Starlight Tools

Boat Trim and Weight Distribution Calculator

Combine a boat's starting weight moment with people, fuel, water, machinery, and gear to calculate loaded longitudinal center of gravity (LCG). Optionally estimate a small change in trim using vessel-specific LCF and MCT data.

Weight moments do not prove a loading condition is safeUse one consistent datum and verified weight data. Stay within the builder's approved weight and LCG limits. This tool does not assess vertical or transverse stability, free surface, freeboard, capacity, structural limits, planing attitude, or conditions underway.

Boat, loads, and stations

Starting condition and datum

Stations are measured forward from an aft datum: 0 at the datum, increasing toward the bow.

Extent of the station system; not necessarily LOA.

Boat before the added items below.

From the same aft datum, increasing forward.

Added onboard weights

Use each item's own center-of-gravity station. Leave station blank when its weight is 0.

ItemWeight (lb)Station (ft)
People

Fuel and tanks

Water and fluids

Gear and cargo

Machinery / batteries

Other load
Optional target and hydrostatic data

Use only a verified target within the approved LCG range.

Used to calculate required shift distance.

Longitudinal center of flotation for the relevant condition.

Enter MCT in the exact displayed units for the expected loaded condition.

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Longitudinal balance

Enter the starting condition and onboard loads, then calculate.

Weight-moment formulas

Item moment = item weight × station

Loaded LCG = total longitudinal moment ÷ total loaded weight

Correction moment = loaded weight × (target LCG − loaded LCG)

Required shift distance = correction moment ÷ movable weight

Optional trim calculation

Added trimming moment about LCF = Σ[added weight × (station − LCF)]

Change in trim = added trimming moment ÷ MCT 1 in (or MCT 1 cm)

A positive result in this tool means change by the bow; a negative result means change by the stern. It is the change in the difference between forward and aft drafts, not the change at either draft mark.

Use one datum and one condition

The origin can be the transom, aft perpendicular, or another documented point. Its physical location matters less than using it consistently for the base LCG, every load station, target LCG, and LCF.

Avoid double counting: The base boat weight and LCG describe one known starting condition. Enter only weights not already included in that condition.

LCF and MCT must come from the boat

The longitudinal center of flotation (LCF) is the approximate fulcrum for small trim changes. Moment to change trim one inch or centimeter (MCT 1 in / MCT 1 cm) changes with displacement and draft.

Use hydrostatic curves, a loading manual, or data from the designer for the relevant loading condition. If your source uses long-ton·ft/in, short-ton·ft/in, or tonne·m/cm, convert it to the exact unit displayed by the calculator.

How to use this calculator

  1. Choose units and define a station line from an aft datum toward the bow. Enter a calculation length that contains every station.
  2. Enter the known weight and LCG of the boat before the listed loads are added. Confirm exactly what the starting condition includes.
  3. Enter each added item's actual weight and its longitudinal center-of-gravity station. For a group of people, use their combined center rather than a seat edge.
  4. Optionally enter a verified target LCG and a realistically movable weight. The result reports the direction and distance of the theoretical shift.
  5. Only enter LCF and MCT together when valid hydrostatic values are available for the relevant condition. Verify the final arrangement against approved documentation.

Example LCG and trim calculation

A 5,200 lb starting boat has an LCG at station 11.2 ft. Added loads total 1,290 lb. Their combined moment, plus the starting boat's 58,240 lb·ft moment, produces:

Loaded LCG = 73,660 lb·ft ÷ 6,490 lb = 11.350 ft from the aft datum

Using an LCF at station 11.5 ft, the added loads create a net 585 lb·ft moment forward of the LCF. If valid loaded-condition MCT 1 in is 2,000 lb·ft/in, the estimated change in trim is 0.293 in by the bow. This does not include parallel sinkage or calculate individual draft changes.

Frequently asked questions

How is a boat's loaded longitudinal center of gravity calculated?

Multiply every weight by its longitudinal station to obtain a moment, add the moments, and divide by total loaded weight. The result uses the same datum and direction as the entered stations.

What is moment to change trim one inch or one centimeter?

MCT 1 in or MCT 1 cm is the vessel-specific moment required to change the difference between forward and aft drafts by one inch or centimeter. It varies with displacement and draft and normally comes from hydrostatic curves or approved loading information.

Can this calculator determine safe boat trim?

No. A calculated LCG only becomes meaningful when compared with the approved LCG range for that boat and loading condition. The tool cannot establish that range or assess freeboard, stability, strength, capacity, or operation in waves.

What station should I use for a fuel or water tank?

Use the center of gravity of the tank contents at their actual fill condition, plus the tank itself if its weight is not in the base condition. An irregular tank's contents center can move as it empties. Slack liquids also produce free-surface effects that this longitudinal calculation does not model.

Does moving weight change total displacement?

No. Shifting an existing weight changes the center of gravity and creates a trimming moment but does not change total weight. Adding or removing weight changes both displacement and weight moment.

Why does the calculator not show new bow and stern drafts?

Dividing a total trim change between draft marks requires their locations relative to LCF. Added weight also creates parallel sinkage, which requires tonnes-per-centimeter or tons-per-inch immersion data. Weight entries alone are not enough.

Does this calculate side-to-side balance or vertical stability?

No. The station system is longitudinal only. Transverse center of gravity, vertical center of gravity, metacentric height, free-surface corrections, righting arms, heel, and dynamic stability require additional vessel-specific data and methods.

Limits and safety disclaimer

  • The weighted-average LCG is only as accurate as the base condition, weights, item centers, station datum, and unit consistency.
  • The forward/aft percentages classify added weight by geometric midpoint; they are descriptive and are not target balance percentages.
  • The target correction assumes an existing weight moves longitudinally without changing total displacement. It does not check whether the proposed position is physically or structurally suitable.
  • The MCT method is a small-change approximation. Large changes in displacement or mean draft require updated hydrostatics and a complete loading calculation.
  • Added-weight trim also includes parallel sinkage, which is not calculated here. The result does not produce new draft, freeboard, or under-keel-clearance values.

Safety disclaimer: This educational planning tool is not a stability assessment, loading instrument, survey, certification, naval-architecture analysis, or operating advice. Use approved loading and hydrostatic data and consult the manufacturer or a qualified naval architect or marine engineer when the loading condition is uncertain.

Methodology and sources

Last reviewed: August 2, 2026. The moment, LCG, trim, and safe-loading explanations were checked against official educational and regulatory references:

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