When you take out a mortgage or car loan, the bank will usually ask you to choose between two repayment methods: equal installments (where your monthly payment stays the same) or equal principal (where the principal portion stays the same). A lot of people pick one without really understanding the difference, only to discover later that the total interest paid — and the monthly pressure along the way — can differ quite a bit. This post breaks down exactly how each method is calculated, so you can figure out which one actually fits your situation.
As the name suggests, "equal installments" spreads the total interest evenly across every payment period, so the amount you pay each month (principal plus interest combined) never changes. But that doesn't mean the principal and interest portions stay fixed individually — early on, most of your payment goes toward interest and only a small slice toward principal; as time goes on, the principal share gradually increases while the interest share shrinks. That's because interest is calculated on the outstanding principal balance — early in the loan you still owe the bank a lot, so naturally more interest accrues.
"Equal principal" works the opposite way — it fixes the amount of principal you repay each period (say, on a 30-year loan, the total principal is split evenly across 360 months, and you pay that same chunk every month), while the interest is calculated fresh each month based on whatever principal remains, so it keeps shrinking. Because you're paying down principal faster early on, the remaining balance drops more quickly, which means interest drops faster too. The result: your monthly payment isn't fixed at all — it starts high and tapers off over time, following a steadily declining curve.
Because equal principal pays down the loan balance faster, the remaining principal shrinks more quickly, which naturally generates less interest overall. So, given the exact same loan amount, term, and interest rate, equal principal will typically end up costing you less in total interest than equal installments — and that gap tends to widen the longer the loan term is.
It comes down to the pressure of early monthly payments. With equal principal, your payments in the early months are noticeably higher than with equal installments (since you're paying down more principal upfront) — which can be a real strain if you've just taken on a mortgage and don't have a lot of cash flow to spare. Equal installments may cost more in total interest, but the fixed monthly payment makes long-term budgeting much easier, and the burden is spread out more evenly — which is exactly why most homebuyers end up going with it.
If your income is stable but what matters most to you is knowing exactly what you'll owe each month and being able to plan around it, equal installments is the better fit. If you have more cash on hand, can handle higher payments early on, and care more about minimizing total interest paid, equal principal will save you money in the long run. One more thing worth noting: if you're planning to pay off the loan early within the next few years (say, once you've saved enough after a few years of work), equal principal usually saves you more interest when you do pay early, since the principal balance has already been reduced faster.
I ran one loan through the same formulas the mortgage calculator uses: principal 1,000,000 yuan, 30 years (360 payments), annual rate 3.95% (monthly rate 3.95% divided by 12). With equal installments you pay a fixed 4,745.37 yuan every month, so the first and last months are identical, and total interest is 708,334.05 yuan. With equal principal you repay a fixed 2,777.78 yuan of principal each month plus interest on the remaining balance, so the first month costs 6,069.44 yuan, the last month only 2,786.92 yuan, and total interest is 594,145.83 yuan. Side by side, equal principal saves 114,188.22 yuan of interest, at the price of paying 1,324.07 yuan more in the first month (6,069.44 minus 4,745.37). The loan calculator uses the same formulas for both methods, so it gives the same results. Note that 3.95% is only the tool's example rate, not any bank's current rate; replace it with the figure on your own contract.
Reading through the logic is one thing, but it's worth plugging in your actual loan amount, term, and interest rate to see the real numbers. A mortgage calculator will let you directly compare the monthly payments and total interest for both methods — and if you're dealing with a car loan or personal loan instead, a loan calculator works just as well. Once the numbers are right in front of you, the choice becomes a lot clearer.