Identify the accrual unit
PTO may accrue per hour worked, per pay period, per month or as an annual grant. The calculation must use the same unit as the policy.
Track the balance as a ledger
A basic balance starts with the opening amount, adds earned hours and adjustments, then subtracts used hours. Keep adjustments separate so corrections remain auditable.
Apply caps and waiting periods explicitly
A cap limits the available balance even when the raw accrual formula would produce more. Waiting periods, carryover limits and eligibility rules should be handled as policy inputs, not guessed by the calculator.
Forecast future leave conservatively
Project expected accrual through the planned leave date, subtract already scheduled usage and show any shortfall. Confirm the result with the employer policy before making commitments.