With interest rates going a sharp downward path, High Yielding Online Savings Accounts and Rewards Checking Accounts are getting very much attention these days. Both these accounts (and several other bank products) pay us interest based on the Average Daily Available Balance (ADAB) on our accounts.
I had been playing with my online savings statements to find out how financial institutions calculate this number and finally I could figure it out yesterday.
Every bank pays you interest at the end of a term. For normal savings accounts this term is typically 1 month, making them to pay interest at the end of every 30 or 31 days period. It is common sense that the more money we have in our accounts the more interest we will get but this is not it.
Banks don't consider our account balance on the last working day of a month for calculating interest, rather what matters to them is how much money was available to them throughout a month. Banks use the term Average Daily Available Balance (ADAB) to mean this, as is clear from the term itself, this number reflects how much money we made available to the bank to be used for their business purposes.
 Formula To Calculate Interest
Amount = Principal (1+r)^{n}
where, Amount is the sum (with interest) you get after n years at r% per year. For eg. if you had $100 in your account for 1 year at 3% per year then
P = $100, r = 3/100 = 0.03, n = 1
This formula will give you Amount you will have after completion of 1 year. When banks calculate interest they replace Principal (P) with the Average Daily Available Balance (ADAB) so the formula becomes:
Amount = ADAB (1+r)^{n}
 Calculating Average Daily Available Balance
I will work with a simple example here. Consider a bank statement below:
Date
 Withdrawl
 Deposit
 Balance

07/01/09

 $100
 $100

07/08/09
 $50

 $50

07/15/09

 $50
 $100

07/22/09

 $100
 $200

07/29/09

 $100
 $300

Assuming there was no transaction after 29^{th}, it would be unfair to the bank if they had to consider $300 as the Principal for calculating interest. And that is true as $300 were not always present in the account. Let's now calculate Average Daily Available Balance for the above account.
The technique is to calculate contribution of an individual account balance towards the months average balance, sum all such contributed balances and divide the sum by number of days in that month. To do so create a table as below:
Account Balance
 Days before next transaction
 Contributed Balance

$100
 (07/01 to 07/07 =) 7
 $700

$50
 (07/08 to 07/14 = ) 7
 $350

$100
 (07/15 to 07/21 = ) 7  $700

$200
 (07/22 to 07/28 = ) 7  $1400

$300
 (07/29 to 07/31 = ) 3  $900

Total

 $4050

July '09, the month under consideration has 31 days. This gives the Average Daily Available Balance (ADAB) equal to:
Average Daily Available Balance = $4050/31 = $130.65
You might be wondering whether this is correct given the account balance at the end was $300. Well it turns out that owing to the transactions made as shown above, the account balance never exceeded $100 for 21 days (07/01/09 through 07/21/09) resulting in a lower average balance.
 Factors Affecting Your Interest
As we saw above there are three main factors affecting the interest we may get on our account balances:
 Average Daily Available Balance
 Rate of interest per year (out of our control)
 Time in years
Factor #2 and #3 are usually out of our control as we can't actually decide the rate and considering per month calculations the time is going to be 1/12 ~ 0.08 years.
Average balance, however, can be controlled and the only way to do that is to leave maximum you can in your account and leave it there for the longest possible.
 Some Ways To Maximize Your Average Daily Balance
 Get your paycheck directdeposited into your savings account, why carry check when it gains you nothing
 Don't withdraw money until it is really needed
 Use automatic money transfers between accounts. My car loan's monthly due date is 2^{nd} of every month. The amount is automatically transferred on the 1^{st} and till then it keeps gaining interest for me.
 Set aside some extra money if you think you are a addict of intermittent withdrawls.
What ways you implement to maximize your interest earnings? Leave a comment and share your views with other readers.
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