Coordinating with CPAs on Tax Projections

Most investors treat their CPA relationship as a backward-looking exercise—hand over documents in March, get a return filed in April, repeat. The cost of this approach shows up as overpaid estimated taxes, missed Roth conversion windows, and surprise IRMAA surcharges that could have been avoided with a single mid-year projection. The right answer: shift from annual compliance to quarterly tax projection coordination, where your CPA models forward-looking liability at each estimated payment deadline.

TL;DR: Quarterly tax projections with your CPA let you optimize Roth conversions, avoid underpayment penalties, sidestep IRMAA surcharges, and harvest losses with precision—turning reactive tax filing into proactive tax management.

What a Tax Projection Actually Is (And Why It's Not Your Tax Return)

A tax projection is a forward-looking estimate of total tax liability for a given tax year, incorporating expected income, deductions, credits, and applicable rates. Your CPA uses it to guide intra-year decisions—estimated payments, Roth conversion sizing, loss harvesting timing—before December 31 locks everything in.

The point is: a tax return tells you what happened. A tax projection tells you what to do about what's happening.

Core terms your CPA will reference in every projection:

• Marginal tax rate: The rate on your last dollar of taxable income. For 2025, the seven federal brackets are 10%, 12%, 22%, 24%, 32%, 35%, and 37%.
• Effective tax rate: Total federal tax divided by total taxable income—always lower than your marginal rate because income is taxed progressively across brackets.
• Bracket-filling strategy: Recognizing income (via Roth conversions or gain realization) up to a target bracket ceiling without spilling into the next bracket. For married filing jointly in 2025, the 24% bracket tops at $394,600 of taxable income.
• Safe harbor: The IRS rule under IRC §6654 that protects you from underpayment penalties if withholding plus estimated payments equal at least 90% of current-year tax or 100% of prior-year tax (110% if prior-year AGI exceeds $150,000).

Why this matters: every threshold above is a decision point in your projection. Miss one, and you either overpay taxes or trigger penalties and surcharges.

How Quarterly Projection Coordination Works in Practice

Effective CPA coordination follows a four-checkpoint cadence aligned with estimated tax payment deadlines: April 15, June 15, September 15, and January 15 of the following year.

Checkpoint → Data exchange → Projection update → Action decision

At each checkpoint, you provide your CPA with:

1. Year-to-date W-2 or 1099 income (including any RSU vests or bonus payments)
2. Realized capital gains and losses from brokerage statements
3. Any Roth conversions already executed (with amounts)
4. Changes in expected income for the remaining quarters (new client, job change, property sale)

Your CPA then runs an updated projection incorporating these inputs against the current bracket structure, the $30,000 MFJ standard deduction (or itemized deductions including the $10,000 SALT cap), and any applicable surtaxes.

The point is: Q1 and Q2 projections set your baseline. Q3 is where the real optimization happens—you have enough data to see the year's trajectory but enough time to act on it.

What the CPA Models at Each Checkpoint

The takeaway: relying on prior-year figures alone (as many investors do) means you're flying blind when income changes materially. A $50,000 capital gain in August changes your optimal Roth conversion amount, your NIIT exposure, and your estimated payment math—all at once.

Worked Example: The Nguyens' Mid-Year Projection Pivot

David and Maria Nguyen file married jointly. Their CPA runs quarterly projections. Here's how the year unfolds.

Phase 1: The Q1 Baseline (April 15)

• Combined W-2 income: $280,000 projected
• Prior-year federal tax: $48,000
• Safe harbor target (110% of prior year, since AGI > $150,000): $52,800
• Initial quarterly estimated payments: $13,200 each ($52,800 ÷ 4)
• Standard deduction: $30,000
• Projected taxable income: $280,000 − $30,000 = $250,000
• Marginal bracket: 24% (well below the $394,600 MFJ ceiling)
• CPA identifies $144,600 of headroom in the 24% bracket ($394,600 − $250,000)

The CPA notes this headroom could support a Roth conversion of up to $144,600 without crossing into the 32% bracket.

Phase 2: The Q3 Trigger (September 15)

Maria sells a concentrated stock position in July, generating a $95,000 long-term capital gain. Updated projection:

• W-2 income: $280,000
• Capital gain: $95,000
• Total gross income: $375,000
• MAGI for NIIT purposes: $375,000 (exceeds the $250,000 MFJ threshold by $125,000)
• NIIT exposure: $125,000 × 3.8% = $4,750
• MAGI for IRMAA (2027 premiums): $375,000 exceeds the $212,000 MFJ threshold—triggering first-tier surcharges of $74/month per person ($1,776/year for two)

The CPA recalculates: with the capital gain, taxable income is now $375,000 − $30,000 = $345,000. Remaining 24% bracket headroom shrinks to $49,600 ($394,600 − $345,000).

Phase 3: The Adjusted Strategy

The CPA recommends three coordinated actions:

1. Roth conversion sized to $49,600 (not the original $144,600)—fills the 24% bracket without triggering the 32% rate. Tax cost on conversion: $49,600 × 24% = $11,904.

2. Tax-loss harvesting in the taxable brokerage account. Maria identifies $18,000 in unrealized losses. Harvesting offsets part of the $95,000 gain, reducing net gains to $77,000. She avoids the wash-sale rule by waiting 31 days before repurchasing (or buying a non-substantially-identical fund immediately).

3. Estimated payment adjustment. With the capital gain and Roth conversion, projected total tax rises to approximately $72,000. Safe harbor (110% of prior-year $48,000) is $52,800—already covered by Q1-Q3 payments of $39,600. The CPA increases Q4 to $32,400 to cover the actual liability and avoid the underpayment penalty (running at approximately 8% annual rate in 2025).

The practical point: Without the Q3 projection update, the Nguyens would have either (a) converted $144,600 at the 32% rate on the excess—costing an extra $7,600 in unnecessary federal tax on the $95,000 that spilled over—or (b) missed the conversion window entirely. The CPA's mid-year recalculation saved them from both mistakes.

Summary Metrics:

Thresholds Your CPA Should Monitor (The Hidden Cliffs)

Several tax thresholds create non-linear cost jumps that only show up in a properly modeled projection:

Net Investment Income Tax (NIIT): A 3.8% surtax on the lesser of net investment income or MAGI exceeding $200,000 single / $250,000 MFJ. These thresholds have been frozen since 2013—not inflation-adjusted. Each $10,000 over the threshold costs $380 in NIIT. Your CPA should flag this at every checkpoint.

IRMAA (Medicare Premium Surcharges): Uses MAGI from two years prior. In 2025, the thresholds start at $106,000 single / $212,000 MFJ. The standard Part B premium is $185/month; surcharges push it up to $628.90/month at the highest tier. The point is: a Roth conversion in 2025 affects your Medicare premiums in 2027—your CPA must model this lag.

SALT Deduction Cap: Limited to $10,000 per return under the TCJA (now permanent under the One Big Beautiful Bill Act, signed August 2025). In high-tax states (NY, NJ, CA, CT, IL), households with property taxes alone exceeding $10,000 lose the excess deduction entirely. This affects timing decisions for state estimated payments and property tax prepayments.

Estimated Tax Safe Harbor: Prior-year AGI above $150,000 requires estimated payments equal to 110% of prior-year tax (not 100%) to avoid underpayment penalties. Missing any single quarterly deadline can trigger per-quarter penalties even if the annual total is sufficient.

Common Pitfalls (And How to Avoid Them)

Pitfall 1: Providing incomplete data to your CPA. You sell a stock in your brokerage account in August but don't mention it until tax-filing season. Your CPA can't optimize what they can't see. The fix: share brokerage statements quarterly, not annually.

Pitfall 2: Ignoring the IRMAA two-year lookback. A $50,000 Roth conversion in 2025 that pushes MAGI over $212,000 triggers surcharges you won't see until 2027 Medicare enrollment. Your CPA should model IRMAA impact on every conversion scenario (and you should ask if they don't).

Pitfall 3: Harvesting losses without checking the wash-sale window. You sell Fund A at a loss and buy Fund B the next day. If Fund B is "substantially identical" to Fund A under IRC §1091, the loss is disallowed for the entire 61-day window (30 days before and after). The disallowed loss gets added to Fund B's cost basis—not lost forever, but useless for this year's projection. The fix: coordinate with your CPA on which replacement securities qualify before executing.

Pitfall 4: Setting estimated payments on autopilot. Using last year's payments without adjustment means you're either overpaying (giving the IRS an interest-free loan) or underpaying (triggering penalties at approximately 8% annual rate). The fix: let Q3 projection results drive Q3 and Q4 payment amounts.

Pitfall 5: Treating asset location as a one-time decision. Placing tax-inefficient investments (bonds, REITs) in tax-deferred accounts and tax-efficient investments (index funds, qualified-dividend stocks) in taxable accounts can add up to 30 basis points annually on a $1M portfolio (Morningstar research). But this only works if your CPA reviews it alongside your projection—a Roth conversion changes which accounts hold pre-tax vs. after-tax dollars.

What Changed: TCJA Permanence Reshapes Projection Strategy

When the TCJA was originally set to expire after December 31, 2025, CPAs advised accelerating Roth conversions to lock in lower brackets before rates reverted (24% back to 28%, 37% back to 39.6%). Fidelity reported Roth conversion activity rose significantly as advisors and CPAs coordinated multi-year bracket-filling strategies.

The One Big Beautiful Bill Act (signed August 2025) made the TCJA's individual rates permanent. The seven brackets—10% through 37%—are no longer scheduled to revert. The $10,000 SALT cap also remains in place.

What matters here: the conversion urgency is gone, but the coordination framework remains essential. CPAs shifted from "convert before sunset" to ongoing multi-year optimization. And the broader principle holds—legislative changes can arrive with little warning (the original TCJA was signed December 22, 2017, giving CPAs weeks to rebuild projection models for the 2018 tax year). Quarterly projections let you adapt to whatever comes next.

Coordination Checklist: Working with Your CPA on Tax Projections

Essential (high ROI):

• Schedule quarterly check-ins aligned with estimated payment deadlines (April 15, June 15, September 15, January 15)
• Share year-to-date brokerage statements showing realized gains, losses, and dividend income at each checkpoint
• Confirm your safe harbor target—100% or 110% of prior-year tax—and verify estimated payments are on track
• Ask your CPA to model IRMAA impact before executing any Roth conversion

High-impact (workflow integration):

• Provide your CPA with projected remaining income (bonuses, RSU vests, expected capital gains) at the Q3 checkpoint
• Review asset location across accounts annually—confirm tax-inefficient holdings are in tax-deferred accounts
• Coordinate tax-loss harvesting candidates with your CPA to avoid wash-sale violations across all accounts (including spouse's accounts and IRAs)

Optional (valuable for high-income households):

• Request a multi-year projection showing Roth conversion impact across 3-5 years, including IRMAA effects
• Model NIIT exposure under different capital gain scenarios before executing large sales
• Review SALT cap impact if you're in a high-tax state—your CPA can advise on timing of state estimated payments

Your Next Step

Before your next estimated tax payment deadline, send your CPA a single email with three attachments: (1) your most recent pay stub showing year-to-date earnings, (2) your brokerage account's realized gain/loss report for the year, and (3) any expected income changes for the remaining quarters. Ask them to run an updated projection and confirm whether your estimated payments need adjustment. That one exchange turns a compliance relationship into a planning partnership.

For related concepts, see Trust Taxation Basics and Tracking Basis in Partnerships and S Corps.