2026-06-25 · Calc-Tools Editorial

Equal Installment vs Equal Principal: Which to Choose?

The difference between equal installment and equal principal is usually compressed into one line: equal principal costs less interest. That is true, and it answers only one of three questions. The other two decide the outcome - what you give up to save that interest, and whether the choice still matters once you plan to prepay.

This article skips the formula derivation. Instead it puts both schedules on one scale, the time the principal spends outstanding, then builds a decision framework with an exact break-even point, and closes with a table that tells you when a prepayment is actually worth making. Every figure uses CNY 1,000,000 at an assumed 3.5% annual rate over 30 years (360 instalments). The 3.5% is an illustration, not a market quote; rebuild the numbers with your own amount and term in the loan comparison calculator.

1. Interest Is Rent on the Money You Still Owe

The interest charged in any period depends on a single number: the balance at the start of that period. Interest = opening balance x monthly rate. In our example the monthly rate is 3.5% / 12, so the first period charges CNY 2,916.67 on a CNY 1,000,000 balance, and as principal is repaid the base for the next period shrinks with it.

Neither method gets a discount on that rule. The only difference between them is how fast the balance falls. Equal installment holds the payment fixed and repays very little principal early - of the first CNY 4,490.45, only CNY 1,573.78 is principal. Equal principal retires a flat CNY 2,777.78 of principal every month, so its balance is a straight line down and its payment falls by CNY 8.10 each period.

Put that on a comparable scale and you get principal-time: the balance owed multiplied by the periods it stays owed, measured in yuan-months. Summing the opening balance across all 360 periods gives 211,392,300 yuan-months under equal installment and 180,500,000 under equal principal, a ratio of 1.1711. The ratio of their total interest is 616,560.88 / 526,458.33, which is also 1.1711 - identical to four decimals. Stated more plainly, the average balance over the 30 years is CNY 587,200.83 under equal installment and CNY 501,388.89 under equal principal.

So "equal principal saves interest" is better read as "equal principal gives up time". On this loan, equal installment costs a flat CNY 4,490.45 a month and CNY 616,560.88 of interest over the term. Equal principal starts at CNY 5,694.44, ends at CNY 2,785.88, and costs CNY 526,458.33. The interest gap is CNY 90,102.54 and it is bought with CNY 1,204.00 more in the first month. Keep those two numbers in view: every trade-off below happens between them.

2. Two Balance Curves on the Same Loan

Slice the 360 periods and look every five years. The gap is not a fixed-width crack; it opens and then closes.

Two things stand out. First, the interest gap compounds: CNY 5,737.82 after five years, CNY 20,969.60 after ten, CNY 64,172.37 after twenty, and CNY 90,102.54 only at the end. It looks small early on because both schedules are still paying interest on a large balance, so most of the gap accumulates in the second half of the term.

Second, the balance gap is widest mid-term: after 180 payments equal installment still owes CNY 628,137.72 while equal principal owes CNY 500,000.00, a difference of CNY 128,137.72. After that the equal-principal payment falls clearly below the equal-installment figure - it drops under CNY 4,490.45 at period 150 and reaches CNY 3,758.10 by period 240 - and the gap narrows again. That is also why the comparison has to run over the whole term: mid-term the two schedules differ by more than CNY 128,000 of outstanding principal, so switching partway through is effectively a repricing.

3. The Decision Framework: Treat the Extra Payment as an Investment

Choosing equal principal means voluntarily paying more every month. Choosing equal installment means keeping that same money in your own hands. So the real question is not which method charges less interest, but whether the money you keep can earn back the interest you gave up.

That question has an exact answer. Compound the difference between the two payments (equal-principal payment minus equal-installment payment) at a single assumed return up to period 360, then compare the household balance under each plan. A positive number means equal installment is ahead:

Note: the returns in this table are assumptions used for illustration only. They are not a product quote and not a market forecast.

The tie lands exactly on 3.5%, the loan's own rate. That is not a coincidence but an identity: choosing equal principal amounts to prepaying into the loan, and the equivalent return on that move is always the loan rate itself. The CNY 90,102.54 you save is simply those early-repaid principal amounts accruing at 3.5% for the remaining life of the loan. It also settles a common argument: when you compare prepaying with investing, the benchmark is the loan rate, not whatever the market did last year.

So the test becomes operational: compare the return that money can actually earn, after tax and after costs, with the loan rate. Below the loan rate, take equal principal. Above it, take equal installment - provided the difference is genuinely invested rather than absorbed by spending. The higher the return, the faster the advantage grows: CNY 83,756.69 ahead at an assumed 5%, CNY 274,437.95 at an assumed 7%. Reverse the sign and equal principal leads by CNY 51,952.69 at an assumed 2%.

The third pillar of the framework is liquidity, and it often decides the outcome more than the return does. The CNY 1,204.00 separating a CNY 5,694.44 first payment from a CNY 4,490.45 one is cash you must find every month for the first several years. Before choosing equal principal, confirm three things: the extra payment does not disturb daily life; the emergency reserve still covers six to twelve months of spending; and income is not expected to fall over the next few years. If any of them fails, equal installment is the safer choice - its payment never changes between period 1 and period 360, and that predictability has value of its own.

4. Why Prepayment Timing Matters More Than the Method

A prepayment always saves interest, but how much depends on when you make it, not on which method you chose. The reason is in the payment structure: under equal installment the interest share falls month by month. Of the first CNY 4,490.45, CNY 2,916.67 is interest, or 64.95%; by period 360 interest is down to CNY 13.06 and the rest is principal. In other words, most of the interest is paid in the first half of the term.

Each percentage uses that method's own total principal or total interest as its denominator. The equal-installment line is the striking one: after 15 years you have paid 70.78% of the interest but repaid only 37.19% of the principal. Cumulative interest passes half at period 118, while the balance does not fall to half until period 226 - a gap of 108 periods, or nine full years. Equal principal front-loads interest too (74.93% of interest paid and exactly half the principal repaid by year 15); the mismatch is simply smaller, because its denominator is smaller.

That mismatch is where the timing effect comes from. The table below converts it into a directly usable number: how much future interest CNY 10,000 of prepayment removes, recomputed with the term unchanged and the payment reduced. If a lender shortens the term instead, the saving is larger; the lender's own recalculation governs.

The same CNY 10,000 saves CNY 5,932.52 in year one and only CNY 915.05 in year 25 - a factor of 6.5, and the decay accelerates late in the term because each yuan has fewer periods left to work. The order of the decision therefore matters: first ask whether the money has a better use right now (section 3), then ask whether waiting will make it worth more later.

One related question is usually answered the wrong way: if I plan to prepay, should I choose equal installment? At any given moment the equal-installment balance is higher (CNY 896,970.73 against CNY 833,333.33 after 60 payments), so a prepayment made then removes more future interest. That is not evidence that equal installment suits prepayment better; it is the same fact seen from the other side, because the higher balance is exactly why its total interest is higher. What determines your saving is always when you prepay, not which method you picked.

5. Combined Loans: Two Contracts, Two Choices

When the housing-fund limit falls short, borrowers commonly combine a housing-fund loan with a commercial loan. These are two separate contracts, so they can usually carry different repayment methods, which creates a third option: put the smaller, shorter portion on equal principal to drive its balance down quickly, and keep the larger portion on equal installment to pin the payment. Total interest comes out below the all-equal-installment version, and the first-month cash requirement does not jump the way it does when both portions use equal principal.

Two cautions. Whether the two contracts may differ, and whether you can prepay only one of them, depends on the local housing-fund centre and the lender's terms; some cities require the methods to match. And a combined-loan prepayment often has to be split proportionally across both portions, so ask about the minimum amount and the order of processing before you act. Estimate the available limit first in the housing fund calculator, then decide the split.

6. Frequently Asked Questions

Exactly how much interest separates the two methods on a CNY 1,000,000 loan over 30 years?

At an assumed 3.5% annual rate, equal installment costs CNY 616,560.88 in interest and equal principal costs CNY 526,458.33 — a gap of CNY 90,102.54, bought with a first payment of CNY 5,694.44 instead of CNY 4,490.45, or CNY 1,204.00 more per month. The gap is about 9% of the principal, and it is not a discount: it is paid for with early cash flow.

If I plan to prepay, should I simply choose equal principal?

Not necessarily. Timing matters far more than the method. With the term kept unchanged, every CNY 10,000 prepaid saves CNY 5,932.52 of interest in year one but only CNY 915.05 in year 25 — a factor of 6.5. At any single point in time the equal-installment balance is higher, so a prepayment against it removes more future interest, but only because it owes more to begin with. Decide when you will prepay first, then pick the method.

Is equal installment a trick that makes the bank more interest?

No. The rate is identical under both methods; the interest gap comes only from how long the principal stays outstanding. Adding up the opening balance across all 360 periods gives 211,392,300 yuan-months under equal installment against 180,500,000 under equal principal, a ratio of 1.1711 — exactly the same as the ratio of their total interest, 1.1711. What equal installment buys with the extra interest is liquidity: less cash out of your pocket in the early years.

What if my money can reliably beat the loan rate?

Then choose equal installment and actually invest the difference. Compounding the payment difference at a single assumed return to period 360 shows the break-even return is exactly the loan rate, 3.5%: below it equal principal wins (CNY 20,218.71 ahead at an assumed 3%), above it equal installment wins (CNY 83,756.69 ahead at an assumed 5%, CNY 274,437.95 at an assumed 7%). The return must be measured after tax and after costs, and it ignores investment volatility.

Can a combined loan mix the two methods?

The housing-fund loan and the commercial loan are separate contracts, so they can usually carry different repayment methods — a common arrangement is equal principal on the housing-fund portion and equal installment on the commercial one. Whether a lender allows this, and whether you may prepay only one of the two, varies by city and contract, so confirm with the local housing-fund centre and the lender.

7. Three Common Misconceptions

Misconception 1: equal principal is always the cheaper choice. Treating total interest as the only cost hides what it is traded for. The CNY 90,102.54 saved is bought with CNY 1,204.00 more in the first month and higher payments for several years after that. If the difference can reliably earn more than the loan rate, equal installment wins instead - CNY 83,756.69 ahead at an assumed 5%. The question is never how much interest was avoided, but what that money could have done.

Misconception 2: prepaying later is fine, since it is still prepaying. Timing is badly underestimated: the same CNY 10,000 saves CNY 5,932.52 in year one and CNY 915.05 in year 25. A related illusion is that equal principal deserves earlier prepayment because its interest share looks higher - after five years it has paid 30.52% of its own total interest against 26.99% for equal installment, but that is an artefact of a smaller denominator, and both methods obey the same rule that earlier is better.

Misconception 3: because the equal-principal payment declines, it suits people whose income will fall. The slope is gentle: the payment drops by just CNY 8.10 per period, so after five years it is still CNY 5,216.44 and after ten years CNY 4,730.32. What needs testing is the first few years, not what the payment eventually falls to. If income really is expected to decline - approaching retirement, for instance - shortening the term is a more effective response than counting on that gentle slope.

8. Three Practical Steps

• Set the payment ceiling first, then pick the method. Put the CNY 5,694.44 first payment back into your household income and test it. If it exceeds what you can carry, equal principal is out before any interest comparison. Run both schedules through the mortgage calculator and compare them side by side.
• Put prepayment timing in the plan, not just the amount. Using the coefficients in section 4, the same CNY 10,000 is worth 6.5 times more in year one than in year 25. Before committing, check three limits in the contract: the annual cap on prepayment amount and frequency, any minimum period after drawdown, and whether a compensation charge applies as a percentage of the amount prepaid.
• Compare repaying and investing on the same basis. Do not weigh the total interest saved against a single year of investment return. Carry both to the same end date: compound the difference at your own realistic return and set it against the loan rate. For the compounding mechanics, the compound interest calculator does the arithmetic.

Seen against the whole household balance sheet, the repayment method is only a switch on the liability side. To keep going, read the side-by-side comparison of loan plans and rates to see how a different rate or term rewrites these conclusions, the housing fund limits and city comparison to plan a combined loan, and compound versus simple interest - the interest a prepayment avoids is that same compounding logic running in reverse.