Illinois Supplemental Tax Regulations for Google (Alphabet) RSUs
Illinois applies a flat 4.95% individual income tax rate with matching 4.95% supplemental withholding.
Illinois Safe Harbor & Underpayment Penalties
IL-1040-ES safe-harbor required if unpaid tax exceeds $1,000.
Employer Stock Plan Specifics: Charles Schwab / Morgan Stanley
Google utilizes a frontloaded monthly vesting schedule (33% across years 1 and 2). Because monthly vests are treated as supplemental wage distributions, Schwab withholds federal tax at flat 22%. Googlers with total comp above $350k find that each monthly tranche underwithholds by 10% to 15%, compounding silently over 12 months.
When vesting at Google (Alphabet), Charles Schwab / Morgan Stanley calculates automatic sell-to-cover withholding using Illinois's statutory supplemental rate of 4.95% combined with the federal supplemental rate (22% on up to $1M). Because top earners in Illinois reach marginal brackets exceeding these rates, you will face an incremental shortfall on April 15.
Localized CPA & Advisory Referral
Advisor Guidance: Work with a Chicago CPA to coordinate Illinois IL-1040-ES payments for large technology RSU vests.