The Structure of Loan Interest Rates — Fixed vs. Variable, Spread, and Understanding DSR

How is a loan rate created?

A loan rate is generally calculated as a structure of adding a spread (add-on rate) to the base rate and subtracting a preferential rate. The base rate is a metric corresponding to the cost of raising funds in the market, using published values like COFIX or bank-bond yields. The portion a financial institution adds on top, reflecting credit risk, operating costs, and target profit, is the spread. The reason people get different rates even when taking loans from the same bank at the same time mostly splits here, at the spread. The preferential rate is the part discounted according to transaction records like salary transfer, card spending, and auto-transfers, but it can disappear if the conditions are not maintained, so you should see it not as "the rate you can get" but as "the rate you actually meet right now." The base-rate portion is hard for you to control, but managing your creditworthiness and actually keeping preferential conditions is an area the borrower can handle. For reference, it is good to also know that in a variable-rate loan, what immediately reflects a change when the base rate shifts is the base-rate portion, while the spread is set at the time of the agreement and rarely changes during the loan period. In other words, even the same 'rate' is a mix of a part that moves over time and a part that is fixed.

Fixed rate and variable rate have different natures

Fixed rate is a method where the rate does not change during the agreed period, and variable rate is a method where the rate is recalculated at set cycles reflecting changes in the base rate. Which is advantageous is not a matter of guessing whether future rates will rise or fall, but of whether each method's nature fits your situation. A fixed rate has high predictability with a constant repayment amount, but usually the price of that stability tends to be reflected in the initial terms. With a variable rate, the burden falls if market rates drop but grows if they rise, so the key judgment standard is whether you can bear the repayment amount even when rates rise. If your repayment period is long and you are sensitive to income changes, lean toward stability; if the possibility of early repayment is high or you have capacity to endure rate changes, lean toward flexibility. There is no correct answer that "one side is unconditionally better."

Total interest changes with the repayment method

Even at the same rate and same period, the amount you repay each month and the total interest change with the repayment method. Organizing the three representative ones as follows.

• Equal principal and interest: The total repaid each month (principal plus interest) is constant. Early on the interest portion is large and the principal portion grows over time. The constant monthly burden makes it easy to plan.
• Equal principal: The principal repaid each month is constant, and interest on the remaining principal gradually decreases, so the initial burden is largest but the total interest is generally the smallest.
• Bullet (maturity lump-sum): You pay only interest during the period and repay the principal all at once at maturity. The monthly burden is light, but since the principal does not decrease, total interest tends to be the largest.

The principle behind the total-interest difference is simple. Interest always accrues on the 'remaining principal,' so the faster you reduce the principal, the smaller the base on which interest accrues. That equal-principal has less total interest, and bullet has more, both stem from this. So 'a small monthly repayment' and 'a small total burden' are different stories, and it is important not to confuse the two. Which method is better is not a matter to decide by total interest alone, but should be judged from the balance between the cash flow you can bear each month and the total burden. For example, if your income is stable and you have initial capacity, equal-principal is advantageous in total interest, but if you want to manage a constant monthly burden, equal principal and interest is convenient for planning.

DSR, LTV, DTI — the rulers that measure repayment ability

Loan screening uses metrics that measure repayment ability and collateral value. LTV (loan-to-value ratio) shows how much you can borrow relative to the price of the home used as collateral, and DTI weighs the principal and interest of the relevant loan and the interest on other loans against income. DSR (debt service ratio) goes one step further, the annual principal-and-interest repayment of all loans you hold, divided by annual income. That is, DSR is a metric that comprehensively looks at "the share of my income used to repay debt," a device to manage the total debt one person can bear. The specific limit ratio differs by region, housing, policy, and time, so do not declare it, check at an official channel. What matters is that these metrics are ultimately safeguards to prevent loans beyond the 'range you can repay.'

The grand principle of every decision — within your repayment ability

The reason to understand the rate structure, repayment methods, and various metrics ultimately converges on one thing: a loan should be decided within the 'range you can repay,' not the 'maximum you can borrow.' Just because a limit comes out does not mean you must borrow that much, and it is safer to leave a buffer, assuming even situations where income falls or rates rise. When choosing a product, do not look only at the single line of the headline rate; weigh together the sustainability of preferential conditions, the monthly burden and total interest by repayment method, and incidental conditions like early-repayment fees. The power to understand the numbers ultimately becomes the power to protect yourself.

DSR and LTV, the actual regulatory numbers

What actually governs the loan limit is DSR. DSR is (annual principal-and-interest repayment divided by annual income) times 100, and the ceiling is generally 40% for banks and 50% for non-banks. For example, with an annual income of 60 million won, under the bank DSR 40% standard, annual principal and interest can hardly exceed 24 million won. This regulation applies in earnest once total loans exceed 100 million won.

On top of this, from July 2025 the stress DSR stage 3 was applied nationwide. As a device that calculates the limit by reflecting future rate rises in advance, it sets the limit by adding a spread of about 1.5%p in the metropolitan area and 0.75%p in provincial areas. That means the limit you can actually get shrinks accordingly. Look at it together with LTV (loan-to-home-price ratio), but these days DSR often tightens the limit first. The exact limit differs by income, existing loans, and region, so calculate it in advance with a calculator.

Frequently asked questions

Q. If I get the maximum preferential rate, does that rate stay?
A. No. The preferential rate applies while you maintain record conditions like salary transfer or auto-transfer, and if the conditions break it can shrink or disappear. So it is safer to calculate the burden accounting for the rate when the conditions drop out, not the after-preferential rate.

Q. Is the method with the smallest monthly repayment the best deal?
A. Not so. Methods with a light monthly burden, like bullet, tend to have more total interest because the principal does not decrease. The monthly burden and the total burden are separate, so you must compare your cash flow and total interest together.

Q. I do not know whether rates will rise or fall, should I choose fixed or variable?
A. It is better to judge by each method's nature than to try to guess the direction. If predictability matters, consider fixed; if you have capacity to bear the increased repayment when rates rise, you can consider variable. Neither is always the correct answer.

This article is for general financial information and does not recommend the use of any specific loan product. Rates, limits, and screening standards differ by individual credit, policy, and time, so before an actual loan please check at the relevant financial institution and official government channels such as the Financial Supervisory Service (FINE). Please decide on a loan prudently, within your repayment ability.