Extra principal and a deposit into an All In One® save you the same interest. Not similar — the same. Both lower the balance the loan charges interest on, and the loan doesn’t care which one you used. What differs is what happens to that dollar afterward: one of them you can spend on Thursday, and one of them is gone.
Why the interest math is identical
A mortgage charges interest on what you owe. Lower what you owe and you lower the interest — that’s the entire mechanism, and it’s the same mechanism in both cases.
Say you have a $400,000 loan at 6.5%, and $50,000 you could put toward it. These figures are illustrative, but the arithmetic holds at any size:
- Pay $50,000 to principal. The balance is now $350,000. Daily interest at 6.5% is about $62 a day instead of about $71.
- Deposit $50,000 into an All In One. The balance is now $350,000. Daily interest is about $62 a day instead of about $71.
Same balance, same rate, same interest. About $9 a day either way, which is roughly $267 over a 30-day month. There is no version of this where one structure charges you less than the other for the same money sitting in the same place.
The difference shows up when you want the money back
Once you pay $50,000 to principal, it’s equity. It’s still yours, but it’s in the house. Getting it back means refinancing, taking a second loan against the home, or selling it — and all three depend on what rates and your credit look like on the day you need it, not the day you sent the money.
The same $50,000 in an All In One is still in your account. It’s offsetting the balance every day it sits there, and the day you spend it, it stops. Your balance goes back to $400,000 and your interest goes back to about $71 a day.
That’s the trade in one line: extra principal buys the savings permanently and takes the liquidity permanently. The All In One rents you both, for as long as the money sits.
The part that isn’t in the All In One’s favor
Extra principal keeps working whether or not you’re paying attention. It’s a decision you make once. An All In One’s benefit is proportional to your average daily balance — if $50,000 lands on the first and is spent by the eighth, you saved a week of interest, not a month.
So the structure rewards money that genuinely sits: a tax reserve, a buffer between commission checks, the savings account that never gets touched. It does very little for money that arrives and leaves.
What to do with this
Two things worth knowing before the structure matters at all:
- Find what your accounts actually average across a month, not what they peak at on payday. That average is the number that decides whether any of this does anything for you.
- If you’re already paying extra principal, ask what you’d do if you needed that money back. If the honest answer is “refinance or borrow it,” the lock is a cost you’re already paying — you just haven’t been billed for it yet.
If you want to run your own averages against both structures, send them over and I’ll show you the arithmetic either way — including when the answer is that a plain fixed mortgage wins.