How a variable rate is built
A variable credit card APR has two parts: an index and a margin. The index is almost always the US Prime Rate as published in The Wall Street Journal, which tracks the Federal Reserve's federal funds target and moves when the Fed moves. The margin is the issuer's own spread, set from your credit profile at approval and generally fixed for as long as you hold the card.
Your APR is the sum. If Prime is 7.50% and your margin is 14.99%, your APR is 22.49%. When the Fed raises rates by a quarter point, Prime goes to 7.75% and your APR becomes 22.74% — usually from the first day of the next billing cycle, with no notice required, because the agreement already disclosed that the rate varies with the index.
A margin is not an APR
This is the distinction that causes the most confusion, and it is why this site treats the two differently. Many agreements, particularly from credit unions, state the pricing as "Prime plus 9.99%" and never state the resulting APR, because the resulting APR depends on a number that changes.
A document quoting a 9.99% margin is not describing a 9.99% card. With Prime in the mid sevens it describes a card around 17.5%. Read as an APR, that filing would look like one of the cheapest in the database; read correctly, it is unremarkable. Where a filing prices this way, every page on this site labels the figure as a margin over Prime and never lists it as an APR — including in the comparison tables, where mixing the two would make the ordering meaningless.
The 45-day rule, and the hole in it
The CARD Act of 2009 requires 45 days' advance notice before an issuer raises your rate, and generally protects your existing balance at the old rate. It is a real protection with a specific exception: it does not apply to an increase caused by the index moving.
If Prime rises, your variable APR rises with it, immediately, on your existing balance, with no notice and no right to opt out. That is not a loophole anyone slipped in — the agreement disclosed the rate as variable, so the change is considered already disclosed. It does mean the protection most people believe they have against rate increases does not cover the most common cause of one.
What a move actually costs
A quarter-point rise on a $5,000 balance is about $12.50 a year. On its own, immaterial. The reason to pay attention is that Prime moves in runs rather than in isolation: between March 2022 and July 2023 it went from 3.25% to 8.50%, a 5.25 point rise that passed straight through to every variable card. On that same $5,000 balance, that run added roughly $262 a year in interest to accounts whose holders had done nothing differently.
What to do with this
If you carry a balance, a variable rate means your cost of borrowing is set by monetary policy and will move without warning. A genuinely non-indexed rate is worth something in that situation, though as covered in what APR actually means, "fixed" does not mean the issuer can never change it.
If you pay in full every month, none of this reaches you. The index can do what it likes.
Next
You can see every agreement in the database that prices this way on the Prime plus margin list, and the broader set of variable rate cards. For how the resulting rate is applied day to day, see what APR actually means.