Wash Sale Calculator

Background.

Use Wash Sale Calculator when you need to quantify how much loss is deferred when replacement shares are bought inside the wash-sale window. The disallowed loss is attached to replacement-share basis rather than disappearing, while replacement-share holding periods can also carry over. Here the arithmetic follows “disallowed loss = realized loss × min(replacement shares ÷ loss shares sold, 1); disallowed amount is added to replacement basis,” rather than silently mixing alternatives.

The editable entries are loss on shares sold, shares sold at a loss, substantially identical replacement shares, original basis of replacement shares. Use values from the document or measurement that governs this wash sale question; the defaults are only the worked fixture below. The most consequential input mistake would be to ignore that this model uses share counts and one loss lot; options, spouse purchases, IRAs and substantially identical securities require transaction-level review.

IRS Publication 550, Investment Income and Expenses; capital losses, wash sales and investment income documents the convention or governing rule used here. The wash sale output is a transparent scenario under those facts: it does not manufacture an unentered market price, professional determination, carrier quote, legal eligibility finding or locally adopted code value.

What is wash sale calculator?

Wash Sale is the relationship behind this decision: the disallowed loss is attached to replacement-share basis rather than disappearing, while replacement-share holding periods can also carry over. On this page it means disallowed loss = realized loss × min(replacement shares ÷ loss shares sold, 1); disallowed amount is added to replacement basis. This model uses share counts and one loss lot; options, spouse purchases, IRAs and substantially identical securities require transaction-level review; that is the line between the reported quantity and a broader tax analysis.

How to use this calculator.

1. 01Confirm that “disallowed loss = realized loss × min(replacement shares ÷ loss shares sold, 1); disallowed amount is added to replacement basis” matches the wash sale convention you need.
2. 02Replace the fixture values for loss on shares sold, shares sold at a loss, substantially identical replacement shares, original basis of replacement shares with dated values from the governing record.
3. 03Keep all currencies, measurement units and time periods on the same basis before calculating.
4. 04Read estimated wash-sale loss disallowed together with this boundary: This model uses share counts and one loss lot; options, spouse purchases, IRAs and substantially identical securities require transaction-level review.

The formula.

The calculation uses disallowed loss = realized loss × min(replacement shares ÷ loss shares sold, 1); disallowed amount is added to replacement basis. In this wash sale model, the entered terms are loss on shares sold, shares sold at a loss, substantially identical replacement shares, original basis of replacement shares. The disallowed loss is attached to replacement-share basis rather than disappearing, while replacement-share holding periods can also carry over, which is why the relationship is presented under this name rather than as a universal alternative. This model uses share counts and one loss lot; options, spouse purchases, IRAs and substantially identical securities require transaction-level review. Calculations keep full decimal precision through the relationship and round only the returned display values.

A worked example.

Frequently asked questions.

References& sources.

1. [1]IRS Publication 550, Investment Income and Expenses; capital losses, wash sales and investment income. Retrieved 2026-08-07. access: open unless marked otherwise.
2. [2]U.S. Internal Revenue Service. About Form 8949, Sales and Other Dispositions of Capital Assets. Retrieved 2026-08-07. independence: primary; access: open.
3. [3]U.S. Internal Revenue Service. Publication 551, Basis of Assets. Retrieved 2026-08-07. independence: primary; access: open.
4. [4]U.S. Internal Revenue Service. Topic no. 409, Capital gains and losses. Retrieved 2026-08-07. independence: primary; access: open.
5. [5]U.S. Internal Revenue Service. Instructions for Schedule D (Form 1040) — includes the Capital Loss Carryover Worksheet. Retrieved 2026-08-07. independence: primary; access: open.