Step-by-Step Breakdown of the Components
It isn't a bonus from your employer. It is a result of the calendar.
A biweekly schedule pays every 14 days, which normally creates 26 pay periods in a year. A month is not exactly four weeks long, so the pay dates move through the calendar. In a typical year, two months contain three biweekly pay dates.
That creates a useful budgeting opportunity, but it is important not to treat those checks as extra annual salary.
Taxes, Deductions, and Adjustments
Suppose your biweekly gross pay is $2,500. Over 26 pay periods, annual gross pay is $65,000. If two months contain three paychecks, those months have $7,500 of gross pay instead of the more common $5,000. The annual total is still $65,000.
The same principle applies to net pay. Your third paycheck can be valuable, but the amount reaching your bank account depends on the same withholding and deduction rules that apply to your regular pay. Some employer benefits are deducted differently depending on the payroll schedule, so don't assume every “extra” check will look exactly like the others.
A sensible use for the extra check is to assign it before it arrives.
You might use it for an emergency-fund contribution, a debt payment, annual insurance, school expenses, travel or another irregular cost. Or you can divide the amount between several priorities.
Estimating and Verifying Your Paycheck
What you should avoid is building your monthly fixed expenses around those two extra checks. They are calendar-driven, not guaranteed monthly income.
There is another reason to understand this when using a paycheck calculator: biweekly and semimonthly are not interchangeable. A $65,000 salary paid biweekly produces a different gross amount per paycheck than the same salary paid twice per month.
If your employer changes pay frequency, your annual salary may stay the same while the individual check amount changes. That can make the transition feel like a raise or pay cut when it isn't.
The extra-paycheck months are best treated as a cash-flow feature of biweekly pay. Plan for them, but don't confuse them with additional annual compensation.