What Is a Metacentric Height (GM) Calculator?
Metacentric height, or GM, is the fundamental measure of a vessel's initial (small-angle) stability. This calculator applies the basic naval architecture relationship — GM equals KM minus KG — to turn your vessel's hydrostatic data and loading condition into a single stability number.
GM is used throughout commercial and recreational vessel stability assessments, from cargo ship loading calculations to small-craft stability checks, because it's the simplest reliable predictor of how a vessel responds to a small heeling force.
How to Read Your Results
Metacentric Height (GM)
The calculated GM value. A positive number means the vessel has a righting tendency at small heel angles; the larger the positive number, the stiffer (and faster-rolling) the vessel tends to be.
Stability Status
A general-guidance label based on common GM ranges. This is a rough educational indicator, not a substitute for your vessel's actual stability booklet or a naval architect's assessment.
KM & KG Entered
Your two inputs echoed back — useful for confirming you didn't transpose or mistype either value before trusting the GM result.
Real-World Example
A vessel with KM (metacenter above keel) of 18.5 ft and KG (center of gravity above keel) of 17.2 ft:
| Item | Value |
|---|---|
| KM | 18.5 ft |
| KG | 17.2 ft |
| Metacentric height (GM) | 1.3 ft |
| Stability status | Low-moderate |
If cargo or fuel is added high up in the vessel, KG rises and GM shrinks — sometimes enough to move the vessel into a much less stable range, which is exactly why loading plans track KG so carefully.
Tips for Working With GM
- Always use current, condition-specific data — KM and KG both change with draft and loading, so don't reuse old figures.
- Track KG carefully during loading — weight added high in the vessel raises KG and lowers GM faster than the same weight added low.
- Never treat a negative or near-zero GM lightly — it indicates a real stability hazard requiring immediate correction.
- Consult your vessel's official stability booklet for regulatory minimum GM requirements, which vary by vessel type and service.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team