Churn Withholding Rate
Percentage of churned gains withheld from taxable accounts to model visible tax drag from portfolio turnover. Applied to realized gains (churn). Should approximate expected capital gains tax rate (0%, 15%, 20%, or 23.8%).
How it works
The gain realization rate comes from the account's basis ratio at entry unless the advisor overrides it — a 70% basis ratio implies a portfolio that turns over enough to keep basis that high, so 70% of future growth is realized and taxed annually. The actual tax calculations are performed on those realized gains. However, because taxes are assumed to be paid from free cash flow (not from the assets themselves), the tax drag is invisible - making taxable accounts appear unrealistically attractive. The Churn Withholding Rate is an optional (but recommended) mechanism to withhold a percentage of the gains realized from portfolio churn to represent paying taxes from the account itself, creating visible tax drag. This elegantly avoids the circular reference problem (withdrawing to pay taxes creates more taxable withdrawals infinitely) by using a fixed withholding percentage applied to churn rather than attempting to reduce the account by exact tax amounts.
1. Understand the Problem: Invisible Tax Drag
The gain realization rate tells us how much is churned (an account entered at 70% basis realizes 70% of its growth). Taxes on this churn are calculated correctly in the tax projections. However, those taxes are paid from free cash flow outside the portfolio. Since cash flow isn't modeled, the tax drag is invisible, and the portfolio value doesn't reflect the cost of paying taxes on churn. This makes taxable accounts unrealistically attractive, often outperforming even tax-deferred accounts.
Example: $1M account, 70% basis, 7% return = $70k growth. 70% churned = $49k realized gains. Tax at 15% = $7,350. But this $7,350 isn't reducing the portfolio - it's paid from free cash flow. Portfolio grows as if tax-free.2. Calculate Churned Gains from Account Growth
Determine the portion of annual growth that represents realized gains (churn) from the account's gain realization rate — set from the basis ratio at entry, or entered directly. An account at 70% realizes 70% of its growth annually. This is the tax base we withhold from: a higher rate means more churn and more tax drag.
Churned Gains = Account Growth × Basis Ratio
Example: $1M account grows 7% = $70k growth. 70% realization rate → $70k × 70% = $49k churned gains subject to taxation3. Apply Withholding Rate to Churned Gains
Withhold a percentage of the churned gains to represent taxes paid from the account itself. The withholding rate should approximate the expected capital gains tax rate for the client's income level. This makes the tax drag visible by reducing the account value, preventing unrealistic outperformance. Withholding happens at year-end after growth but before compounding to the next year.
Year-End Withholding = Churned Gains × Withholding Rate
Example: $49k churned gains × 15% withholding rate = $7,350 withheld → Account grows from $1M to $1.0627M (not $1.07M)4. Why Not Withdraw Exact Tax Amounts?
You might ask: why not calculate the exact tax on churn and withdraw that amount? There are three reasons: 1) This is not necessarily more realistic. Capital gains taxes are commonly paid by cash flow, so we need to retain that option. 2) In reality, taxes are paid in the calendar year following the year of the realized gains. While not impossible, this level of complexity is often difficult for people to understand in normal planning contexts. 3) Withdrawals taken to pay taxes likely generate additional taxes. Using a fixed withholding percentage applied to churn elegantly avoids all these problems.
Example: Attempt exact: $49k gains → $7,350 tax → withdraw $7,350 → has ~$2,205 gains (30% unrealized) → $331 more tax → withdraw $331 → has ~$99 gains → $15 more tax → withdraw... never ends!5. Calibrating the Withholding Rate
To model capital gains taxes paid entirely from the taxable account, set the withholding rate to the expected capital gains tax rate. To model capital gains taxes paid from free cash flow, enter 0.0%. Enter any percentage in between to model partial funding from taxable accounts.
Withholding Rate ≈ Expected Capital Gains Tax Rate
Example: Low income (0% LTCG bracket): 0% withholding | Moderate income (15% LTCG bracket): 15% withholding | High income (20% LTCG bracket): 20% withholding | Very high income (20% + 3.8% NIIT): 23.8% withholding6. Real-World Parallels: Income Tax Withholding
This mechanism mimics income tax withholding. With income tax withholding, you estimate your tax rate and withhold that percentage of income throughout the year. With churn withholding, you estimate your capital gains tax rate to be paid from the account and withhold that percentage of churned gains throughout the year. For stock option exercises, this withholding is mandatory (22-37%). For regular brokerage accounts, it's a modeling choice to make the tax drag visible.
Example: Taxable Account Value: $1.0M; entered at $700K basis (70%), so a 70% realization rate; Growth: 7%; End-of-Year Value: $1.07M. A Churn Withholding Rate of 20% reduces account value by 20% of the churned gains: $70K × 70% = $49K churn × 20% = $9.8K Tax Withheld.7. When to Use vs Not Use Withholding
Use withholding (non-zero rate) when: modeling accounts where churn taxes reduce account value, comparing taxable vs tax-deferred account performance, or preventing unrealistic taxable account outperformance. Use zero withholding when: taxes truly paid from outside sources that don't affect net worth, or intentionally modeling best-case scenario where churn taxes don't impact account growth.
Example: High-net-worth client with external income to cover taxes: 0% withholding | Moderate income retiree in 15% bracket: 15% withholding | High income client in 20% bracket: 20% withholding
Real-world context
Elegantly Modeling Reality
This is an elegant solution to a complex problem. Simplistic algorithms and modeling tools typically tax portfolio growth each year to model tax drag on the portfolio. However, this is arguably more erroneous than not taxing the taxable account at all since long-term holds in taxable accounts are common. The model used here facilitates both extremes (annual taxation or long-term hold) and anything in between.
Quarterly Estimated Tax Payments
For typical brokerage accounts, realized gains from churn may generate quarterly estimated tax payments. The Churn Withholding Rate models this scenario rather appropriately. Whether withholding is actually required for a given client is less important than the ability to model and discuss the tax drag on taxable accounts.
Why Taxable Accounts Can Outperform (Realistically)
With a low realization rate (20-30% = buy-and-hold, little churn) and a low (or 0.0%) Churn Withholding Rate (taxes paid from external income), taxable accounts can legitimately outperform tax-deferred accounts due to: (1) preferential capital gains rates (0-20% vs 22-37% ordinary), (2) tax deferral until sale (like tax-deferred accounts), (3) step-up in basis at death (eliminates gains). But with a high realization rate (70%+ = high churn) and zero withholding, the outperformance becomes unrealistic - that's when withholding is essential.
Withholding Does NOT Change the Tax Calculation
A common source of confusion: changing the Churn Withholding Rate does NOT change the tax totals in the projection. The tax calculation on realized gains (churn) is unchanged regardless of the withholding rate — that tax is always computed correctly and appears in the year's capital gains tax. What the withholding rate actually controls: WHERE the tax comes from. At 0%, the tax is assumed paid from free cash flow outside the portfolio (invisible; the taxable account balance is unaffected). At 15%, the tax is modeled as paid from the taxable account itself (visible; the account balance drops by the withheld amount). Either way, the tax dollar amount is the same. This means the advisor cannot adjust the withholding rate to make the portfolio 'more tax-efficient' — withholding is a modeling visibility knob, not a real-world tax lever. To model a genuinely more tax-efficient portfolio, LOWER the gain realization rate on the account (Base Data tab → Assets → account configuration modal), or enter a lower basis ratio, which sets that rate by default. Either changes the tax calculation itself, not just the visibility of where the tax comes from.
Distinction from Stock Option Exercise Withholding
The Churn Withholding Rate is NOT the same as stock option exercise withholding. Both involve withholding from an account, but they model different things: Stock option exercise withholding: mandatory 22–37% federal supplemental rate applied at exercise time. Represents actual tax payment to the IRS at the time of exercise, just like W-2 withholding on a paycheck. Set on each grant (Base Data → Stock Option Grants); default 22%. The withheld dollars are real tax payment. Churn withholding: optional modeling mechanism to make the ongoing tax drag of portfolio turnover visible in the account balance. The withheld dollars represent the accumulation of taxes paid on churn over the year, not a discrete tax event. It's a visualization tool, not a mandatory tax mechanic. For clients with both stock options and taxable accounts, both withholdings operate independently — stock option withholding happens at exercise (real tax), and churn withholding happens year-end on churn gains (visibility).
What drives the result
Percentage of churned gains withheld from taxable accounts to model tax drag from portfolio turnover. Applied to realized gains (churn), NOT account value. Should approximate expected capital gains tax rate. ZERO withholding (0%) assumes churn taxes paid from free cash flow (invisible cost) - may cause unrealistic taxable account outperformance. NON-ZERO withholding (15%, 20%) makes tax drag visible by reducing account value - creates realistic relative performance vs tax-deferred accounts.
0% withholding: Taxes paid externally, no account impact | 15% withholding: Moderate income client in 15% LTCG bracket | 20% withholding: High income client in 20% LTCG bracket | 23.8% withholding: Very high income with NIIT
The gain realization rate determines how much churn occurs — the tax base. It comes from the basis ratio at entry (70% basis → 70% of future growth realized and taxed annually) unless the advisor sets it directly on the account. The withholding rate then determines what share of those churned gains is withheld from the account. Together they set total withholding: more churn plus a higher tax rate means more withheld.
30% realization = low churn → $70k × 30% = $21k gains from $70k growth | 70% realization = high churn → $70k × 70% = $49k gains from $70k growth
Higher returns generate higher growth, which generates higher churned gains (scaled by the realization rate), which generates higher withholding amounts. The withholding RATE stays constant (expected tax rate), but the withholding AMOUNT increases with returns.
4% return, 30% basis, 15% rate → $1M × 4% × 30% × 15% = $1,800 withheld | 8% return, same basis/rate → $1M × 8% × 30% × 15% = $3,600 withheld (doubled)
Assumptions
- Withholding rate is constant annually (reality: varies with actual income and tax brackets)
- Withheld amounts represent taxes paid and removed from net worth (not reinvested elsewhere)
- Tax rates remain constant (reality: brackets change, tax law changes)
- All churn generates long-term capital gains (reality: some may be short-term or ordinary income)
Limitations
- Does not account for tax-loss harvesting reducing tax drag
- Assumes linear withholding (reality: taxes are lumpy and depend on other income)
- Does not model state income taxes on capital gains (federal only)
- Withholding is not reconciled afterward to account for under- or over-withholding. This is absorbed in free cash flow.
- Does not adjust for changes in income level or tax bracket over time
Related
This page explains how Stratum models this calculation. It is educational material for financial professionals, not tax or legal advice, and tax law changes. Verify current figures against primary IRS sources before relying on them with a client.