Utilization is calculated from what a card issuer reports, and issuers report a snapshot rather than an average. The day that snapshot is taken decides the figure entirely.
Reporting is a monthly snapshot
Issuers send balance information to credit bureaus on a schedule, commonly at the close of the statement cycle. The reported balance is the balance on that day.
Activity before and after the snapshot is invisible to the calculation. A balance run up and paid down within a cycle may never be reported at all if the timing falls right.
Conversely a single large purchase sitting on the card on the closing date is reported in full, even if it is paid two days later.
Paying in full does not mean reporting zero
A cardholder who pays the statement balance by the due date pays after the statement closed, which means after the balance was already reported.
The credit file therefore shows the statement balance, not the zero that the account reached shortly afterward. This is why disciplined payers see utilization they did not expect.
Paying before the statement closing date rather than by the due date changes what gets reported, though it does not change the interest position for someone already paying in full.
The ratio has two components
Utilization compares reported balances to credit limits, and it is generally assessed both per card and across all revolving accounts together.
A single card carrying most of a household's balance can show high utilization even when the total across all cards is modest, and both readings appear on the file.
Limit changes therefore move the ratio without any change in spending, which is one reason closing an unused card affects the figure at all.
It carries no history
Unlike payment history, utilization is generally assessed on current reported figures rather than accumulated over time. Last year's high balance is not part of today's ratio.
That makes it one of the fastest-moving inputs on a file, changing with the next reporting cycle after a balance changes.
It also makes it volatile, since a single unusual month, such as a large purchase made for reimbursement, moves the figure as much as a genuine change in borrowing would.
Timing is the part a cardholder controls
The statement closing date is printed on every statement and is generally fixed, while the reporting date follows it closely.
Knowing those dates is what allows a cardholder to understand why a file shows what it shows, rather than inferring behavior from the number.
Anyone reviewing their own file will find the reported balances dated, which makes it straightforward to match each entry to the cycle that produced it.