How case odds actually work (and what the return rate means)
The five rarity tiers, the real weight of the rare slot, and how a case price is chosen so the house still wins.
Five slots, not one
A CS2 case does not roll uniformly. It first decides a rarity tier, then picks uniformly inside that tier. The classic weights are roughly 79.92% Mil-Spec, 15.98% Restricted, 3.2% Classified, 0.64% Covert, and 0.26% for the rare special slot that holds knives and gloves.
That last slot is why knives feel rarer than the numbers suggest: you have to hit 0.26% and then land the specific knife you want out of a dozen.
Return rate is not a promise per case
Expected value is the sum of every item's price times its weight. Divide that by the case price and you get the return rate. On this site it sits in the 0.67 to 0.78 range depending on the case.
That number only converges over hundreds of opens. Ten opens is a coin flip wearing a spreadsheet costume.
How to read a case before opening it
Look at three numbers: price, expected value, and the single most expensive item. A case with a huge top item and a low EV is a lottery ticket. A case with a flat EV and a cheap price is entertainment with a small tax.
If you want the maths done for you, the calculator page lists every case with its live return rate.