Checked against 46 CFR Part 69 and Coast Guard tonnage guidance · September 2026
“I work on a 75-foot boat. What tonnage license do I need?”
The vessel's length alone cannot answer that question. A 75-foot vessel is not necessarily 75 gross tons, and a 100-ton license does not mean you can operate any vessel up to 100 feet long.
For Coast Guard licensing, you need to know the vessel's official gross tonnage, along with the operating route and the type of service involved.
What does tonnage mean for your license?
For this discussion, only two tonnage terms need your attention:
| Term | What you need to know |
|---|---|
| GRT — Gross Register Tonnage | A measure of vessel volume under U.S. measurement rules. This is the tonnage used in familiar national license designations such as 100, 200, 500, and 1,600 tons. |
| GT — Gross Tonnage, often identified as ITC | A separate volume-based measurement under international rules. You may encounter it on vessel documents and STCW endorsements. Higher-tonnage national endorsements may show both, such as 1,600 GRT / 3,000 GT. |
Neither figure tells you how much the vessel weighs. A vessel may have both GRT and GT assigned, and those numbers can differ considerably.
Always keep the letters with the number. “99 GRT” and “250 GT” describe different measurements, even when they appear on the same vessel's paperwork.
You may also hear displacement, deadweight, or net tonnage discussed aboard a vessel. Those figures serve other purposes. When checking your deck-license tonnage limit, look for the applicable gross tonnage.
Why doesn't length determine tonnage?
Picture two boats of equal length. One has a narrow hull and a small cabin. The other has a broad hull, greater depth, and several enclosed decks. They are equally long, but they do not contain the same amount of space.
Tonnage measurement considers more than length. The Coast Guard's simplified system, for example, multiplies a vessel's length, breadth, and depth. Here is a rough illustration using that formula for two 75-foot boats:
| Boat A: narrow and shallow | Boat B: broad and deep | |
|---|---|---|
| Length | 75 ft | 75 ft |
| Breadth | 20 ft | 26 ft |
| Depth | 8 ft | 12 ft |
| Approximate gross tonnage | about 80 GRT | about 157 GRT |
Illustration only. Based on the simplified formula for a vessel not designed for sailing (0.67 × length × breadth × depth ÷ 100, from 46 CFR Part 69, Subpart E). Your vessel's documented tonnage is the figure that counts.
A shorter, broader vessel can have more gross tonnage than a longer, narrower one. There is no dependable feet-to-tons conversion for choosing a license.
Where do you find your vessel's tonnage?
Start with the captain, owner, or company office. Ask:
“What are this vessel's official gross tonnage figures, and can I see the document showing them?”
| Document | What to look for |
|---|---|
| Certificate of Documentation (COD) | The gross and net tonnage entries. For licensing, identify the applicable gross figure and its measurement system. |
| Certificate of Inspection (COI), if the vessel is inspected | Vessel tonnage information and the required credentialed crew. Read the manning requirements as well as the tonnage. |
| Tonnage certificate, including an International Tonnage Certificate where applicable | The assigned gross tonnage and the measurement system used. The international certificate identifies GT. |
Do not substitute a sales listing, the boat's model number, or someone's estimate of its weight for its documented gross tonnage.
What if it's a small boat without tonnage paperwork?
Some small, state-registered boats will not have a Certificate of Documentation or an assigned tonnage readily available.
For qualifying vessels, the Coast Guard provides a simplified measurement procedure using the vessel's dimensions. Ask the National Maritime Center how to report the vessel for your sea-service application, and use the Coast Guard's instructions rather than guessing from length.
Common Coast Guard license designations
These familiar designations refer to national deck-officer endorsements. The vessel examples are illustrations; actual authority depends on the endorsement and the vessel's documented particulars.
| Common designation | Examples of vessels within the appropriate tonnage limit |
|---|---|
| 25 GRT | Small launches and sightseeing boats |
| 50 GRT | Charter boats and small excursion vessels |
| 100 GRT | Inspected passenger boats, charter vessels, and yachts |
| 200 GRT | Larger passenger vessels, yachts, and workboats |
| 500 GRT | Coastal vessels, larger yachts, and offshore vessels |
| 1,600 GRT | Larger offshore, research, and coastal vessels |
| Unlimited tonnage | Large tankers, containerships, bulk carriers, and passenger ships |
The standard lower-tonnage levels are 25, 50, 100, and 200 GRT, followed by the 500 and 1,600 GRT categories and the unlimited track. Credential wording commonly specifies “less than” the stated tonnage, so read your actual endorsement carefully.
When joining a vessel, record its name, official or registration number, gross tonnage, and whether the figure is GRT or GT. Ask that your sea-service documentation clearly identify the tonnage.
The Coast Guard considers the tonnage of the vessels you served on when setting certain license limits. Knowing the correct figure early helps you understand how that service supports your next endorsement.
Before accepting an officer position, check the complete match: your capacity, tonnage limit, route, and any required specialized or STCW endorsements, together with the vessel's manning requirements. A tonnage number alone does not establish that you are qualified for the job.
Official sources
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