Seafarers' Earnings Deduction: How the 365-Day Rule Actually Works (2026)

The Seafarers' Earnings Deduction is one of the most valuable reliefs in the UK tax code: a 100 per cent deduction against your employment earnings. It is also one of the most misunderstood, because qualifying does not depend on a single number. It depends on building an eligible period of at least 365 days that survives two separate tests applied every time you come home.

Accurate as of 24 August 2026. Every figure on this page is quoted from the primary source linked in Sources and was checked on that date.

The first test: no single visit over 183 days

Per HMRC helpsheet HS205, Seafarers' Earnings Deduction, an eligible period is a chain of days abroad and permitted UK return visits. The first hard rule is that no single return visit to the UK may last more than 183 consecutive days. In HMRC's own words, the condition is that "no single return visit lasts for more than 183 consecutive days". Break it, and the chain snaps at that point.

The second test: the half-test, and why it catches people

This is the one that quietly disqualifies otherwise careful seafarers. At the end of each return visit, your cumulative days in the UK must not exceed one half of the days elapsed from the first day of the eligible period to the end of that visit.

HMRC's own worked example makes the trap explicit: 183 days abroad followed by 182 days in the UK does not qualify. Count it through. The elapsed period is 365 days, and 182 UK days is just under half, so it looks safe. It is not, because the test is applied at the end of the return visit and the arithmetic fails. The lesson is that "roughly half my time abroad" is not the standard. The standard is a specific ratio, tested at a specific moment.

Building the period in practice

• The 365 days are a minimum, not a target. An eligible period can run for years, and a longer period gives the half-test more headroom.
• The tests are applied at each return visit, not once at the end. A period that looks fine in the aggregate can fail at a specific point mid-chain.
• You must keep evidence. Discharge books, contracts of employment, travel documents and dated records of every departure and return are what an enquiry will ask for.
• Residency of an EEA state can also qualify you, not only UK residence. Check your own position against HS205.

Two common misconceptions

First, working on a vessel is not the same as being a seafarer for this relief. If you work on an offshore installation, you are excluded outright regardless of your days, which is a separate article because so many rotational workers assume the opposite.

Second, the SED governs UK tax on your earnings. It does not decide your UK tax residency, which runs on the Statutory Residence Test and a different clock, the 6 April to 5 April tax year. Seafarers routinely need to track both at once.

Counting these days automatically

A 365-day chain, tested at every return visit, with a ratio that has to hold at each one, is not something to reconstruct from memory at the end of a tax year. The Days Monitor iPhone app logs your days by country and state automatically, runs custom rules with rolling windows of any length, warns you before you cross a line, and exports a timestamped PDF or CSV record if you are ever asked to evidence it. Download it on the App Store.

Sources

• HMRC helpsheet HS205, Seafarers' Earnings Deduction (accessed 24 August 2026)
• HMRC Employment Income Manual EIM33101 (accessed 24 August 2026)

All sources checked 24 August 2026.

This article is general information, not tax, legal or immigration advice. Day-counting rules interact with treaties, your immigration status and your individual facts. Verify current rules with the relevant authority and take advice from a qualified professional before relying on any threshold.