The spreadsheet has 5 sheets: (1) "CompInt": computes interest (with no extra deposit/payments). Here is the notation: * P: principal (initial amount) * r: rate (usually APR) over some period (which is a year in the case of APR). Remember that if it is percentage, you need to divide by 100. (So, if the rate is 12.8%, you enter 0.128 for r.) * n: number of times the interest is compounded in the period of the rate. (Usually 365 fro APR, in other words, the interest is compounded daily. If it were monthly, n would be 12, as we have 12 months in a year.) * t: number of periods (of the rate). * F (RESULT!): final value, in other words, amount of money with the interest with the given data. For example, the first example (row 2) of this sheet shows that if you have/owe \$100 (P=100), and have an APR of 1.09% (r=0.0109), compounded daily (n=365), and wait 30 years (t=30), you will have \$138.68 (F is computed for you automatically). (2) "APY": this sheet converts APR to APY and vice-versa. In this case, n should be left as 365. Enter the APR in column C and you get APY in column D. Enter APY in column F and you get APR in column G. (So, from the first example (row 2) shows that APR of 19.8% is the same as APY of 21.88... and APY of 1.10% is the same as APR of 1.094....) Columns I/J and L/M are similar, but they use the percentages already divided by 100, which is more useful in computations. (3) "Paym": this sheet checks how much you will owe/have if you make payment/deposits regularly. Here is the notation: * P: principal (initial amount) you have or owe. * r: rate (usually APR) over some period (which is a year in the case of APR). Remember that if it is percentage, you need to divide by 100. (So, if the rate is 12.8%, you enter 0.128 for r.) * n: number of times the interest is compounded in the period of the rate. (Usually 365 fro APR, in other words, the interest is compounded daily. If it were monthly, n would be 12, as we have 12 months in a year.) * d: how many times interest is paid between payments. (If interest is paid daily, as usual, this means how many days. If you pay every first of the month, the number of days change according to how many days we have in a month. You can average that by 365/12=30.42...) * A: amount paid/deposited (in the regular intervals of "after every d payments of interest"). If you are paying off, use NEGATIVE number (as it will subtract from what you owe (P)). * t: number of payments made. * Aux (automatically computed): don't touch this. This is just an auxiliary computation. * F (RESULT!): final value, in other words, what you have/owe after t deposits/payments. * Int. (RESULT!): How much interest was paid/charged in the period. For example, the first example (row 2) shows that if I have \$100 (P=100) in a savings account with APR of 1.09% (r=0.0109) compounded daily (n=365), and make regular deposits every month (d=365/12=30.42...) of \$100 (A=100) for 30 years (so, t=30*12=360 payments), I will have \$42,702.88 (the result for F), of which \$6,602.88 was the interest paid to me (the result of Int.). The second example (row 3) shows that if I owe \$1,000 (P=1000) and am charged an APR of 12.9% (r=0.129) compounded daily (n=365) and make regular payments every month (d=365/12=30.42...) of \$20 (A=-20 -- NOTE THE NEGATIVE, as I am subtracting from what I owe) for 3 years (so, t=3*12=36 payments), I will still owe \$598.03 (the result for F), and will have been charged (so far) \$318.03 in interest (the result of Int.). (4) "deposit": This sheet tells you how much to deposit to obtain a certain amount (for instance, if you want to buy something for which you know the price) or to pay off some debt after a fixed period of time. NOTE: this assumes you do not make a payment in the first month. (The difference would be very small if you do.) The notation is the same as (3) above, but now you know what is the final value (how much you want to get) and how long you want it to take, and the spreadsheet tells you how much you need to deposit. For example, the first example (row 2) tells you that if you start with nothing (P=0), have an APR of 3.5% (r=0.035) compounded daily (n=365), making payments every month (n=365/12=30.42...), and wants to get one million dollars (F=1000000) after 50 years (t=50*12=600 payments), you need to deposit \$614.37 a month (result of A). (Again, Aux and Aux2 should not be changed, as they are used to help the computation.) The second example is the same as the first, except you start with \$10,000 (P=10000), and in this case you need to deposit \$578.94 a month. The third example is a pay off one. In this case you owe \$5,000 (P=5000), the APR is 19.8% (r=0.198) compounded daily (n=365) and you want to make monthly payments (d=365/12=30.42...) to pay it off in a year, in other words, make 12 payments (t=12). Since you want it paid off, the final amount is 0 (F=0). In this case the spreadsheet tells you that you need to make payments of \$463.07 (A=-423.82, and the negative sigh is because you want to subtract that value from what you owe). (5) "time": this sheet computes how long it will take to either pay off or save a specific amount with a specific regular payment. The notation is the same as (3) and (4), but now you know both F (how much you want to have or owe) and A (what you want to pay every month). The first example (row 2) shows that if you start with \$200 (P=200) in a saving account with APR of 3.5% (r=0.035) compounded daily (n=365) and you want to make monthly deposits (d=365/12=30.42...) of \$200 (A=200), you will get \$10,000 (F=10000) after 46 payments (round up the result t=45.73...). The last column just divide the previous number by 12, saying that it would take 3.81 years. The second example (row 3) shows that if I owe \$5,000 (P=5000) and am paying an APR of 19.8% (r=0.198) compounded daily (n=365) and want to make monthly payments (d=365/12=30.42...) of \$100 (A=-100, negative as I want to subtract it from what I owe), you pay off the debt (F=0) after 108 payments (round up the result t=107.99...). The last column just divide the previous number by 12, saying that it would take 9 years.