The 3.8% nobody warned you about: net investment income tax from Britain

Figures relate to tax year 2025 (US)

Your salary is taxed in Britain at the top rate, and your foreign tax credits cover the American income tax in full. Then a separate line appears on the return: 3.8% on your investment income. The net investment income tax is the one charge those credits cannot touch.

It surprises people every year, and it surprises them again in a catch-up filing covering several years at once. So this guide covers who it hits, what counts as investment income, and the few things that genuinely reduce it.

Key takeaways

• The charge is 3.8% on investment income above an income threshold.
• Thresholds start at $200,000 single and $250,000 for a joint return.
• Foreign tax credits cannot offset it, because it sits outside the income tax rules.
• The foreign earned income exclusion does not reduce it either, though it affects the threshold test.
• A separate 0.9% Medicare charge applies to wages and self-employment above the same thresholds.
• Back-year filings often produce several years of this charge at once.

What is the net investment income tax?

It is a 3.8% charge on investment income for higher earners, introduced separately from the income tax system. According to IRS guidance, it applies to the smaller of your net investment income or the amount by which your income exceeds the threshold for your filing status.

The thresholds are not indexed. They have stayed at $200,000 and $250,000 since the charge began, so more people cross them every year as incomes rise.

What counts as investment income?

Interest, dividends, capital gains, rental profit and royalties, broadly. Income from a business you actively run usually falls outside. British sources count in exactly the same way as American ones, so a UK savings account and a UK rental property both sit inside the charge.

Why foreign tax credits do not help

Because the charge sits outside the part of the code that allows those credits. The credit rules relieve income tax, and this is a separate levy with its own section. So British tax on the same dividend does nothing to reduce it.

That is the whole reason the charge feels unfair to people in Britain. They pay more tax here than they would at home, and this amount survives anyway.

In our practice it is the most common reason a return still shows a balance due after credits. Nothing is wrong with the figures.

How the exclusion interacts

The foreign earned income exclusion does not reduce the charge, and it does not remove excluded income from the threshold test. So someone excluding salary can still cross the line and pay 3.8% on their investments.

That catches people who assume the exclusion solved everything. It shelters income tax on earnings, and it leaves this charge untouched.

The extra 0.9% on earnings

A separate Medicare charge applies to wages and self-employment income above the same thresholds. It runs at 0.9%, and it applies to earnings rather than investments. Between them, the two charges reach most parts of a high earner's income.

Employers withhold it automatically in America. A British employer does not, so the amount usually appears as a balance on your return rather than being collected through the year.

Where a certificate of coverage puts you inside British National Insurance, self-employment income escapes the American social security charges. Our guide to the certificate of coverage explains how that works.

Who actually pays it?

Higher earners with investments outside a pension. A single filer over $200,000 or a couple over $250,000, holding dividends, interest, rental profit or gains. Below those lines the net investment income tax simply does not arise, whatever the investments look like.

One-off events push people over the line more often than salaries do. A share sale, a property disposal or a large bonus year can all cross the threshold for a single year.

That is why the charge feels random. It appears in a good year and vanishes in a quiet one.

How is it calculated?

On the smaller of two figures: your net investment income, or the amount your income exceeds the threshold. So a couple £10,000 over the line with £70,000 of investment income pay 3.8% on £10,000, not on the larger figure.

That cap protects people who cross the threshold narrowly. It also means a modest pay rise can bring the charge into view for the first time.

A worked example

The figures below are illustrative. Take an example: an American couple in London with £220,000 of combined salary, £30,000 of UK dividends and a £40,000 gain on selling shares in 2025.

Their income comfortably exceeds the joint threshold. Investment income of roughly £70,000 therefore falls inside the charge, producing about £2,660 of additional American tax.

British tax on the dividends and the gain reduces their American income tax to nothing. It does nothing at all to this 3.8%, which stands whatever they paid here.

Why back-year filings hurt more

Because the charge repeats for every year in the filing. A catch-up covering three years can produce three separate amounts, each with interest running from the original due date. Nothing about a compliance program removes the underlying tax.

High earners feel this most. A streamlined filing that looked like a formality can produce a real bill once investment income is added across several years.

Our guide to what a streamlined filing costs covers the wider budget for a catch-up.

Does a UK company change the position?

It can, and not always helpfully. Dividends you take from your own British company are investment income, so they sit inside the charge. Salary from the same company does not, though it counts toward the threshold and carries the separate Medicare charge instead.

So the salary and dividend mix matters more for an American owner than for a British one. The British answer favours dividends. The American answer often does not.

Model both before setting the mix for the year. A split that saves £3,000 here can cost more than that across the Atlantic.

What about jointly held investments?

The answer follows how you file. A joint return uses the $250,000 threshold and pools the income. Filing separately drops the threshold sharply, which often makes the charge worse for a couple with one American spouse.

Many couples in Britain file separately for other reasons, particularly where one spouse is not American. Check what that choice does to this charge before settling on it.

What actually reduces the charge?

Fewer things than people hope. Investment expenses properly allocated against the income reduce the base, and losses can offset gains. Holding assets inside a pension keeps income outside the charge entirely, which is the cleanest structural answer.

1. Total your investment income for the year, from both countries.
2. Check whether your income crosses the threshold for your filing status.
3. Allocate any investment expenses properly against that income.
4. Apply capital losses against gains before calculating the charge.
5. Consider whether assets belong inside a pension rather than a taxable account.
6. Complete Form 8960 with the return, and keep the allocation workings.
7. Model the timing of large disposals, because one year can carry the whole charge.

Does a pension help?

Yes, and it is the most reliable planning tool here. Income and gains inside a qualifying pension are not investment income for this purpose, so they sit outside the charge. Distributions later are also excluded, although they count toward the threshold test.

That makes a workplace pension or a personal pension more attractive than a taxable account for anyone near the thresholds. It also avoids the fund reporting problems that British investments create.

What about selling a home?

The gain can fall inside the charge once any main residence exclusion is used up. A large gain on a British property sale is a classic trigger, because it often lands in a single year and pushes income well past the threshold.

Timing can help where a sale is flexible. Splitting a disposal across tax years, or pairing it with realized losses, sometimes keeps part of the gain below the line.

Does the treaty override the charge?

No. The treaty allocates taxing rights over income, and the saving clause preserves America's right to tax its citizens. Nothing in it removes this particular levy for someone who holds a US passport and lives in Britain.

Some argue the position should be different, and the point has been debated for years. In practice, returns are prepared on the basis that the charge applies.

What records should you keep?

A schedule of investment income by type and source, with the tax paid on each. Add any investment expenses you allocate, and the basis records behind every disposal. The net investment income tax calculation depends on all three.

Keep them by calendar year rather than by British tax year. That saves rebuilding the figures each spring when the American return comes round.

When should you plan for it?

Before the year ends, not when the return is prepared. Once a disposal has completed or a dividend has been paid, the charge is fixed. The planning levers all sit in the months before that point.

Three questions cover most of it. Will you cross the threshold this year? Can any income sit inside a pension instead? And can a large disposal wait until January?

We run those questions with clients each autumn. They take twenty minutes and they occasionally save several thousand pounds.

Mistakes and penalties we see with this charge

• Assuming foreign tax credits cover it, then missing the balance due entirely.
• Leaving UK bank interest out because the amounts looked trivial.
• Forgetting that excluded salary still counts toward the threshold.
• Treating rental profit as active without the facts to support it.
• Ignoring the charge in back years, which produces interest on top.
• Holding funds in a taxable account when a pension would have sheltered them.

The penalties follow the unpaid amount rather than the charge itself. Interest runs from the original due date, and an understated return can carry an accuracy penalty as well.

None of it is hard to avoid once the charge is on the checklist. The problem is that most returns never put it there until a notice arrives.

How US UK Tax Accountants helps

We calculate the charge properly, allocate expenses where the rules allow, and plan the timing of large disposals. For catch-up filings we show the whole multi-year position before you commit. If your return keeps showing tax despite heavy British tax, get in touch and we will review it alongside your US federal returns.

Last reviewed 24 September 2026. This article is general information and not personal tax advice. Thresholds and rules change, so check the current position for your filing year.