The interest on loans is calculated on a reducing balance method
lets say you took 100000 loan for a 1 year period @ 12% interest. 12% annual interest means monthly interest rate comes to 1%.
so every month you pay 1% simple interest on principal remaining at that time.
First month you pay 1,00,000 x 1% = 1000 rupees as interest and some amount as principal.
Second month, you pay 1% on the remaining principal and so on.
The EMI (Equated Monthly Installment) is calculated such that that each month you pay a fixed amount which contains both a fixed principal as well as the 1% monthly interest on the balance amount so that the the remaining principal becomes nil at the end of the loan duration.
Basically, it boils down to a sum of series whose simplification leads to below formula for EMI.
For the example taken above, the EMI comes to 8885/-
First month :: Interest (1% of 100000) = 1000 , Principal Repaid (8885 - 1000) = 7885, Balance (100000 - 7885) = 92115
Second Month :: Interest (1% of 92115) = 921, Principal Repaid = (8885 - 921) = 7964, Balance (92115 - 7964) = 84151
and so on.
When you make Fixed deposits, the bank gives you compound interest which is compounded quarterly. If we take the same example as above, quarterly interest rate is 3%.
First quarter:: Interest (3% of 100000) = 3000, Balance (100000 + 3000) = 103000
Second quarter:: Interest (3% of 103000) = 3090, Balance (103000 + 3090) = 106090
Third quarter :: Interest (3% of 106090) = 3183, Balance (106090 + 3183) = 109273
Forth quarter :: Interest (3% of 109273) = 3278, Balance (109273 + 3278) = 112551
So in comparison, if you take a loan, you pay 6619 as interest to bank because of the reducing principal every month, but if you put an FD in bank for 1 year, they pay you 12,551 as interest.[DOUBLEPOST=1514540623][/DOUBLEPOST]Just want to add that FD interest rates are always much lower than loan interest rates. For example, I pay 8.65% interest on my home loan while I get 6.75% (or lower) interest on FD's. Personal loans have much higher interest rates ranging from 11 to 17%.
Also, the interest earned from FD is deemed to be income and taxable based on your income bracket.