Interest Calculator Guide
Calculate deposit or loan interest after tax, with flat, effective, annuity, compound, tiered, step-up and floating methods.
The same annual rate can produce different returns depending on the interest method and day-count basis. Interest Calculator puts the formula beside the result for simple, compound, tiered, step-up, floating, flat, effective and annuity calculations. Enter a principal, a rate and a tenor and the page shows the formula it used, the result as gross, tax and net where tax applies, and a period-by-period breakdown. The product presets follow the tax and day-count conventions commonly used in Indonesia and are marked ID; everything is computed in the browser.
Work out a one-month deposit
The page opens on Bank deposit (deposito) — ID with Simple interest (flat) — ID, a principal of 10000000, a rate of 8% a year, a tenor of 1 month, Tax on interest 20% and an Actual/365 basis. Result fills in as you type: Gross interest Rp 65,753, Tax 20% − Rp 13,151, Net interest Rp 52,603, Ending balance Rp 10,052,603, plus net per day, per month and per year. How the method works prints the formula the page used, so you can check it by hand: 10,000,000 × 0.08 × (30 ÷ 365).
A tenor in months counts as 30 days each, and Day-count basis decides whether a year is 365 or 360 days — which is why one month is not exactly one twelfth of the year. Switch Tenor to 12 months and the gross comes to about Rp 789,041; switch the basis to 30/360 and it rises, because every month is now exactly one twelfth.
Change Interest method to Compound interest and two controls appear: Compounding frequency (monthly by default) and Roll over interest (ARO). With ARO ticked each period’s net interest is added to the balance so the next period earns interest on it, and Gross EAY / Net EAY show the annualised effect. Change Product type to Government bond (SBN) — ID and the preset moves the tax to 10% and the tenor to one year; Bank deposit brings it back to 20%. Tiered interest opens a Balance brackets box (upper_limit rate per line, * for the top bracket), while Step-up and Floating open a rate-per-period box; the flat, effective and annuity loan methods replace the deposit rows with a monthly Instalment schedule.
The tax defaults (deposit 20%, bond 10%, money market 0%) and the Actual/365 basis are Indonesian conventions marked ID; the formulas are the same everywhere. Copy result takes the summary as plain text.
Where your input goes
Processing runs in your browser. The tool does not upload your input to ilham.dev.
Related guide
For more background, read Calculators and Unit Conversion.
Questions you might have
- What is the formula for simple interest?
- I = P x r x (days / basis), where P is the principal, r the rate per year as a decimal, and basis the day count (365 or 360). The interest is paid once and never earns interest itself, which is how a deposit that pays at maturity works.
- What is the formula for compound interest?
- A = P x (1 + r / n)^(n x t), where n is the number of compounding periods per year and t the tenor in years. Each period’s interest is added to the balance so the next period earns interest on it. When tax is withheld at each payment, the amount that compounds is the net interest.
- How are tiered, step-up and floating interest calculated?
- All three add up per-slice amounts instead of applying one rate to the whole balance. Tiered splits the balance into brackets and computes slice x rate x (days / basis) for each. Step-up and floating apply a list of rates per period and add P x r_i x span_i. The only difference is that step-up rises on a schedule while floating can move either way.
- What is the difference between flat, effective and annuity?
- Flat charges interest on the original principal for the whole term: I = P x r x (months / 12), split evenly across the instalments, so the true cost is higher than the headline rate. Effective charges interest on the remaining balance each month: interest = remaining x r / 12, so the interest share falls over time. Annuity keeps the total instalment fixed with A = P x i / (1 - (1 + i)^-n), where i = r / 12 and n is the number of months, while the split shifts from interest to principal.
- Why does one month not equal one twelfth of the yearly rate?
- Because the day-count basis decides how a month becomes a fraction of a year. Actual/365 counts a month as 30/365, a little less than a twelfth; 30/360 treats every month as exactly 1/12, a little more. The page lets you switch basis so the choice is visible instead of hidden.
- Which parts are specific to Indonesia?
- The product presets and the ID badges: the final tax on deposit and bond interest, the Actual/365 basis, the ARO rollover, and the loan structures quoted as flat, effective or annuity. The formulas themselves are the same everywhere; only the defaults and the naming are local.