a 15,000 gallon pool floats three at a time.

Three tablet rules people learn the hard way. Never throw tablets straight into the pool; they sink, sit on the liner, and bleach a permanent white ring into it. Pull the floater out, or tether it away from the swim area, while people are in the water, since it is a concentrated acid puck a curious kid can grab. And never load trichlor into a feeder or floater that has ever held cal-hypo. The two react, and the reaction is violent enough that product labels warn about it in bold. Different products, different containers, always.

Test before you pour, and fix pH first

A chlorine dose is only as good as the test that preceded it. Strips are fine for a Tuesday check. A drop-based kit (the Taylor K-2006 is the one pool forums have canonized, for good reason) is worth owning for anything you plan to act on, because strips read badly at the exact moments that matter: very low chlorine and very high chlorine.

The reading that outranks chlorine is pH. Chlorine kills as hypochlorous acid, and the share of your chlorine in that active form collapses as pH rises: roughly half at pH 7.5, down near 20 percent at pH 8.0. Pour a full dose into high-pH water and most of it sits there in a lazy, ineffective form while the algae carries on. The order of operations is always the same. Test. Adjust pH into the 7.2 to 7.6 window. Then chlorinate. Then retest after 30 to 60 minutes of pump circulation before trusting the number.

Where pool chlorine goes wrong

The test reads zero the morning after a full dose. This is almost never a testing error. Something in the water consumed the chlorine overnight: an early algae bloom you cannot see yet, ammonia from heavy bather load, or debris after a storm. A pool that loses all its chlorine overnight is telling you the demand is higher than the dose. Shock it, then retest at sunrise. If it holds overnight, you are done. If it zeroes again, shock again. Chlorine demand always wins until it is satisfied.

Total chlorine is fine but the water smells and stings. Test kits report two numbers, and the difference between them is the diagnosis. Free chlorine is the active sanitizer. Total chlorine includes chloramines, the used-up combined form that causes the sharp "chlorine smell" and red eyes. A strong pool smell means too little active chlorine, not too much. The fix is counterintuitive: a shock dose high enough to burn the chloramines off, which the MAHC annex describes as breakpoint chlorination.

Chlorine lock. Months of trichlor tablets push cyanuric acid past 100 ppm, and at that level CYA binds chlorine faster than the chlorine can work. The test shows adequate free chlorine while algae grows anyway. No product pours CYA back down. The only realistic fix is draining a third of the pool, refilling, and switching the routine to liquid. Test CYA monthly if tablets are your main feed; it climbs about 3 ppm for every ppm of chlorine the tablets deliver.

Granular poured straight in. Cal-hypo scattered dry across the surface clouds the water and can bleach vinyl liners where the grains settle. Pre-dissolve it in a bucket of pool water, stir with a stick that is not your hand, and pour the slurry around the perimeter. Always chemical into water, never water into chemical.

How long before you can swim

Does chlorine go bad? Yes, and faster than you think.

Liquid chlorine is the perishable one. At 10 percent strength it loses potency month by month, and a jug that spent a summer in a hot garage can be down to half strength by the next season. Buy liquid in quantities you will use within 4 to 8 weeks, store it cool and dark, and if you are dosing correctly with old liquid and the numbers still will not move, the jug is the suspect. Trichlor tablets hold 3 to 5 years sealed and dry. Cal-hypo lasts a couple of years but is the touchiest to store: it must stay bone dry and nowhere near anything organic or flammable, per its own label. Old algaecide, since people ask: it keeps for years, but separated or gelled product goes in the hazardous waste drop-off, not the pool.

The rest of the pool project

Chlorine math is usually one line item in a bigger backyard plan. If the pool surround is on the list, the paver calculator covers deck pavers and their base depth, and the concrete calculator handles a poured deck or equipment pad in cubic yards. Landscaping the perimeter runs through the mulch calculator. And if you are capturing roof runoff to top the pool off, the rainwater calculator will tell you what an inch of rain on your roof is actually worth in gallons.

Run your numbers in the calculator above, write the result on the test kit lid, and stop guessing. A season of exact doses costs less, smells like nothing, and never once turns green.