Boat Trim Ballast Calculator
Correct trim improves boat performance, handling, and safety. This calculator uses the moment principle: to correct a trim error, the correcting moment (weight times distance from the center of flotation) must equal the existing trim moment causing the imbalance. Enter the existing imbalance weight (the load causing incorrect trim), its distance from the center of flotation, and the planned ballast placement distance from the center of flotation. The calculator returns the ballast weight required to restore trim. Positive distances are aft of the CF; negative distances are forward.
Trim moment formula
Trim moment = Weight * Distance from center of flotation (lb-ft)
To restore trim:
Correcting moment = Ballast weight * Ballast distance
Ballast weight = Trim moment / Ballast distance
The center of flotation is the pivot point for trim changes. Weight aft of CF depresses stern; weight forward depresses bow.
Practical trim advice
- Move heavy gear (fuel, water, anchor chain) longitudinally to adjust trim before anchoring.
- On planing powerboats, trim tabs adjust dynamic trim while under way more effectively than ballast.
- Excessive stern trim on a sailboat increases weather helm and slows the boat.
- Consult your vessel's stability booklet for precise trim data including MT1 and center of flotation position.
Boat trim and ballast: frequently asked questions
What is boat trim?
Boat trim refers to the fore-and-aft attitude of the vessel. A boat is in proper trim when it floats on its designed waterline with neither bow nor stern depressed. Poor trim causes increased resistance, reduced speed, reduced visibility, and uncomfortable motion.
How is trim correction calculated?
The moment (weight times distance from a reference point) needed to restore trim equals the trim error in length times the vessel's trim moment per unit length. For a simpler approach: weight to add (lbs) = trim moment needed / distance of ballast from center of flotation (ft).
What is the center of flotation?
The center of flotation (CF) is the geometric center of the waterplane area. When weight is added anywhere on the vessel, the vessel trims about the CF. Weight added aft of the CF depresses the stern; weight forward lifts the stern.
How much does 1 lb of ballast change trim?
Trim change per pound depends on the vessel's trim moment coefficient (MT1: moment to trim 1 inch), which is proportional to waterplane area and length. This varies greatly between vessel types. For small planing powerboats, the effect per pound is much larger than for large displacement vessels.
Can I use gear shifting to correct trim?
Yes. Moving heavy items like coolers, gear, and fuel between forward and aft locations is the simplest way to adjust trim without ballast. Trim tabs on powerboats provide dynamic trim adjustment under way.
Official sources
- US Coast Guard Marine Safety: USCG National Maritime Center.
- National Geospatial-Intelligence Agency: Bowditch American Practical Navigator.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.