Wash Sale Adjustment Calculator
Use this calculator to estimate how much of your loss is disallowed under the wash sale rule and how that disallowed loss adjusts your replacement-share basis.
How Is Wash Sale Adjustment Calculated?
The wash sale adjustment is calculated by identifying how much of a realized loss is disallowed when you buy substantially identical securities within the wash sale window. Under U.S. federal tax rules, if you sell stock or securities at a loss and acquire substantially identical stock or securities within 30 days before or after the sale, the matched portion of your loss is not deductible right away. Instead, that disallowed loss is added to the tax basis of the replacement shares.
In practical terms, the wash sale rule changes timing, not necessarily total lifetime tax benefit. You usually do not lose the loss forever in a taxable account. The loss is deferred by increasing basis in replacement shares and potentially adjusting holding period treatment. This is why precise lot matching matters for active investors, swing traders, and anyone tax loss harvesting near year end.
Core Formula Used in Most Scenarios
For a basic one lot sale and one replacement purchase, the adjustment can be estimated with this framework:
- Realized loss per share = Original basis per share minus sale price per share.
- Total realized loss = Realized loss per share multiplied by shares sold.
- Matched shares = Smaller of shares sold at loss and replacement shares bought within the wash sale window.
- Disallowed loss = Realized loss per share multiplied by matched shares.
- Currently deductible loss = Total realized loss minus disallowed loss.
- Replacement basis adjustment = Add disallowed loss to replacement shares that created the wash sale.
If there is no realized loss, there is no wash sale disallowance. Gains are not disallowed under wash sale rules.
Step by Step Example
Assume you bought 100 shares at $50 and sold them for $42. Your realized loss per share is $8, so total realized loss is $800. If you buy 100 replacement shares within the 61-day window (30 days before sale date, sale date itself, and 30 days after), then all 100 shares are matched. The entire $800 loss is disallowed immediately and added to basis of the replacement lot.
- Original replacement cost: 100 × $43 = $4,300
- Disallowed loss added to basis: +$800
- Adjusted replacement basis: $5,100
- Adjusted basis per matched share: $51
That basis increase means when you eventually sell those replacement shares in a non-wash sale transaction, the deferred loss can reduce future taxable gain or increase future loss.
Partial Wash Sales: The Most Common Real World Case
Partial wash sales occur when the number of replacement shares is less than the number of shares sold at a loss, or when only some replacement purchases fall within the wash window. Example: you sell 200 shares at a $5 loss per share (total loss $1,000), then buy only 80 shares back within 30 days. Matched shares are 80, so disallowed loss is $400. The remaining $600 is generally deductible now, subject to capital loss limitation rules.
This is why lot-level data matters. Broker 1099-B reporting often captures many wash sales, but investors with multiple accounts, spouse accounts, or manual lot management should independently verify tax treatment.
Key IRS Numbers and Limits (Reference Table)
| Rule Metric | Current Federal Figure | Why It Matters for Wash Sale Planning |
|---|---|---|
| Wash sale identification window | 30 days before and 30 days after loss sale date (61-day span including sale date) | Any substantially identical purchase in this window can disallow part or all of the loss. |
| Net capital loss deduction against ordinary income | $3,000 per year ($1,500 if married filing separately) | Even allowed losses may be limited in current year and carried forward. |
| Capital loss carryforward | No fixed federal expiration for individuals, carried forward until used | Deferred or unused losses can still have long-term tax value. |
2024 Long Term Capital Gain Brackets (Real IRS Thresholds)
These rates do not determine whether a wash sale exists, but they help quantify why basis adjustments are valuable. A deferred loss can offset future gains taxed at 0%, 15%, or 20% rates depending on taxable income and filing status.
| Filing Status (2024) | 0% LTCG Rate Up To | 15% LTCG Rate Up To | 20% LTCG Rate Above |
|---|---|---|---|
| Single | $47,025 | $518,900 | Over $518,900 |
| Married Filing Jointly | $94,050 | $583,750 | Over $583,750 |
| Head of Household | $63,000 | $551,350 | Over $551,350 |
| Married Filing Separately | $47,025 | $291,850 | Over $291,850 |
What Counts as Substantially Identical?
The tax code does not provide a simple universal checklist, which is why this area can be nuanced. Selling one ETF and buying the exact same ETF again clearly triggers wash sale treatment when done inside the timing window at a loss. Questions become harder when switching between similar ETFs tracking related indexes, share classes, options, or convertibles.
- Same CUSIP or same exact security is high risk for wash sale treatment.
- Identical options on the same underlying can also create wash sale issues.
- Cross account activity can matter, including spouse activity in some fact patterns.
- IRA repurchases after taxable loss sales can be especially unfavorable because basis adjustment may not provide recoverable benefit in the same way.
Holding Period Impact
In many wash sale cases, the holding period of the original shares is added to the replacement shares whose basis is adjusted. This can influence whether a later sale becomes short term or long term. For investors near long-term qualification thresholds, this detail can materially alter tax outcome. Your broker statements and tax software may partially track this, but complex patterns still require manual review.
Common Mistakes That Cause Reporting Problems
- Only checking one brokerage account and ignoring transactions in another account.
- Ignoring automatic dividend reinvestments that buy small replacement lots within 30 days.
- Tax loss harvesting in taxable account while simultaneously buying equivalent exposure in retirement accounts.
- Assuming every broker reports all cross-account wash sales for you.
- Not keeping lot-level records when using specific share identification methods.
Best Practices for Accurate Wash Sale Tracking
- Use a transaction log with trade date, settlement details, quantity, price, and account name.
- Tag every loss sale and check for substantially identical purchases in the 30 days before and after.
- Disable dividend reinvestment temporarily during planned tax loss harvesting windows.
- Review broker 1099-B wash sale adjustments and reconcile with your own records.
- When complexity rises, involve a CPA or EA with active trader experience.
Why This Calculator Is Useful
This calculator gives you a fast estimate of the disallowed loss and adjusted basis impact, which helps with planning before you place replacement trades. It is especially useful when deciding whether to replace a position immediately, wait outside the wash window, or use a non-identical substitute security to maintain market exposure while preserving tax loss recognition.
Keep in mind this tool is educational. Real tax reporting may involve multiple lots, corporate actions, option overlays, and broker-specific lot matching. Treat this as a planning engine, then validate with your tax records.
Authoritative References
For official guidance, review IRS and legal references directly:
- IRS Publication 550, Investment Income and Expenses
- IRS Topic No. 409, Capital Gains and Losses
- 26 U.S. Code Section 1091 (Cornell Law School)
Final Takeaway
To calculate wash sale adjustment correctly, focus on three elements: realized loss per share, number of matched replacement shares within the wash window, and basis transfer amount. Once you calculate disallowed loss precisely, you can determine what is deductible now and what carries into replacement basis. Done correctly, you avoid unpleasant filing surprises and make cleaner tax-aware portfolio decisions.