What is the flat mortgage formula?

Installment = (Principal / tenor) + (Principal × monthly rate). Example: IDR 500M, 0.8%/month, 180 months → (500M/180) + (500M×0.8%) = IDR 2.78M + IDR 4M = IDR 6.78M/month.

Flat vs effective difference?

Flat: interest on initial principal (fixed installment). Effective/annuity: interest on remaining balance (starts high, then drops). Akad Credit uses flat for transparency — what you simulate is what you pay.

Minimum mortgage down payment?

10% for first home, 15–20% for second home. Refinancing can top up without extra DP if the certificate is already in your name.

Mortgage · 6 min read · 2026-08-30

How to Calculate Mortgage Installments Correctly (Flat vs Effective)

By Akad Credit — OJK-licensed financing platform. Updated 2026-08-30.

Summary

Flat mortgage = (Principal ÷ tenor) + (Principal × monthly rate). Example: IDR 500 million, 0.8% per month, 180 months (15 years) → IDR 6.78M/month fixed until payoff. Flat is transparent: your simulation equals what you pay.

Flat vs effective — what's the difference?

Flat: interest always on the initial principal, so the installment stays fixed. Effective/annuity: interest on the remaining balance — interest is high at first, then shrinks. Flat is easier to predict; effective may cost less in total interest for long tenors.

Mortgage simulation (flat 0.8%/month)

Principal	Tenor	Monthly	Total paid
IDR 300M	120 mo	IDR 4.90M	IDR 588M
IDR 500M	180 mo	IDR 6.78M	IDR 1.22B
IDR 750M	240 mo	IDR 9.13M	IDR 2.19B

Formula (manual)

Installment = Principal/tenor + Principal × monthly_rate. Total paid = installment × tenor. Total interest = total paid − principal. For effective annuity: PMT = P×r×(1+r)^n / ((1+r)^n −1).

Tips to keep installments light

Keep installments below 35% of income.
Match tenor to your age (max 65 at payoff).
Budget 1% provision + life insurance.

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FAQ

Flat mortgage formula?

Installment = (Principal/tenor) + (Principal × monthly rate). Example 500M/180 + 500M×0.8% = IDR 6.78M/month.

Flat vs effective?

Flat from initial principal (fixed). Effective from remaining balance (declining). Akad Credit uses flat for transparency.

Minimum down payment?

10% for first home, 15–20% for second. Refinancing can skip extra DP.