The principal is the original sum of money that is invested or borrowed, before any interest is added. Every interest calculation starts from it: interest is a percentage of the principal (and, with compounding, of the interest it has already earned).
Principal in savings and investments
When you open a savings account with $5,000, that $5,000 is your principal. If you later add more, your principal grows by the amount you deposit. Interest is what the bank adds on top. Keeping the two separate matters for several reasons:
- It shows how much of your balance came from your own money and how much from growth.
- In many countries interest is taxable while the return of your own principal is not.
- Deposit insurance limits usually apply to principal plus accrued interest, so it helps to know both.
Our regular contributions calculator shows principal, contributions and interest separately for each year.
Principal in loans
For a loan, the principal is the amount you borrowed — or, later on, the amount you still owe, not counting interest. Each payment on a typical mortgage or car loan has two parts:
- Interest — this month’s interest on the outstanding principal.
- Principal repayment — the rest of the payment, which reduces the balance.
Worked example: the first mortgage payment
A $200,000 mortgage at 6% over 30 years has a monthly payment of $1,199.10.
- Monthly rate: 6% ÷ 12 = 0.5%.
- First month’s interest: 200,000 × 0.005 = $1,000.00.
- Principal repaid: 1,199.10 − 1,000.00 = $199.10.
- New principal balance: $199,800.90.
Early payments are mostly interest. As the principal falls, each month’s interest falls too, and more of the fixed payment goes to principal. Extra payments “towards principal” reduce the balance immediately and cut the total interest over the life of the loan.
Principal vs interest vs balance
| Term | Meaning |
|---|---|
| Principal | Money originally invested or borrowed (or still owed, excluding interest). |
| Interest | The cost of borrowing or the reward for saving. |
| Balance | Principal plus any interest added and not yet paid out or repaid. |
| Capitalised interest | Unpaid interest added to a loan’s principal, which then earns interest itself. |
How principal affects interest
Interest is proportional to principal. Double the principal and, at the same rate and time, you double the interest. That is why the first step in any interest calculation is to identify P correctly — for example, a deposit after an account-opening fee, or a loan amount after a fee that is added to the balance.
Finding the principal you need
If you know a target amount, the rate and the time, you can work backwards to the principal you must invest today:
P = A ÷ (1 + r/n)n × t
To have $50,000 in 8 years at 5% compounded monthly, you would need to invest $33,543.87 today. The Principal Calculator does this instantly.
Key takeaways
- Principal is the starting amount, before interest.
- In loans, payments split between interest and principal; only the principal part reduces what you owe.
- Interest scales directly with principal.