Equal Principal vs Equal Instalment: The Real Difference

Two repayment structures, same loan amount, same rate, same term — and a total interest difference that can reach 20% of the principal.

The two structures

Equal instalment (annuity) — the payment is constant. Early payments are mostly interest; principal repayment accelerates over time.

Equal principal — the principal repaid each month is constant (P / n), interest is charged on the declining balance, so the payment starts high and decreases every month.

	Equal instalment	Equal principal
Monthly payment	Flat	Starts high, declines
Total interest	Higher	Lower
Early-stage cash pressure	Lower	Higher
Predictability	High	Payment changes monthly

Where the difference comes from

Interest is charged on the outstanding balance every period. Anything that reduces the balance faster reduces total interest.

Equal principal reduces the balance by a fixed amount from month one. Equal instalment reduces it slowly at first, so the average balance — and therefore the interest — is higher.

For a 1,000,000 loan at 4.5% over 30 years:

	Equal instalment	Equal principal
First payment	~5,067	~6,528
Last payment	~5,067	~2,788
Total interest	~824,000	~677,000

Equal principal saves roughly 147,000 in interest, and costs about 1,460 more per month for the first several years.

The correct comparison is not "total interest"

Total interest is the wrong metric on its own. The money you do not pay early stays in your pocket and can earn something.

The right frame: the extra payment is an investment with a guaranteed return equal to the loan rate. Paying 1,460 extra per month to save 147,000 is equivalent to earning 4.5% tax-free, risk-free, for 30 years.

That is a good deal if your alternative is a savings account, and a bad deal if your alternative is a diversified portfolio earning more. Neither answer is universal.

Overpaying: shorten the term or reduce the payment?

Most lenders offer both, and the difference is large:

Option	Effect	Interest saved
Shorten the term, payment unchanged	Loan ends earlier	More
Reduce the payment, term unchanged	Same end date, lower outflow	Less

Shortening keeps the balance falling at the accelerated rate for the whole remaining term, which is why it saves more. Reducing the payment keeps the original schedule and just shaves the balance once.

The trade-off is flexibility: shortening commits you to the higher payment. If cash flow might tighten, reducing the payment is safer even though it saves less.

Fixed vs floating

If the rate is floating, the payment recalculates at each repricing date. Two things people get wrong:

1. A rate change does not apply immediately. It takes effect on the repricing date in the contract (often 1 January or the anniversary), not the day the reference rate moved.
2. The remaining term usually stays fixed, so the payment adjusts rather than the schedule stretching.

What actually matters over 30 years

1. The rate — 1 percentage point on a 30-year loan costs more than the equal-principal/equal-instalment choice.
2. The term — 30 years vs 20 years changes total interest by far more than the repayment structure.
3. The structure — a real but second-order effect, mostly a cash-flow decision.
4. Overpayment timing — early overpayments save more, because more interest is still ahead.

Loan Calculator computes both structures and prints the full schedule, so you can see exactly which month the crossover happens. Mortgage Calculator adds the early-repayment model: enter an amount and a month, and it shows the interest saved and the shortened term.

A note on rounding

Two calculators can disagree by a few units because lenders accrue interest daily (rate / 365 × days) rather than monthly, and the first period is often a partial month. Differences under about 10 units are almost always this, not a bug.

If the difference is larger, check: is the quoted rate the effective annual rate or a nominal one? Are fees rolled into the principal? Is the term in months or years?

Summary

Equal principal trades early cash flow for less interest. Equal instalment trades interest for predictability. Neither is cheaper in an absolute sense — the extra payment is an investment at the loan rate. Choose based on whether you can deploy that money better elsewhere, and whether the higher payment is safe if income drops.