Gross Tonnage Explained: Why 25 / 50 / 100 / 200-Ton Matters

When a captain says they hold a "100-ton Master," that number isn't how much the boat weighs — it's a measure of volume, and it sets the ceiling on the vessels you're allowed to command. Tonnage confuses almost everyone at first. Here's how it actually works and why it shapes your career.

Tonnage Is Volume, Not Weight

The word "ton" here comes from "tun" — an old unit for a large cask. Gross tonnage (or Gross Register Tonnage, GRT) is a measure of a vessel's total internal volume, converted into "tons" by formula. A bigger, roomier vessel has more gross tonnage; it has nothing directly to do with how heavy she is. (Displacement is weight; tonnage is space.)

For licensing, what matters is simple: your credential authorizes you to operate vessels up to a stated gross tonnage on a given route. A 100-Ton Master can run vessels up to 100 GRT; a 200-Ton Master, up to 200.

How Your License Tonnage Is Set

Here's the part that surprises people: you don't just pick your tonnage. The Coast Guard sets it based on the tonnage of the vessels your documented sea time was earned on. The formula looks at the size of the boats you've actually operated and grants a license tonnage scaled to that experience — typically up to about three times the tonnage of your qualifying vessels, capped at the license level you're testing for.

The practical takeaway: if you want a higher-tonnage license, log time on bigger boats. Three hundred sixty days entirely on a 30-foot charter boat won't qualify you for a 100-ton Master, no matter how many you accumulate.

The Common License Tonnages

• 25 / 50 / 100-Ton Master: the inspected-vessel licenses most charter, tour, and small commercial captains hold. The number reflects your sea-time tonnage.
• 200-Ton Master: a meaningful step up — bigger vessels, and the exam adds celestial navigation and radar.
• Higher tonnages and MMC officer endorsements: for larger commercial vessels, with progressively more sea time, route, and exam requirements.

OUPV Is the Exception

The OUPV (6-pack) license isn't tonnage-rated the same way — it's defined by passengers (up to six) on uninspected vessels, not by gross tonnage. That's why it's the common entry point: you qualify on days and passengers, then step into the tonnage-rated Master world as your experience on larger, inspected vessels grows.

Why It Matters for Your Path

Tonnage is the rung-spacing on the career ladder. Your first license is shaped by the boats you've run; each upgrade asks for more time on larger vessels. If you have a direction in mind — bigger charters, tour vessels, commercial work — start logging time on the size of boat that qualifies you for where you want to be.

Map your route from here with the upgrade tracker, keep your qualifying days documented in the sea time tracker, and study the Master modules — including the deck and chart-plotting material that separates OUPV from tonnage-rated licenses — in the question bank.

Binnacle School is a study and planning resource and is not affiliated with the USCG or the National Maritime Center. Tonnage determinations are made by the NMC — confirm specifics at uscg.mil/nmc.