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.