NIIT calculator and formula
By Jude Wallis
NIIT in this identity is the supplied net investment income slice times 3.8 percent. A $20,000 slice produces $760 of tax. The input already represents the amount subject to the rate.
Net investment income tax
$760.00
3.8 percent of a $20,000.00 slice.
- NII slice
- $20,000.00
- Rate
- 3.8%
- NIIT
- $760.00
The lesser-of slice already chosen. This page does not apply the MAGI gate.
The federal NIIT rate is the default. The taxable slice must already reflect the lesser-of test.
Policy inputs checked August 20, 2026
Net investment income tax rate
United States federal. Effective January 1, 2013 until superseded. Checked August 20, 2026. Review due by November 20, 2026.
The tool applies 3.8 percent to a supplied taxable slice. The lesser-of test and filing threshold are determined before that slice is entered.
- IRS Publication 550, Investment Income and Expenses: NIIT is 3.8 percent of the lesser of net investment income or the amount by which MAGI exceeds the filing threshold.
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On this page
The formula
NII is the supplied net investment income slice. Multiply it by the entered 3.8 percent rate.
Apply 3.8 percent to the supplied slice
$20,000 times 0.038 is $760. How NIIT works explains how the relevant slice is determined.
A smaller supplied slice
$5,000 times 0.038 is $190. The sheet supplies the slice already, so this identity starts from that amount.
The base is not total income
The input is the net investment income amount subject to this calculation. Net investment income tax is separate from the ordinary capital gains tax calculation.
Scope of the tax
This page multiplies a supplied slice by the entered rate. Threshold testing and classification happen before this identity. This is educational material, not financial advice.
Worked examples
\$20,000 supplied slice
The supplied net investment income slice is $20,000 and the rate is 3.8 percent. What is NIIT?
- Write 3.8 percent as 0.038.
- Multiply , so $760.
$20,000 of supplied net investment income at 3.8 percent produces $760 of NIIT.
\$5,000 supplied slice
The supplied net investment income slice is $5,000 and the rate is 3.8 percent. What is NIIT?
- Write 3.8 percent as 0.038.
- Multiply , so $190.
$5,000 of supplied net investment income at 3.8 percent produces $190 of NIIT.
Applying the rate to an unscreened total
This identity expects the applicable net investment income slice. Feeding it a different income measure answers a different question.
Common questions
Does this page find the applicable slice?
No. The input already supplies the slice for the multiplication.
What rate do the examples use?
Both use 3.8 percent.
Is this financial advice?
No. It is educational material for the NIIT multiplication.
Keep reading
This page is educational material, not financial advice. The figures come from the formula shown and assume the inputs you enter hold for the whole term. Your own rate, fees, taxes and timing will differ, so treat the output as arithmetic to check a decision against, not as a recommendation.