If you're running a business that touches both the UK and the US, one of the first things that trips people up isn't tax rates or forms — it's the calendar itself. The UK tax year runs from 6 April to 5 April the following year, with Self Assessment returns due by 31 January. The US tax year follows the standard calendar year, from 1 January to 31 December, with federal returns typically due by 15 April. This single difference cascades into nearly every part of how you plan, report, and file across both countries.
For international entrepreneurs, freelancers, and business owners operating across the Atlantic, understanding this gap isn't optional — it directly affects your bookkeeping, your deadlines, and how you avoid paying tax twice on the same income. This guide breaks down exactly how the two systems differ, why they diverge the way they do, and what it practically means for anyone managing a UK-US business.
Why This Matters for International Business Owners
If you're a freelancer invoicing US clients from the UK, a UK-based founder with a US LLC, or an American entrepreneur running a UK limited company, you're not dealing with one tax calendar — you're dealing with two, running on different clocks, governed by two entirely separate tax authorities. Get the timing wrong, and you risk mismatched accounting periods, missed deadlines, or worse, double taxation on income that should only be taxed once.
Understanding both systems side by side isn't just useful — it's foundational to running a compliant, well-organized cross-border business.
The UK Tax Year: Dates and Structure
The UK tax year — sometimes called the fiscal year — runs from 6 April to 5 April the following year. So the "2025/26 tax year" spans 6 April 2025 to 5 April 2026. This unusual start date traces back centuries to calendar reforms in the 1700s, and while there have been periodic calls to modernize it, it remains firmly in place today.
Key UK tax year facts:
- Governing body: HM Revenue and Customs (HMRC)
- Individual filing method: Self Assessment (SA100 form)
- Online filing deadline: 31 January following the end of the tax year
- Paper filing deadline: 31 October following the end of the tax year
- Corporation Tax: UK companies set their own accounting period, often but not always aligned with the tax year, with Corporation Tax due nine months and one day after the end of that period
If you need a full breakdown of every UK filing date, our detailed guide on UK tax return deadlines for 2026 covers registration, filing, and payment dates in depth.
The US Tax Year: Dates and Structure
The US tax year, by contrast, follows the standard calendar year — 1 January to 31 December — for the vast majority of individual taxpayers and many businesses. This is far simpler to remember, since it aligns with the year everyone already uses in daily life.
Key US tax year facts:
- Governing body: Internal Revenue Service (IRS)
- Individual filing method: Form 1040, U.S. Individual Income Tax Return
- Standard filing deadline: 15 April of the following year
- Extension available: Form 4868 grants an automatic six-month extension to 15 October
- Business tax year: Many businesses use the calendar year, but corporations can elect a fiscal year ending on the last day of any month, provided it's used consistently
If you're specifically weighing extension options for a missed or looming US deadline, our guide on filing Form 4868 step-by-step walks through the exact process.
UK Tax Year vs US Tax Year: Side-by-Side Comparison
Here's the complete picture at a glance:
Start Date — UK: 6 April | US: 1 January
End Date — UK: 5 April (following year) | US: 31 December
Governing Authority — UK: HMRC | US: IRS
Individual Filing Deadline — UK: 31 January (online) | US: 15 April
Individual Filing Form — UK: SA100 (Self Assessment) | US: Form 1040
Extension Option — UK: Reasonable excuse appeal only | US: Automatic 6-month extension via Form 4868
Business Tax Year — UK: Company's chosen accounting period | US: Calendar year or elected fiscal year
Corporate Filing Deadline — UK: 9 months + 1 day after accounting period ends | US: 15th day of the 4th month after year-end (typically 15 April for calendar-year filers)
Why Does the UK Tax Year Start in April?
This is a genuinely common question, and the answer is historical rather than practical. The UK originally used 25 March (Lady Day) as the start of the legal year under the Julian calendar. When Britain switched to the Gregorian calendar in 1752, eleven days were "lost" to align the calendars, and to avoid short-changing the Treasury on a full year's tax revenue, the start of the tax year was pushed to 5 April — later adjusted again to 6 April in 1800 to account for a leap year discrepancy. It's stuck ever since, despite periodic proposals in Parliament to modernize it to align with the calendar year.
Why Does the US Tax Year Follow the Calendar Year?
The US tax year's alignment with the calendar year is far more straightforward — it was simply designed for administrative simplicity when the modern federal income tax system was established in 1913 following the 16th Amendment. Using the calendar year that everyone already tracks in daily life made recordkeeping, payroll, and reporting more intuitive for both taxpayers and the newly formed IRS.
How the Tax Year Difference Affects Business Accounting
For a business operating in just one country, this is a non-issue — you simply follow your local tax year and move on. But for international business owners, the mismatch creates real accounting complexity:
- Income earned in the UK's 2025/26 tax year spans two different US calendar years (part of 2025 and part of 2026), meaning it may need to be split and reported differently across each country's return.
- Bookkeeping software and internal reporting periods often need to run dual calendars — one for UK compliance, one for US compliance — rather than a single unified ledger.
- Estimated tax payments in the US follow a quarterly schedule based on the calendar year, while UK payments on account follow the January/July split tied to the UK tax year, so business owners juggling both need to track two entirely separate payment rhythms.
A common approach among cross-border business owners is to maintain accounting records on a monthly basis internally, then map that data into whichever tax year format each country's filing requires — rather than trying to force one calendar to fit both systems.
Do International Business Owners Need to File Taxes in Both Countries?
This depends heavily on your residency status, citizenship, and where your income is sourced — but broadly, yes, many international business owners do have filing obligations in both countries simultaneously.
US citizens living or operating in the UK generally must file a US tax return regardless of where they live, due to the United States' citizenship-based taxation system — one of the few countries in the world that taxes based on citizenship rather than residency alone.
UK residents with US business interests — such as owning a US LLC — typically have US filing obligations tied to that entity, separate from their UK personal tax obligations.
Non-US citizens with US-sourced income (say, a UK freelancer invoicing American clients) may have limited US tax obligations, often addressed through withholding rather than a full US return, depending on the nature of the income and any applicable treaty provisions.
If you're structuring a US business from outside the US, our guides on registering a US LLC from Pakistan and EIN vs ITIN for US LLCs cover the foundational registration and tax ID requirements non-US founders typically need.
The US-UK Tax Treaty: Avoiding Double Taxation
Fortunately, business owners aren't left to sort out overlapping obligations entirely on their own. The US-UK Tax Treaty (formally the Double Taxation Convention) exists specifically to prevent the same income from being taxed twice by both countries. It does this primarily through two mechanisms:
- Foreign Tax Credit — Allows US taxpayers to credit tax paid to the UK against their US tax liability on the same income, reducing double taxation.
- Foreign Earned Income Exclusion — Allows qualifying US citizens living abroad to exclude a portion of foreign-earned income from US taxation entirely, up to an annually adjusted limit.
The treaty also addresses which country has primary taxing rights over specific income types — business profits, dividends, royalties, and pensions are treated differently depending on residency and the nature of the income. Because treaty provisions are genuinely complex and depend heavily on your specific circumstances, this is an area where professional guidance consistently pays for itself in reduced tax liability and avoided penalties.
FATCA and Reporting Requirements for Cross-Border Business Owners
Beyond income tax itself, US citizens and certain US-connected businesses face additional reporting obligations under the Foreign Account Tax Compliance Act (FATCA). This requires disclosure of foreign financial accounts exceeding certain thresholds, and non-compliance carries steep penalties. Related to this, US persons with foreign bank accounts exceeding $10,000 at any point during the year may also need to file an FBAR (Foreign Bank Account Report) with the US Treasury, separate from their standard tax return.
These requirements apply regardless of which country's tax year you're using for your primary filings, so it's worth building them into your annual compliance checklist from the start rather than discovering them after the fact.
UK Corporation Tax vs US Corporate Tax Filing
If you're operating through a company rather than as an individual, the structural differences deepen further.
UK Corporation Tax:
- Based on the company's own accounting period, which the company chooses (often, but not always, matching the tax year)
- Due nine months and one day after the end of that accounting period
- Filed via Company Tax Return (CT600), separate from the Self Assessment system used for individuals
US Corporate Tax:
- Calendar-year filers must file by 15 April (or the 15th day of the 4th month after fiscal year-end for non-calendar-year filers)
- Filed via Form 1120 for C-corporations, with different forms for S-corporations and partnerships
- Extensions available via Form 7004, similar in spirit to the individual Form 4868 extension
If you're deciding between operating as an S-corp or C-corp for a US-facing business, our guide on S-corp vs C-corp tax deadlines breaks down how these filing structures and dates differ further.
Practical Tips for Managing Both Tax Years
If you're running a business across both jurisdictions, a few practical habits make a meaningful difference:
- Maintain monthly, not annual, bookkeeping. This lets you slice your data into either country's tax year format without redoing your records from scratch.
- Mark both sets of deadlines on one shared calendar. Treat 31 January (UK) and 15 April (US) as equally critical dates, since missing either creates separate penalty exposure.
- Track currency conversion consistently. Use a consistent methodology (average annual rate, spot rate, or another IRS/HMRC-accepted method) for converting income and expenses between GBP and USD.
- Work with an accountant experienced in both systems. Cross-border tax rules — treaty benefits, foreign tax credits, entity structuring — are genuinely complex enough that DIY approaches often cost more in missed savings than professional fees would.
- Understand your entity structure's tax year flexibility. Some structures (like certain LLCs) offer more flexibility in aligning fiscal periods than others.
If you're still deciding on the right business entity for a cross-border operation, our comparison of LLC vs Ltd for e-commerce businesses is a useful starting point for understanding how structure affects both tax year and tax filing obligations.
Frequently Asked Questions
What is the UK tax year and when does it run? The UK tax year runs from 6 April to 5 April of the following year. For example, the 2025/26 tax year spans 6 April 2025 to 5 April 2026, with Self Assessment returns typically due by 31 January 2027.
What is the US tax year and when does it run? The US tax year follows the standard calendar year, from 1 January to 31 December, with federal individual tax returns typically due by 15 April of the following year.
Why is the UK tax year different from the US tax year? The UK tax year's April start dates back to historical calendar reforms in the 1700s, while the US tax year follows the calendar year for administrative simplicity, established when the modern federal income tax system began in 1913.
Do international business owners need to file taxes in both countries? Often, yes. US citizens generally must file US returns regardless of residency due to citizenship-based taxation, while UK residents with US business interests or US-sourced income may also face filing obligations in both jurisdictions.
Is there a tax treaty between the UK and US to avoid double taxation? Yes. The US-UK Tax Treaty (Double Taxation Convention) allows mechanisms like the Foreign Tax Credit and Foreign Earned Income Exclusion to prevent the same income from being taxed twice by both countries.
How does the tax year difference affect business accounting? It requires maintaining records that can be mapped into two different reporting periods, since UK and US tax years don't align, and managing two separate sets of deadlines, estimated payment schedules, and filing requirements simultaneously.
Final Thoughts
Navigating two different tax years isn't just a scheduling inconvenience for international business owners — it shapes how you structure your bookkeeping, plan your cash flow, and stay compliant across two entirely separate tax authorities. The UK's April-to-April calendar and the US's January-to-December calendar rarely align conveniently, which is exactly why proactive planning, monthly-level recordkeeping, and treaty awareness matter so much for anyone operating across both systems.
If you're building or managing a cross-border business, our guides on sole proprietorship vs LLC tax differences and federal tax vs state tax offer further groundwork for understanding how US structures interact with your obligations. For authoritative details on each system, you can also refer directly to HMRC on GOV.UK and the IRS's official tax year guidance.
Managing tax obligations across two countries doesn't have to be overwhelming. Explore Advanced Services at Brandora Services to get expert cross-border tax support tailored to your UK-US business, so you stay compliant and confident on both sides of the Atlantic.