Seafarers Earnings Deduction: The Six Tests, in Order

The Seafarers Earnings Deduction, usually shortened to SED and sometimes called the seafarers allowance, is a 100% deduction from the employment earnings you get for working as a seafarer. It sits in the Income Tax (Earnings and Pensions) Act 2003 at sections 378 to 385. Qualify, and the income tax on those earnings comes down to nil. It is entered on your own self assessment tax return, in the same way as any other deduction from employment income.

Whether you qualify comes down to six tests. HMRC's helpsheet HS205 walks them in a fixed order, and so does everything below: the 365-day eligible period, the midnight test, the 183-day cap on a single UK visit, the half-day rule, the requirement that your duties are performed on a ship rather than an offshore installation, and the requirement that you are an employee. Each one is a separate hurdle. Failing any single one ends the claim for that period, so take them one at a time.

Test one: an eligible period of at least 365 days

The condition is a period of at least 365 days made up mainly of days on which you were absent from the United Kingdom. That period is the thing HMRC tests. It is not a rule that you must spend a set number of days abroad, and it is not a tax-year rule either: an eligible period can start on any day and run across as many tax years as it needs to.

An eligible period begins on the first day of a period of absence and continues for as long as the arithmetic in tests three and four holds. Once it has run for 365 days or more, you have an eligible period, and the earnings from the qualifying seafaring employment that fall within it attract the deduction. If the arithmetic breaks on day 300, that period is dead and the count restarts from the next qualifying day of absence. Nothing is banked.

Two consequences follow that catch people out. First, a period can run for far longer than 365 days, and often does: a career at sea with short leave periods can hold one continuous eligible period across several years. Second, because the period runs across tax years, the return you file for any one year depends on days that fall in the years either side of it, which is why the running worksheet matters more than the annual return does.

Test two: the midnight test decides what counts as a day of absence

A day counts as a day of absence from the UK only if you are outside the UK at midnight at the end of that day. That is the whole rule. Where you were at noon, how long you spent in the air, and how much of the day you worked make no difference at all.

So a flight home that lands at 22:00 turns that whole day into a UK day. A flight out that leaves at 22:00 and clears UK airspace before midnight makes it a day of absence. Over a long eligible period, the pattern of departure and arrival times matters as much as the pattern of contracts, and crew who join and leave ships at odd hours can find their day count comes out very differently from their rota count.

Counting your days

The practical work is a running list of date pairs: the date you left the UK and the date you returned, for every trip in the period, with midnight as the boundary in each case. From that list you get three numbers you need at any moment: the number of days elapsed since the eligible period began, the total UK days inside it, and the length of the longest single UK visit. The HS205 worksheet is laid out to hold exactly that, and a spreadsheet with one row per crossing does the same job. Update it when you travel rather than in January, because reconstructing a year of border crossings from memory is where most claims come apart.

Test three: no single UK visit longer than 183 consecutive days

Return visits to the UK are allowed and expected. The limit is that no single visit to the UK may exceed 183 consecutive days. Exceed 183 consecutive days in the UK at any point and the eligible period ends there.

This is the figure most often quoted back at us as if it were the condition itself, usually as "you have to be out of the country for 183 days". It is not. It is a ceiling on one visit, and on its own it decides nothing about whether you qualify. A seafarer can be in the UK for well under 183 consecutive days on every single visit and still fail, because the test that runs alongside it is the next one.

Test four: the half-day rule, tested at every return

Across the whole eligible period, your total UK days must not exceed one half of the days elapsed since the period began. The comparison is made at the end of each return visit to the UK, so it is a rolling test, not a year-end one.

Written as a fraction: at the end of any UK visit, take the total number of days you have spent in the UK since the period started and divide it by the total number of days that have elapsed since the period started. If that comes to more than one half, the eligible period breaks on the last day of that visit. If it comes to one half or less, the period continues.

Worked in numbers: suppose 400 days have elapsed since your eligible period began and you have spent 190 of them in the UK. One half of 400 is 200, and 190 is below it, so the period survives. Take one more leave period of 15 days: 205 UK days against 415 elapsed, against a half of 207.5, and it still survives, but with very little room left. That is the shape of the rule. It tightens when you take leave and loosens with every day at sea, and it is the reason a claim can fail after several successful years without anything about the job changing.

Test five: a ship, not an offshore installation

Your duties must be performed wholly or partly on a ship, on a voyage that begins or ends outside the United Kingdom. And an offshore installation is not a ship. Rig, platform, floating production storage and offloading vessel, accommodation unit: if you work on an installation, you are excluded, however seagoing the job feels and however far offshore you are stationed.

This is the single most common disqualifier, and it disqualifies people who have every other test comfortably covered. It also catches vessels that change role. A drillship or a construction vessel can be a ship on passage and an installation while it is on station, and what matters is what the vessel was doing during the period you worked on it, not what it is called.

The voyage condition catches a second group. For this deduction, the UK sector of the North Sea counts as the United Kingdom. A vessel working only between UK ports, or only within that sector, is not on a voyage that begins or ends outside the UK, so the days spent doing it do not support a claim even though the vessel is plainly a ship.

Test six: employees only

The deduction is available only to employees. If you work at sea on a self-employed basis, as many marine contractors, delivery skippers and freelance engineers do, you cannot claim it. You are an ordinary self-employed trader, your income is taxed as trading profit, and your position is governed by the ordinary rules on business expenses rather than by anything in this deduction.

Being paid gross, being paid in a foreign currency, or being paid by an employer with no UK presence does not make you self-employed, and it does not stop you being an employee for these purposes either. Status turns on the substance of the engagement. If your contract calls you a contractor but the working reality is employment, that is worth resolving properly before a claim is entered, because status is the foundation the whole claim rests on.

Residence: UK or EEA

You also have to be resident in the United Kingdom or in a European Economic Area (EEA) state for the year of the claim. Residence is decided by the statutory residence test, and if you leave or return part-way through a tax year, split-year treatment can change the picture. Residence has its own body of rules and it is not taught here. What matters for the deduction is that it is a condition in its own right: a seafarer who has passed all six tests above but is not UK or EEA resident for the year does not have a claim for that year.

Worked example: a full year at 2026/27 rates

Bilal is a second engineer based in Plymouth, employed on a merchant vessel on deep-sea trades. In 2026/27 he earns £54,000 from that employment and has no other income. His eligible period started in an earlier year and is still running: his longest UK visit in it was 41 days, and at his last return home his UK days stood at well under half the days elapsed.

The figures below use a personal allowance of £12,570, a basic rate of 20% on the first £37,700 of taxable income and a higher rate of 40% above that. Those bands are locked at their 2025/26 levels and were still current when this page was checked in August 2026.

Without the deduction, Bilal's taxable income is £54,000 minus the £12,570 personal allowance, which is £41,430. The first £37,700 of that is taxed at 20%, giving £7,540. The remaining £3,730 is taxed at 40%, giving £1,492. His income tax for the year is £9,032.

With the deduction, the full £54,000 of seafaring earnings is deducted from his employment income. That leaves him with nil taxable income for the year and an income tax liability of £0. If his employer operated PAYE on those earnings through the year, roughly £9,032 will have been paid over already, and the self assessment calculation shows the year's liability as nil, so HMRC settles the difference through the normal self assessment process.

The arithmetic hides something on each side. The deduction applies to the seafaring earnings, so if Bilal also had £6,000 of UK rental profit, that income would still be taxed in the ordinary way and would use his personal allowance. And nothing above says anything about his National Insurance, which is the next section and a different question entirely.

NIM29000: the mariners National Insurance rules the deduction does not touch

Qualifying for the deduction tells you nothing about your National Insurance position, and it is a mistake to read one across to the other. Mariners have their own National Insurance rules: special Class 1 provisions that turn on matters such as the ship's flag, where your employer has its place of business, and where the employment is domiciled. HMRC sets them out in its National Insurance manual from NIM29000 onward.

There is no general answer here, and anyone giving you one without reading your contract and knowing your employer's position is guessing. Foreign-flag and foreign-employer cases in particular need checking on their own facts. What we will say is that the National Insurance question should be asked separately and answered from the documents, because it affects your state pension record and any contributions your employer is or is not obliged to make.

Putting the claim on the return

The deduction goes on the employment pages of your self assessment return, against the seafaring employment, with the box confirming a Seafarers Earnings Deduction claim ticked. Work from HS205 and its worksheet, which is where the eligible period is recorded and where the arithmetic in tests one, three and four is shown.

Keep the evidence for the day count with your records: the worksheet itself, your discharge book or equivalent record of sea service, contracts of employment, and travel documents for every departure and return. HMRC can and does test the day count, and the deduction stands or falls on being able to show where you were at midnight on each day in the period.

We prepare self assessment returns for seafarers and enter the deduction as part of the return, on a fixed fee for the work. We do not process claims on your behalf, take an assignment or nomination, or charge a percentage of anything. If you want the audience-level view of how we work with crew, including how the position differs for self-employed marine contractors, see our page for seafarers and yacht crew. For the mechanics of the return itself, deadlines and payments on account, start with our self assessment page.