Every travel hacker knows that interest charges (and annual fees) are the flip side of credit card rewards. You may earn 2% on the front end when paying with a credit card, whether you're buying a cup of coffee or manufacturing spend, but if you don't pay off your entire balance in full by the due date on your statement, you'll give it all back and more as your remaining balance accrues interest. On my credit cards rates typically start at 12.99% APR annually and go way, way up from there.
All over the travel hacking blogosphere you'll find variations of the mantra, "if you don't pay your credit cards off in full every month, travel hacking isn't for you." There's an ironclad kernel of truth to that (your interest charges will far exceed the value of your rewards) but also a deep illogic: even if you have to pay interest, you're strictly better off earning the most valuable rewards possible on any purchases you have to make. So I'll skip the lectures and stick to the facts.
Warning: trailing interest is like interest, but worse
Just like the interest earned on your savings, the interest paid on your credit card balances compounds, which gives rise to a (deliberately) confusing concept: trailing interest. Trailing interest is the product of a mismatch between the pace at which interest accrues (daily) and the pace at which it posts to your outstanding credit card balance (monthly).
Here's the description of trailing interest given on my American Express credit card statements:
"About Trailing Interest
You may see interest on your next statement even if you pay the new balance in full and on time and make no new charges. This is called "trailing interest." Trailing interest is the interest charged when, for example, you didn't pay your previous balance in full. When that happens we charge interest from the first day of the billing period until we receive your payment in full. You can avoid paying interest on purchases by paying your balance in full and on time each month."
The takeaway from this statement is that, if you failed to pay for your purchases in full and thus have a balance that's accruing interest, the date your credit card statement closes is the only date when your outstanding balance accurately reflects the amount you owe. Every subsequent day, a hidden amount of trailing interest accrues which will only post on the following statement closing date.
To avoid paying interest, you have to pay your balances off on time. To avoid paying trailing interest, you have to pay any interest-bearing balances off early, preferably on the statement closing date, to avoid giving trailing interest a chance to accrue.
Bonus warning: know how your banks calculate interest charges
Since I pay off my credit cards in full every month (preferably before my statement closes, to ensure as low a credit utilization as possible is reported to the credit bureaux), I never took the slightest interest in how banks calculate interest charges.
Until a few months back, that is, when due entirely to my own negligence I paid $5 less than my statement balance on my US Bank Flexperks Travel Rewards Visa Signature card:
Mint, the website I use to track my bank accounts, credit cards, and investments alerted me that interest had been charged on one of my accounts, so I pulled up my statement and was horrified to see an interest charge of $20.35. Naturally my first move was to call in and ask a representative reverse the interest charge:
But while I was on the phone, I asked her to explain how it was possible that I was charged $20.35 in interest on a $5 unpaid balance. The representative explained that at US Bank, they charge interest on your entire balance if any part of it is unpaid on the statement's due date.
So in case you were wondering how credit card companies pay for the rewards they shower on us, this is how: by aggressively charging customers who are anything less than totally and utterly vigilant about paying off their credit cards in full and on time.
I hope credit card interest charges are an issue that will remain completely and utterly academic for all my readers. Realistically, that's not going to be the case, but the more information you have about the kinds of interest charges and the way they're calculated, the more lucrative I hope your relationship with your credit card issuers will be.