Estimate boat gross tonnage from length, beam, depth, vessel shape factor, or enclosed volume using register or ITC 1969 methods for boats.
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Boat Gross Tonnage Formula
The calculator runs one of three formulas depending on which tab and method you choose.
By dimensions (simplified):
GT = (L × B × D × C) / 100
By volume, register method:
GT = V_ft3 / 100
By volume, ITC 1969 method:
GT = (0.2 + 0.02 × log10(V_m3)) × V_m3
- L = length overall (ft)
- B = breadth or beam (ft)
- D = molded depth from deck to underside of hull, not draft (ft)
- C = shape factor (0.67 powerboat, 0.50 sailboat, custom 0 to 1)
- V_ft3 = enclosed volume in cubic feet
- V_m3 = enclosed volume in cubic meters
Gross tonnage is a unitless volume index, not a weight. The dimensional formula is the U.S. Coast Guard simplified method used for documentation of recreational and small commercial vessels. The ITC 1969 formula is the international standard required for vessels engaged in international voyages and gives a smaller K1 multiplier for smaller volumes.
Reference Tables
Use these to sanity-check the number you get.
| Vessel type | Shape factor (C) |
|---|---|
| Powerboat, motor yacht, fishing vessel | 0.67 |
| Sailboat with auxiliary power | 0.50 |
| Barge or scow (block-shaped) | 0.84 |
| Fine-hulled rowing or racing craft | 0.35 to 0.45 |
| Typical boat | L × B × D (ft) | Approx GT |
|---|---|---|
| 22 ft center console | 22 × 8 × 3 | ~3.5 |
| 35 ft sailboat | 35 × 11 × 6 | ~12 |
| 50 ft motor yacht | 50 × 15 × 8 | ~40 |
| 75 ft commercial fishing | 75 × 22 × 11 | ~120 |
The 5 GT and 100 GT thresholds matter for U.S. documentation and licensing requirements.
Example and FAQ
Example. A 40 ft powerboat with a 13 ft beam and 7 ft molded depth:
GT = (40 × 13 × 7 × 0.67) / 100 = 2,439.4 / 100 ≈ 24.4 GT
Is depth the same as draft? No. Depth is measured from the main deck down to the bottom of the hull on the inside. Draft is only the part below the waterline. Using draft will undersize your result.
Why does GT not equal pounds or tons? Gross tonnage measures internal volume, not weight. One GT corresponds to roughly 100 cubic feet of enclosed space.
Which method should you use? For U.S. recreational documentation, the simplified dimensional formula is accepted. For international or commercial certification, use the ITC 1969 method with a measured enclosed volume.
Why does the 5 GT line matter? Vessels at or above 5 GT are eligible for U.S. Coast Guard documentation. Below that, state registration applies.
