A Colorado RSU tax calculator estimates the federal + 4.4% Colorado flat tax owed on each vest. Default employer withholding (22% federal, 4.4% state) usually under-collects on the federal side — the calculator below shows the dollar shortfall for your bracket.
Calculate RSU Withholding
Estimate your RSU tax withholding and net proceeds after vesting.
Try Calculator →Open the Colorado RSU tax calculator with these inputs prefilled →
How Colorado taxes your RSU vest
Colorado treats RSU income exactly like ordinary W-2 wages: when shares vest, the fair-market value on the vest date is added to your Form W-2 Box 1 in the year of vest, taxed at the flat 4.4% Colorado individual income tax rate, and reported alongside federal withholding (IRS Publication 525). Because Colorado uses a flat tax — not progressive brackets — your Colorado liability scales linearly with RSU income; the bracket you fall into doesn't matter on the state side, only the dollar amount of the vest does.
The two numbers that matter on every vest:
- Federal supplemental withholding rate: 22% on RSU income up to $1M (IRS Publication 15). Above $1M, the rate jumps to 37%.
- Colorado supplemental withholding rate: 4.4% flat. Colorado does not publish a separate "supplemental" rate the way California or New York do — employers withhold at the same 4.4% rate that applies to all Colorado wage income, so the default withholding and your actual state liability are usually identical on the state side.
If your marginal federal bracket is higher than the 22% supplemental rate, your employer is under-withholding on the federal side — and you owe the difference at filing time. The Colorado piece typically washes, but the federal gap shows up on every CO vest.
Worked examples (Colorado, 2025)
These three scenarios use the EquityTax calculator with the inputs shown. The math is identical to what you'll see when you click through to the calculator.
Example 1 — Junior IC, single filer
Inputs: $120K salary, single filer, 100 RSUs vesting at $80 FMV ($8,000 RSU income).
Result:
- Federal withholding: $1,760
- Colorado withholding: $352
- Estimated total tax on the vest: $2,884
- Shortfall vs default withholding: $160
The 22% federal default closely matches the 24% federal marginal rate at this income level, so the gap is small — $160. Colorado is a flat tax, so default state withholding (4.4% × $8,000 = $352) equals the actual Colorado liability on the vest. No state shortfall.
Run this scenario in the calculator →
Example 2 — Mid-level IC, married filing jointly
Inputs: $200K salary, married filing jointly, 400 RSUs vesting at $120 FMV ($48,000 RSU income).
Result:
- Federal withholding: $10,560
- Colorado withholding: $2,112
- Estimated total tax on the vest: $13,564
- Shortfall vs default withholding: $196
Total household income is $248K, which puts this couple in the 24% federal bracket. The 22% supplemental rate is close but still short by 2 percentage points on the RSU income. Colorado again contributes a clean $2,112 (4.4% × $48,000), with no state-side gap.
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Example 3 — Senior IC, married filing jointly
Inputs: $350K salary, married filing jointly, 1,500 RSUs vesting at $200 FMV ($300,000 RSU income).
Result:
- Federal withholding: $66,000
- Colorado withholding: $13,200
- Estimated total tax on the vest: $113,685.50
- Shortfall vs default withholding: $27,435.50
This is where Colorado RSU earners get hurt. Total household income lands at $650,000, deep into the 35% federal bracket. The employer withholds at 22%, so on $300,000 of RSU income the federal gap alone is $27,435.50. Colorado state tax is a clean $13,200 (flat 4.4%), and there's also an estimated $1,920.49 underpayment penalty if no quarterly estimate is made. Total owed at filing: $29,355.99.
Run this scenario in the calculator →
Why employer withholding usually isn't enough in Colorado
The gap is almost entirely federal, not state. Colorado's flat 4.4% means employer supplemental withholding lines up with actual state liability for every vest size, every filing status, every income level. The federal supplemental rate, by contrast, is a fixed 22% — and the moment your total income crosses into the 24%, 32%, 35%, or 37% brackets, your employer is under-withholding on RSU income.
The fix is straightforward and depends on the size of the gap:
- Adjust your W-4 so your paycheck withholding picks up the difference across the year (IRS W-4 instructions).
- Make a quarterly estimated payment if the shortfall is concentrated around a single vest date.
- Sell-to-cover at vest — if your broker offers it, this can absorb the federal gap automatically; the state gap (4.4%) usually doesn't need separate action.
The 22% federal supplemental rate is a default — not your actual marginal rate. If your total income (salary + RSU) puts you in the 32%, 35%, or 37% bracket, the federal under-withholding alone can dwarf your Colorado state liability.
How the Colorado RSU tax calculator handles your vest
The Colorado calculator uses Colorado's actual 2025 flat 4.4% rate from the Colorado Department of Revenue, not a generic state placeholder. Because Colorado has no progressive brackets, the state portion of the engine is a straight multiplication on vest-day RSU income — but the federal side still walks the full 2025 IRS bracket table for your filing status, which is where the real shortfall comes from.
Open the calculator with your inputs →
FAQ
Does Colorado charge a higher rate on RSU income than on regular salary? No. Colorado uses a single flat 4.4% rate on all individual income, including RSU vesting income. There is no separate supplemental rate or surtax on equity compensation at the state level.
Why is my Colorado RSU shortfall almost always zero? Because Colorado withholding (4.4% flat) equals Colorado liability (4.4% flat) on the RSU income. The shortfall you see on Colorado RSU vests is almost entirely federal — driven by the 22% supplemental rate being lower than your real marginal federal bracket.
Do I owe Colorado tax on RSUs that vest while I live out of state? Generally only on the portion attributable to Colorado workdays during the vesting period — sourcing rules apply. Verify your specific situation with the Colorado Department of Revenue and a CPA.
When do I need to make a quarterly estimated payment in Colorado? If you expect to owe more than $1,000 at filing after withholding and credits, the IRS safe-harbor rules generally require estimated payments. For most Colorado RSU earners, the federal gap is the trigger — the 4.4% state piece is already covered by default withholding.
Sources
- IRS Publication 525, Taxable and Nontaxable Income — RSU treatment, fair-market-value rules.
- IRS Publication 15, Employer's Tax Guide — federal supplemental withholding rate (22% / 37%).
- Colorado Revenue Department — state flat-tax rate and equity-comp guidance.
- EquityTax Colorado RSU Calculator (internal engine, last verified 2026-05-09).
Tax Disclaimer: This content is for educational purposes only. Always consult with a licensed tax professional or certified public accountant before making financial decisions related to equity compensation, tax planning, or investment strategies.
Estimate only — not financial or tax advice. Consult a qualified CPA before making decisions about exercising stock options, selling equity, or other financial moves.