Deposit 1000 yuan, interest due in three months:1000x1.8%/4 = 4.5 yuan (before tax, interest tax is 20%, after tax 3.6 yuan); /kloc-0 can be deposited four times a year. If compound interest is not calculated, the interest of 1 year is 18 yuan (before tax); After calculating compound interest (that is, the actual interest after tax is deposited as the principal), you can get the actual interest of 1 year as follows:

3 months later: the sum of after-tax interest and 3.6 yuan's principal and interest: 1003.6 yuan.

After 6 months: after-tax interest is 3.6 1 yuan, total principal and interest: 1007.5438+0 yuan.

After 9 months: the after-tax interest is 3.62 yuan, and the total principal and interest is 10 10.83 yuan.

After 12 months, the after-tax interest is 3.64 yuan, and the sum of principal and interest is 10 14.45 yuan.

The actual accumulated after-tax interest is 14.45 yuan (compound interest, simple interest is 14.4 yuan).

First, the basic knowledge of interest calculation

(1) The interest rate conversion formula for RMB business is (note: common for deposits and loans):

1. daily interest rate (0/000)= annual interest rate (%)÷360= monthly interest rate (‰)÷30.

2. Monthly interest rate (‰) = annual interest rate (%)÷ 12

(two) banks can use the product interest method and the transaction interest method to calculate interest:

1. Accumulate the account balance daily according to the actual number of days, and multiply the accumulated product by the daily interest rate to calculate the interest. The interest-bearing formula is:

Interest = cumulative interest-bearing product × daily interest rate, where cumulative interest-bearing product = total daily balance.

2. Transaction-by-transaction interest calculation method calculates interest one by one according to the preset interest calculation formula: interest = principal × interest rate × loan term, with three details:

If the interest-bearing period is a whole year (month), the interest-bearing formula is:

① Interest = principal × year (month )× year (month) interest rate

If the interest-bearing period is a whole year (month) and days, the interest-bearing formula is:

② Interest = principal × annual (monthly) × annual (monthly) interest rate+principal × odd days × daily interest rate.

At the same time, banks can choose to convert all interest-bearing periods into actual days to calculate interest, that is, 365 days per year (366 days in leap years), and each month is the actual number of days in the Gregorian calendar of the current month. The interest-bearing formula is as follows:

③ Interest = principal × actual days × daily interest rate

These three formulas are essentially the same, but because the interest rate conversion is only 360 days a year, when calculating the actual daily interest rate, it will be calculated as 365 days a year, and the result will be slightly biased. Which formula is used specifically, the central bank gives financial institutions the right to choose independently. Therefore, the parties and financial institutions can agree on this in the contract.

(3) Compound interest: Compound interest means adding interest at a certain interest rate. According to the regulations of the central bank, if the borrower fails to repay the interest at the time agreed in the contract, it will be charged with compound interest.

(4) Penalty interest: If the lender fails to repay the bank loan within the prescribed time limit, the penalty interest paid by the bank to the defaulter according to the contract signed with the parties is called bank penalty interest.

(V) loans overdue liquidated damages: penalties for the defaulting party with the same nature as penalty interest.