Every application for a Certificate of Competency, an endorsement or a Notice of Eligibility turns on one number: how many days you have actually served on board. Being out by a few days is enough to have the paperwork sent back. This sea time calculator adds up any number of contracts, counts both the sign-on and the sign-off day, converts the total into months at the standard 30-day rate, and flags overlapping dates before an examiner does.
Everything runs in your browser. Your dates are never sent anywhere, and they are remembered on this device so you can build up a full service record across several sittings.
Add one row per contract. Both the sign-on and sign-off dates are counted as days on board.
This tool is for checking and planning. Your discharge book (CDC) entries and sea service testimonials remain the official record.
How to use this sea time calculator
- Enter the sign-on and sign-off dates for your first contract, exactly as they appear in your discharge book.
- Add the vessel name and your rank if you want the summary to match your service record. Both are optional.
- Press + Add another voyage for each further contract. There is no limit on how many you add.
- Read the totals: days at sea, the same figure expressed in months, and the decimal equivalent that most application forms ask for.
- Use Copy summary to paste the full breakdown into an email or an application draft.
How sea time is counted under STCW
Both end dates count. A contract running from 1 March to 30 March is 30 days of sea service, not 29. The sign-on day and the sign-off day are each counted as a full day on board, which is the convention applied by most administrations. If yours works differently, untick the option below the voyage list and the calculator will count the difference between the dates instead.
A month means 30 days. STCW defines a month as a calendar month, or 30 days made up of periods of less than one month. In practice that means part-contracts are aggregated at 30 days to the month, so 180 days of service is treated as six months. The calculator shows both the whole-month figure and the decimal equivalent, because application forms ask for one or the other.
Overlapping contracts are counted once. If two periods you have entered share any days, the total is reduced accordingly and a warning appears. Overlaps are almost always a transcription error made while copying dates out of a discharge book, and they are exactly the sort of thing that gets spotted at the counter.
Gaps are shown but not counted. Time between contracts appears in the breakdown as days ashore, so you can see the shape of your service at a glance without it inflating the total.
What this sea service calculator does not decide
It does the arithmetic. It does not tell you whether you qualify. How much sea service each certificate requires differs by administration and by grade, and the requirements are revised from time to time — so check the current circular from the authority you are applying to rather than any figure quoted on a website, this one included. In India that is the Directorate General of Shipping; in the UK, the Maritime and Coastguard Agency.
The same applies to what counts as qualifying service. Watchkeeping time, periods in dry dock, lay-up, standby and time served on vessels below a given tonnage or power are all treated differently depending on the administration and the certificate. This tool totals whatever periods you decide to enter — the judgement about which periods belong there is yours.
Sea time calculator FAQs
Does the sign-on day count as sea time?
Under the convention used by most administrations, yes — both the sign-on and the sign-off day are counted as full days on board. A contract from 1 March to 30 March is therefore 30 days, not 29. This calculator counts both by default, and you can switch it off if your administration tells you otherwise.
How many days is one month of sea time?
Thirty. STCW defines a month as a calendar month, or 30 days made up of periods of less than one month, so part-contracts are aggregated at 30 days to the month. That is why 180 days is treated as six months of sea service.
Is this an STCW sea time calculator?
It applies the STCW definition of a month — a calendar month, or 30 days made up of periods of less than one month — so the totals line up with the way sea service is expressed on STCW-based applications. It is not tied to any one administration, though. What counts as qualifying service is decided by whoever issues your certificate, and you should confirm that against their current circular.
What happens if two of my contracts overlap?
Overlapping days are counted once, not twice. The calculator flags the overlap and shows the corrected total, because a duplicated period is usually a typing error when copying dates out of a discharge book.
Does time in dry dock, lay-up or standby count?
It depends on your administration and on the certificate you are applying for. Some count it in full, some at a reduced rate, and some not at all. This calculator totals whatever periods you enter — decide what qualifies by checking the current circular from your administration before you rely on the figure.
Does leave count towards sea service?
Generally no. Sea service is the time actually served on board as recorded in your discharge book, so leave between contracts is excluded. The calculator shows the gaps between your contracts separately so you can see the periods ashore at a glance.
Can I submit this as proof of sea time?
No. Your Continuous Discharge Certificate entries, sea service testimonials and the company letters remain the official record. Use this tool to check the arithmetic and to see where you stand before you apply.