In FBAR, there is a checkbox (Line 15a) that allows you to check "Maximum account value unknown" and not report the exact value of your account. I don't know my account's maximum value for the entire year (the bank does not keep statements for that long, and I no longer have them) but can make a rough guess based on my memory (like, it's within +/- $5000 precision).
Is it better to check the box, or fill it with the rough estimate? Is there a drawback from checking this box?
The answer above is not correct. I just called the IRS FBAR and Title 31 Helpline (contact information).
I was told it is very simple. Either you know the maximum value, or you don't. If you don't know it then check the box that says maximum value unknown. There is no negative consequence associated with that box. If you do know then put the value in. DO NOT TRY TO GUESS the value! Either you know it or you don't.
From the IRS guidelines here: https://www.irs.gov/irm/part4/irm_04-026-016.html
If a person has one or more but fewer than 25 reportable accounts and is unable to determine whether the maximum value of these accounts exceeded $10,000 at any time during the calendar year, the FBAR instructions state that the person is to complete the applicable parts of the FBAR for each of these accounts and enter "value unknown" in Item 15.
EDIT: I found another link here which has more descriptive information:
Monetary amounts: When recording the maximum value of accounts, record all amounts as U.S. Dollar amounts rounded up to the next whole dollar. The amount $15,265.25 would be recorded as $15,266. The maximum value of the account can be determined using the following steps.
NOTE: After determining the value of the account, as described below, if the value results in a negative (minus) value, enter zero (0) in item 15, “Maximum account value.”
Step 1. Determine the maximum value of each account (in the currency of that account) during the calendar year being reported. The maximum value of an account is a reasonable approximation of the greatest value of currency or nonmonetary assets in the account during the calendar year. Periodic account statements may be relied on to determine the maximum value of the account, provided that the statements fairly reflect the maximum account value during the calendar year. For Item 15, if the filer had a financial interest in more than one account, each account must be valued separately. For an account denominated in U.S. Dollars, the maximum value of the account is the largest U.S. Dollar value of the account during the report year.
Step 2. In the case of non-United States currency, convert the maximum account value for each account into United States dollars. Convert foreign currency by using the Treasury's Financial Management Service rate (http://www.fiscal.treasury.gov/fsreports/rpt/treasRptRateExch/treasRptRateExch_home.htm) for the last day of the calendar year. If no Treasury Financial Management Service rate is available, use another verifiable exchange rate and provide the source of that rate. In valuing currency of a country that uses multiple exchange rates, use the rate that would apply if the currency in the account were converted into United States dollars on the last day of the calendar year.
If the maximum account value of a single account or aggregate of the maximum account values of multiple accounts exceeds $10,000, an FBAR must be filed. An FBAR is not required to be filed if the person did not have $10,000 of maximum value or aggregate maximum value in foreign financial accounts at any time during the calendar year.
For United States persons with a financial interest in or signature authority over fewer than 25 accounts that are unable to determine if the aggregate maximum account values of the accounts exceeded $10,000 at any time during the calendar year, complete the appropriate Part II, III, IV, or V section for each of these accounts and check the “amount unknown” box, item 15a.
No, it is not, and "bank doesn't keep statements" is not an excuse. It is your responsibility to keep the statements, not the bank's.
This check box is for the cases when you know there's an account in your name, but you don't know what the value is and cannot reasonably find out. For example, you may have signature authority on a company account, but you do not have the access to the account statements.
According to the instructions:
15a. Amount unknown. Check this box if the value of the account cannot be determined.
If you have access to the account statements - you can most definitely determine the value of the account.
