what are the downsides of rolling credit card debt in this fashion
Assuming you can get keep getting credit cards like this forever, you open yourself up to risk in short term losses. Stock/bond prices fluctuate. If you need to pay the money back for some reason (at the end of the 15 months) your investment may be less than the 5,000 you started with. Awesome, you are a math guy.  Very good for you.  In theory, what you are proposing, should work out great as the math works out great.  However have you taken a economics or finance coursework?  The math that they do in these class will leave a most math guys uncomfortable with the imprecision even when one is comfortable with chaos theory. Personal finance is worse.  If it were about math things like reverse mortgages, payday lenders, and advances on one income tax returns would not exist.   The risk derived from the situation you describe is one born out of behavior.  Sometimes it is beyond control of the person attempting your scheme.  Suppose one of these happen: In my opinion the market is risky enough without borrowing money in order to invest.  Its one thing to not pay extra principle to a mortgage in order to put that money in play in the market, it is another thing to do what you are suggesting.  While their may be late fees associated with a mortgage payment, a fixed rate mortgage will not change if you late on payment(s). On these balance transfer CC schemes they will jack your rate up for any excuse possible.  I read an article that the most common way to end up with a 23%+ credit card was to start out with a 0% balance transfer.  One thing that is often overlooked is that the transfer fee paid jacks up the stated rate of the card.   In the end, get out of consumer debt, have an emergency fund, then start investing.  Building a firm financial foundation is the best way to go about it.  Without one it will be difficult to make headway.  With one your net worth will increase faster then you imagined possible.
"I've been using YNAB4 for the last few years, and I like it so much that I haven't switched to the web version (new YNAB) yet.  However, I have played around with the web version a little, and here is what I have discovered. Despite the different look of the credit card account and the lengthy dissertation on the credit card differences in the Transition Guide, credit cards are handled almost exactly the same in the new YNAB as they were in YNAB4. You enter credit card spending transactions in the same way as YNAB4.  When you enter a transaction, money is pulled out of the budget category you select.  The only difference is that in YNAB4, this money was considered ""gone.""  Now, that money moves from your budget category into the new credit card category. When it comes time to pay the credit card bill, you also enter this transaction in the same way as before.  It is entered as a transfer of money from your checking account to your credit card account.  The only difference here is that with new YNAB, the funds are deducted from your credit card category.  This is handled automatically, so you don't have to think about it if you don't want to.   If you always pay your credit card bill in full, you never have to budget money manually into the credit card category.  The money will already be there from when you entered the credit card spending transactions.  The only time you would manually budget money into the credit card spending category is if you have old credit card debt that you are trying to pay off. A quick example, in pictures: I start out with $10,000 in my checking account, and no credit card debt:  I've got all $10,000 in my ""Fun Money"" category:  Now, I spend $100 at the Store:  You can see that, just like in YNAB4, the credit card account is now in the red $100, and the checking account balance has not changed. In the categories, my Fun Money category is down $100 to $9,900, just like it would be in YNAB4.  The only difference is that there is now $100 in the new Credit Card Payments category.  When it is time to pay the bill, I enter an account transfer, just like in YNAB4:  Note that the Credit Card balance is back to $0, and the Checking Account balance is now down to $9,900. The Credit Card Payment budget category is now magically back to $0:  The above example starts with a zero balance on the credit card.  However, most people will have a non-zero balance on their credit card when they first start a budget.  In YNAB4, when you added a credit card with a (negative) balance, the debt was shown in a budget category called ""Pre-YNAB Debt.""  You then added money to this budget category until it went to zero, and then you didn't need this budget category anymore. With new YNAB, credit card balances are not shown in budget categories.  If you add a credit card account with a balance, the debt is not shown in the budget categories.  To pay off this debt, you can fund the Credit Card Payments category.  After this existing balance amount is paid off, you won't need to fund the Credit Card Payments category anymore as long as you properly assign each new credit card purchase to a funded budget category." Lets say Debit is what you pay and Credit is what somebody else pays for you. A Debit card will charge your bank account directly. A Credit card will charge your bank account some time later. In both cases the shop owner has the money available directly. It's called Credit because somebody believes you will be able to pay your debt on time (or later with an interest). As for purchase and sales. A customer buys = makes a purchase. A shopkeeper sells = makes a sale. Note from a bar: I made an agreement with the bank. They don't sell beer, I don't give Credit. "First to actually answer the question ""how long at these rates/payments?""-  These is nothing magic or nefarious about what the bank is doing. They add accrued interest and take your payment off the new total.  I'd make higher payments to the 8.75% debt until it's gone, $100/mo extra and be done. The first debt, if you bump it to $50 will be paid in 147 months, at $75/mo, 92 months. Everything you pay above the minimum goes right to the principal balance and gets you closer to paying it off.  The debt snowball is not the ideal way to pay off your debt. Say I have one 24% credit card the bank was nice enough to give me a $20,000 line of credit on. I also have 20 cards each with $1000 in credit, all at 6%. The snowball dictates that the smallest debt be paid first, so while I pay the minimum on the 24% card, the 6% cards get paid off one by one, but I'm supposed to feel good about the process, as I reduce the number of cards every few months.  The correct way to line up debt is to pay off the (tax adjusted) highest rate first, as an extra $100 to the 24% card saves you $2/mo vs 50 cents/mo for the 6% cards. I wrote an article discussing the Debt Snowball which links to a calculator where you can see the difference in methods. I note that if the difference from lowest to highest rate is small, the Snowball method will only cost you a small amount more. If, by coincidence, the balances are close, the difference will also be small.  The above aside, it's the rest of your situation that will tell you the right path for you. For example, a matched 401(k) deposit should take priority over most debt repayment. The $11,000 might be better conserved for a house downpayment as that $66/mo is student loan and won't count as the housing debt, rather ""other debt"" and part of the higher ratio when qualifying for the mortgage. If you already have taken this into account, by all means, pay off the 8.75% debt asap, then start paying off the 3% faster. Keep in mind, this is likely the lowest rate debt one can have and once paid off, you can't withdraw it again. So it's important to consider the big picture first.  (Are you depositing to a retirement account? Is it a 401(k) and are you getting any matching from the company?)" Patience is the key here, I hate to say! There are five factors to FICO credit scores:  Payment history is adversely affected by late payments - so always pay on time, otherwise your report will be haunted for seven years! 👻 Credit utilization has to do with how much of your available credit is currently in use - lower is better, but 0% isn't good either because they want to see that you're using credit. 10% or less is a good goal, and try to keep any single card balance to 30% or less when its statement close date rolls around. Credit history is based on the average age of all of your accounts, cards or otherwise, the older the better. Don't close either of your other cards (because that would cause your average account age to fall), and make sure to use the store card at least occasionally, because lenders sometimes decide to close unused lines of credit. Credit mix has to do with the different types of credit you hold and is why your bank's website suggested taking out a loan. It also has to do with the number of accounts overall; I've never found a satisfactory answer for what the sweet spot is, but I suspect it's in the 6-12 range? You wouldn't want to get several new ones at the same time because... New credit is affected by the credit inquiries (hard pulls) that occur when you apply for new cards or loans. Inquiries stay on your report for two years before falling off. This is almost certainly where your score dropped. You also mentioned not knowing if some hospital bills are still affecting your score. You'll want to review your credit reports and find out, plus checking your credit reports regularly is a really great habit to get into because errors (and fraud) can and do happen. There are three credit reporting agencies: Experian, Equifax, and TransUnion, and you'll want to review all three. You can get one free report from each of them every year: https://www.usa.gov/credit-reports It can take a couple of months for a new credit account to show up on your credit report, so your score should recover and go even higher once that happens. Sit tight, as annoying as that is! If you don't carry a balance, there is no disadvantage. Merchants pay less for their in-house credit, so there are often incentives for you to use the store card. The perils of opening a credit card hurting your credit score are way overblown in general, if you have good to excellent credit. If you have excellent credit, there is no material effect on your ability to borrow. You'll get knocked down a few points when you open the card, but as long as you're not on a credit application frenzy there isn't an issue. "Typically you can use credit card balance transfers to consolidate some, or all, of your other loan balances in one place.  The interest rate might be lower.  Some prefer to make one payment rather than multiple payments.  There is typically a fee that is imposed by the card that is originating or creating the loan.  This would be the credit card you are transferring the balances.  That fee is typically in the 3% to 5% range.   While tempting and attractive on the surface, this plan typically leads to a worse situation then you are now.   It's a ""tough pill to swallow"", but your problem is that you spend too much money.  Transferring money will not change this problem, it is your behavior that has to change in order to not accumulate more debt.  It has to change further if you want to get rid of the debt in a timely fashion.   You would be far better served to forget about this transfer and get your life into control.  Spend a lot less, earn more.  Pay off the cards you have now and cut them up.  Make a goal to be done in a year and figure out how to earn enough money to make that happen. BTW I am a reformed over-spender that now owes nothing.  Yep my house, cars, and rental property are all paid for.  You can get there too." Except for the fact that canceling credit cards will hurt your credit, either by increasing your utilization ratio or decreasing the length of your credit lines if you cancel the oldest card, which makes it that much harder to get the good interest rates available on other loans. "@Joe's original answer and the  example with proportionate application of the payment to the two balances is not quite what will happen with US credit cards. By US law (CARD Act of 2009), if you make only the minimum  required payment (or less), the credit-card company can choose which part of the balance that sum is applied to. I am not aware of any company that chooses to apply such payments to anything other than that part of the balance which carries the least interest rate  (including the 0% rate that ""results"" from acceptance of balance transfer  offers). If you make more than the minimum required payment, then the excess must, by law, be applied to paying off the highest rate balance. If the highest rate balance gets paid off completely, any remaining amount must be applied to second-highest rate balance, and so on.  Thus, it is not the case that that $600 payment (in Joe's example) is applied proportionately to the $5000 and $1000 balances owed. It depends on what the required minimum payment is. So, what would be the minimum required payment? The minimum payment is the total of (i) all finance charges incurred during that month, (ii) all service fees and penalties (e.g. fee for exceeding credit limit, fee for taking a cash advance, late payment penalty) and other charges (e.g. annual card fee) and (iii) a fraction of the outstanding balance that (by law) must be large enough to allow the customer to pay off the entire balance in a reasonable length of time. The law is silent on what is reasonable, but most companies use 1% (which would pay off the balance over 8.33 years).  Consider the numbers in Joe's example together with the following assumptions: $5000 and $1000 are the balances owed at the beginning of the month, no new charges or service fees during that month, and the previous month's minimum monthly payment was made on the day that the statement paid so that the finance charge for the current month is on the balances stated). The finance charge on the $5000 balance is $56.25, while the finance charge on the $1000 balance is $18.33, giving a minimum required payment of $56.25+18.33+60 = $134.58. Of the $600 payment, $134.58 would be applied to the lower-rate balance ($5000 + $56.25 = $5056.25) and reduce it to $4921.67. The excess $465.42 would be applied to the high-rate balance of $1000+18.33 = $1018.33 and reduce it to $552.91. In general, it is a bad idea to take a cash advance from a credit card. Don't do it unless you absolutely must have cash then and there to buy something from a merchant who does not accept credit cards, only cash, and don't be tempted to use the ""convenience checks"" that credit-card companies send you from time to time. All such cash advances not only carry larger rates of interest (there may also be upfront fees for taking an advance) but any purchases made during the rest of the month also become subject to finance charge. In other words, there is no ""grace period"" for new charges, and this state of affairs will last for one month beyond the first credit-card statement whose statement is paid off in full in timely fashion. Finally, turning to the question asked, viz. "" I am trying to determine how much I need to pay monthly to zero the balance, ...."", as per the above calculations, if the OP makes the minimum required payment of $134.58 plus $1018.33, that $134.58 will be applied to the low-rate balance and the rest $1018.33 will pay off the high-rate balance in full if the payment is made on the day the statement is issued. If payment is made later, but before the due date, that $1018.33 will be accruing finance charges until the date the payment is made, and these will appear as 22% rate balance on next month's statement. Similarly for the low-rate balance. What if several monthly payments will be required? The best calculator known to me is at https://powerpay.org (free but it is necessary to set up a username and password).  Enter in all the credit card balances and the different interest rates, and the total amount of  money that can be used to pay off the balances, and the site will lay out a payment plan.  (Basically, pay off the highest-interest rate balance as much as possible while making minimum required payments on the rest). Most people are surprised at how much can be saved (and how much shorter the time to be debt-free is) if one is willing to pay just a little bit more each month."