Vessel Trim Calculator
Trim is the longitudinal tilt of a vessel, the difference between how deep it sits at the stern and how deep it sits at the bow. From a forward draft reading and an aft draft reading you can find the trim, whether the vessel is trimmed by the head or by the stern, and the mean draft amidships. These values matter for steering, propeller immersion, fuel efficiency, and the draft surveys used to weigh bulk cargo. Enter your two draft readings in any consistent unit (meters or feet) and this calculator returns trim, mean draft, and the trim direction.
Vessel trim formula
Trim = aft draft - forward draft
Mean draft = (forward draft + aft draft) / 2
Trim > 0: by the stern
Trim < 0: by the head
Trim = 0: even keel
A positive trim means the stern sits deeper (trim by the stern); a negative trim means the bow sits deeper (trim by the head). Mean draft is the average of the two readings and approximates the draft amidships on a vessel without hog or sag.
Trim in practice
- Most vessels are designed to run with a small trim by the stern for propeller immersion and steering.
- Trim by the head usually increases resistance and reduces directional stability.
- Mean draft feeds into the displacement read from the vessel hydrostatic tables.
- Keep readings in one consistent unit so trim and mean draft share that unit.
- Always check final trim against the limits in the vessel stability booklet before sailing.
Vessel trim: frequently asked questions
What is vessel trim?
Trim is the difference between the draft measured at the stern (aft) and the draft measured at the bow (forward) of a vessel. If the aft draft is greater, the vessel is trimmed by the stern; if the forward draft is greater, it is trimmed by the head. Trim is normally expressed in the same length unit as the draft readings.
How is mean draft calculated?
Mean draft is the simple average of the forward and aft draft readings: (forward draft + aft draft) divided by 2. This gives the draft amidships for a vessel that has no hog or sag and helps estimate the volume of water displaced.
What does trim by the stern mean?
Trim by the stern means the aft draft is greater than the forward draft, so the vessel sits deeper at the back. Most vessels are designed to operate with a small trim by the stern for steering and propeller immersion. Trim by the head (bow deeper) is generally undesirable for steering.
Why does trim matter for a ship?
Trim affects steering response, propeller and rudder immersion, fuel efficiency, and the accuracy of draft survey cargo calculations. Excessive trim by the head can reduce directional stability and increase resistance. Loading must be planned so that the final trim stays within the limits in the vessel stability booklet.
What units should I use for draft?
Use any single consistent length unit for both draft readings, such as meters or feet. The trim and mean draft outputs will be in that same unit. This calculator does the arithmetic only; refer to your vessel's hydrostatic tables and stability booklet for displacement and stability assessment.
Official sources
- U.S. Coast Guard Navigation Center: Navigation Center.
- NOAA Office of Coast Survey: Nautical Charts.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.