England or Scotland, 2026/27
Is your RSU withholding enough?
Employers usually sell enough shares to cover a flat assumed rate, not your real one. This checks the gap on a single vest.
Scotland uses its own income tax bands; NI is UK-wide
Your gross annual salary, before this vest
Salary stays in GBP. Results are always shown in GBP.
Falls in tax year 2026/27
What gets sold or withheld to cover tax. Default 47% (45% additional-rate income tax + 2% upper-band NI).
Result
- Gross RSU income
- £15,000
- Real tax (income tax + NI)
- £8,300
- Employer withheld
- £7,050
- Effective rate on this vest
- 55%
£1,250 still owed
Your employer's flat 47% withholding falls short of your real marginal rate on this vest, so this amount is worth setting aside before it's spent.
Why employer withholding often falls short
When RSUs vest, most UK employers run “sell to cover”: they sell a fixed share of the vesting shares (commonly around 47%, covering the additional-rate income tax band plus the upper-band employee National Insurance rate) and remit that to HMRC via PAYE. That flat rate is a reasonable default for someone whose marginal rate genuinely sits there, but it is not personalised to you.
An RSU vest is employment income that stacks on top of your salary. If that stacking pushes you through the personal-allowance taper (between £100,000 and £125,140 of adjusted net income, where you lose £1 of personal allowance for every £2 over £100,000) your real marginal rate on the vest can briefly exceed 60%, well above the flat withholding rate. The shortfall becomes a true-up you owe HMRC directly, usually via self-assessment, due 31 January after the tax year.
This tool checks a single vest in isolation. For multiple tranches across a tax year, capital gains tax on a later sale, and a sell-versus-hold comparison, use the full RSU + CGT planner.
This is a generic illustration. It is not financial, tax, or investment advice and does not account for your full circumstances.