Principal

Principal is the amount of money borrowed or invested, separate from any interest that accrues on top. A $400,000 mortgage starts with principal of $400,000; after one month of payments under standard amortisation, principal is roughly $399,670 (and the remaining $2,330 of the payment went to interest).

Two distinctions worth keeping straight:

• Original principal: the amount you borrowed at the start. Fixed for the life of the loan.
• Remaining principal (or outstanding principal / principal balance): the unpaid portion at a given moment in time. This is what interest is computed against each period.

For investments, principal means the original amount put in. “Principal-protected” investments guarantee that you won’t get less back than you invested (modulo issuer credit risk). “Return on principal” is the gain on the original amount, distinct from compound returns on accumulated gains.

For loans, principal-only payments are extra payments applied directly against remaining principal, bypassing the normal interest portion. They shorten the loan and reduce total interest paid. See our mortgage calculator for the math.

Why principal-only prepayments are so powerful early on: early in a standard 30-year amortisation, roughly 85% of each payment goes to interest. A $1,000 extra principal payment in month 1 of a 7% mortgage saves roughly $6,300 of future interest over the loan’s life — every dollar paid early avoids compounding for the remaining 359 months. The same $1,000 paid in month 300 saves only ~$130 of future interest, because there’s much less time left for compounding to bite. This is why financial-planning literature so heavily weights “extra payments in the first ten years” for fixed-rate debt.

Watch out for the prepayment-penalty trap: some loans — historically subprime mortgages, today some auto and business loans — carry prepayment penalties that claw back a percentage of any principal paid above scheduled amounts in the first few years. The penalty is usually disclosed in the loan documents (CFPB Reg Z in the US requires it) but rarely emphasised. Before making any principal-only payment, confirm the loan has no prepayment penalty for the period — for a US conforming mortgage post-2014, this is generally guaranteed by federal regulation. Related: amortisation, APR. Reference: CFPB — Prepayment penalties.

Worked example

Take a $400,000 mortgage at 7.0% for 30 years. Monthly payment (P+I) = $2,661. Month 1: interest = $400,000 × 0.07/12 = $2,333; principal paid = $2,661 − $2,333 = $328; new balance = $399,672. Month 12: balance has fallen to $396,000; interest = $396,000 × 0.07/12 = $2,310; principal = $351. After 5 years (60 payments), the borrower has paid $2,661 × 60 = $159,660 total — of which only ~$32,000 reduced principal. Now imagine making one extra $10,000 principal payment in month 13 instead of saving it for later. The amortisation table recalculates: that single payment removes ~$31,000 of total interest from the remaining schedule and shortens the loan by roughly 21 months. The same $10,000 paid in month 313 would save only about $1,200 of interest.

When and why it matters

Confusing “principal” and “payment” leads to several predictable money mistakes. People judge debt by monthly payment and ignore the principal-balance trajectory — extending a loan from 15 to 30 years reduces the payment but more than doubles total interest. People assume their mortgage balance is “the home price minus 5 years of payments” and are surprised to find it’s barely moved. People treat investment “principal” as immutable and confuse it with current portfolio value (your $10,000 invested ten years ago is principal; its current $18,000 value isn’t). For any decision involving debt or interest-bearing accounts, ask “what is the current principal balance, and what fraction of my next payment reduces it?” — the answer often surprises and reframes the choice. Reference: SEC Investor.gov — Principal.