Methodology

Where the numbers come from

Every price on this site comes from an automated check of the retailer's own product page — the same page you'd land on if you clicked through. A crawler runs nightly, fetches each product page, and records the price and stock status it finds at that moment. We never estimate, average, or carry a price forward: if a page can't be read cleanly on a given night, that offer is simply skipped for that night rather than shown with a guessed number. Every table on this site reflects the single most recent successful reading for each offer, whatever night that happened to be.

Step 1: Delivered price

Delivered price is the item price plus whatever shipping you'd actually pay at that retailer, given that item's price and that retailer's shipping rules:

delivered price = item price + shipping fee

The shipping fee is $0 if the retailer offers free shipping above a threshold and this item's price meets or exceeds it. Otherwise it's that retailer's flat shipping fee (also $0 if the retailer doesn't charge one). We use each retailer's real published shipping rules, not a flat industry-wide assumption — two retailers selling an identical price can end up with different delivered prices once shipping is factored in.

Step 2: Pounds of available chlorine

A 50 lb bucket of tablets isn't 50 lb of chlorine — it's 50 lb of product, some percentage of which is the active, sanitizing ingredient. We call that the package's pounds of available chlorine:

lbs of available chlorine = package weight (lb) × active ingredient % ÷ 100

The active ingredient percentage is the "available chlorine" figure stated on the product's own label or spec sheet, not the purity of the raw chemical compound. A tablet that's 99% pure trichloro-s-triazinetrione but rated for 90% available chlorine is recorded here as 90% — that's the number that actually predicts how much your pool's chlorine level will rise.

Step 3: Dollars per pound of available chlorine

This is the number every table on this site is ranked by — the true apples-to-apples cost of chlorine, independent of package size or list price:

$/lb available chlorine = delivered price ÷ lbs of available chlorine

A smaller bucket with a lower list price can easily lose to a larger bucket once you divide out the shipping and the actual chlorine content — that's the entire point of computing it this way instead of comparing sticker prices.

Out-of-stock listings

If a retailer's page reported an item out of stock at the time of the most recent check, we still show that row — muted, and marked Out of Stock — rather than deleting it, so you can see the full picture of what a retailer carries and how it was priced the last time it was available.

What we don't do

We don't average multiple readings, don't smooth over gaps with a stale price, and don't insert a placeholder number when extraction fails. A missing observation for a night is acceptable; a wrong one is not. We also don't currently use affiliate links — the retailer URLs on this site are plain, unmodified links to the product page. If we join an affiliate program in the future we'll say so here.