How to Use the Capital Gains Tax Calculator (2026)

Why Use a Capital Gains Tax Calculator

Selling an investment can trigger two very different tax treatments: short-term gains (held under a year) are taxed as ordinary income, while long-term gains (over a year) get preferential 10%/15%/20% rates — plus the 3.8% NIIT above threshold. The Capital Gains Calculator untangles this in seconds.

How to Use the Capital Gains Calculator in 5 Steps

1. Enter Your Cost Basis

What you paid, including commissions and improvements. The calculator subtracts this from proceeds to find the gain.

2. Enter the Sale Proceeds

The gross amount you received. Gain = proceeds minus basis.

3. Set the Holding Period

Under 12 months → short-term (ordinary rates). Over 12 months → long-term (preferential rates). This single toggle changes the result dramatically.

4. Add Your State

State treatment varies wildly: Texas and Florida add nothing, while California taxes gains as ordinary income at 13.3%, and Washington levies its own 7% capital gains tax. See the capital gains guide.

5. Review After-Tax Proceeds

The calculator shows tax owed, effective rate, and what you keep. Pair with the Federal Bracket tool to see your ordinary-rate alternative.

Worked Example

Sell $50,000 of stock bought for $20,000, held 18 months, with no state gain tax:

• Gain: $30,000.
• Federal long-term tax: $4,500 (15%).
• NIIT (if income over threshold): +$1,140 (3.8% of $30,000).
• You keep ≈ $44,360.

Common Mistakes

• Mislabeling the holding period. A one-day miss under a year flips you to ordinary rates.
• Ignoring state tax. CA, NY, and WA can add double digits.
• Forgetting NIIT. The 3.8% surtax hits high earners on top of the bracket rate.

Who Should Use It

Investors planning a sale, retirees managing withdrawals, and anyone deciding whether to hold past the one-year mark should run the numbers first. See the capital gains guide for strategy.