How student loan interest works (and capitalization) ⚙

The situation

You haven’t made a single payment yet — maybe you’re in the grace period after graduating, maybe the loan is in deferment. It’s easy to assume the loan is just… waiting. It isn’t. On an unsubsidized loan, it’s growing every day, and there’s one specific moment where that quiet growth gets locked in permanently. This is the lesson where you learn to see it.

The idea

Two mechanics, and you know the second one already.

Mechanic 1: daily simple interest. Federal student loans accrue simple interest, every day:

daily interest = balance × (rate ÷ 365)

Take our $27,000 loan at about 6.5%: $27,000 × 6.5% ÷ 365 ≈ $4.81 a day. About $146 a month. About $1,755 a year. It’s simple interest — for now — meaning it accrues only on the principal. The unpaid interest piles up next to the loan, not inside it.

(Why “about 6.5%”? Federal rates are fixed for the life of each loan, but the rate new loans get is reset every July 1 — so any specific number ages fast. The studentaid.gov interest-rates page always has the current one.)

Mechanic 2: capitalization. Here’s the trap. Capitalization is the moment that pile of unpaid interest joins your principal. From that day forward, daily interest is calculated on the bigger base — you’re paying interest on interest. Remember the snowball from lessons 6.2 and 6.3, the one that grew your savings? And remember 7.4, where the same engine ran against you on a credit card? This is its third appearance: a capitalized student loan is the snowball rolling against you, locked in.

When does it happen? A deferment is a legal pause on payments (in school, unemployed, hardship); a forbearance is a similar pause for financial difficulty. On an unsubsidized loan, interest keeps accruing through both. Since rule changes in 2023, capitalization mostly happens in two cases: when you exit a deferment on an unsubsidized loan, and when you consolidate (more on that in 9.5). The first one is exactly the situation a first-job grad walks into.

The rule of thumb: unpaid interest is a snowball waiting to be locked in. If you can pay even the interest during a pause — about $146 a month on this loan — you stop capitalization from ever happening.

By the numbers

Here’s one 12-month unpaid deferment on our $27,000 unsubsidized loan at ~6.5%, before and after:

	Before	After a 12-month unpaid deferment
Principal	$27,000	$28,755
Interest accrued per year @ ~6.5%	$1,755	$1,869.08

The year of unpaid interest — $1,755 — capitalizes into the principal. The new balance is $28,755, and the loan now accrues $1,869.08 a year instead of $1,755. That’s about $114 more interest every single year going forward, until the loan is gone — the permanent price of one unpaid year. The deferment felt free. It wasn’t.

Drawn as a loop, it’s the snowball from 6.2 and 6.3 — same engine, pointed the other way. And notice the exit: pay just the interest during the pause and the loop never closes.

flowchart LR
  accTitle: How capitalization turns unpaid interest into principal
  accDescr: Your loan balance accrues simple interest every day. During a pause such as a deferment, that interest goes unpaid and piles up beside the loan. When the pause ends it capitalizes — the unpaid interest is added to your principal. It is principal now, so it accrues daily interest itself, which makes the balance bigger, and the loop goes round again. The way out is to pay just the interest during the pause, so capitalization never happens.
  B["Your loan balance"] --> I["Interest accrues on it every single day"]
  I --> U["The interest goes unpaid during a pause, such as a deferment"]
  U --> C["It capitalizes: the unpaid interest is ADDED to your principal"]
  C --> P["It IS principal now, so it accrues daily interest itself"]
  P --> B
  U --> S["Pay just the interest during the pause, and capitalization never happens"]

Now, the calculator below. It’s the same generic snowball machine from lesson 6.3, showing its savings default: $1,000 plus $100 a month at 5% growing to $17,175.26 over ten years. The labels say savings — it doesn’t know about loans. But the engine is the engine. To feel what a balance does when it grows untouched, set the starting amount to $27,000 and the monthly contribution to $0, and watch a balance expand entirely on its own. That widening band is what “I’ll deal with it later” looks like — the only thing that changes between saving and owing is which side of the snowball you’re on.

Where this fits

You now know what the loan does on its own: $4.81 a day, and a one-time lock-in if interest goes unpaid through a deferment. The next lesson is about the thing you control — the monthly payment — and the two very different shapes it can take.