What is ballast in a ship?

Ballast is weight — typically seawater pumped into dedicated tanks — added to a vessel to lower the center of gravity, improve stability, achieve adequate propeller immersion, and reach a safe operating draft when carrying little or no cargo. Some vessels also use solid ballast (concrete, iron) in the keel.

How is required ballast weight calculated?

Required ballast = (Target displacement) − (Light ship displacement) − (Cargo weight). Target displacement is found from the target mean draft and TPC: if the target mean draft differs from the current draft, the difference in centimeters multiplied by TPC gives the mass change needed.

What is TPC and why does it matter?

TPC (Tonnes Per Centimetre Immersion) is the mass required to change the mean draft by exactly 1 cm. It is read from the ship's hydrostatic tables at the relevant displacement. A higher TPC means the hull has a large waterplane area and draft changes slowly per tonne added.

What is LCG of the ballast tank?

LCG (Longitudinal Centre of Gravity) of the ballast tank is its longitudinal distance from the aft perpendicular, using the same datum as LCF. When ballast is added, the overall ship LCG shifts toward the tank LCG, changing trim. Entering the tank's LCG allows the calculator to predict the trim effect.

Can I use this for multiple ballast tanks?

This tool calculates for a single effective ballast position (the combined LCG of all tanks you plan to fill). For multiple tanks, use the weighted-average LCG: sum each tank's (mass × LCG) and divide by total ballast mass, then enter that weighted LCG here.

What if I want even keel (zero trim)?

Enter 0 as the target trim. The tool will show the ballast mass for the target draft plus indicate whether the tank LCG can achieve even keel. If the required trim correction exceeds what the tank position can deliver, you need to redistribute ballast between forward and aft tanks.

Ballast Calculator

Enter hull displacement, cargo weight, and draft targets to calculate how much ballast is needed to achieve your desired draft and trim.

Vessel Parameters

Light Ship Displacement (t)

Vessel mass without cargo or ballast

Cargo Weight (t)

Total cargo currently aboard (0 if empty)

TPC — Tonnes Per cm Immersion (t/cm)

From hydrostatic tables at current loading

Current Mean Draft (m)

Target Mean Draft (m)

Trim Parameters (optional)

AP / aft
x = 0 FP / fore
x = LBP

LCF is the trim pivot. Signed trim is aft draft minus fore draft: positive is by the stern, negative is by the bow.

LBP — Length Between Perpendiculars (m)

LCF from Aft (m)

Longitudinal Centre of Flotation from aft perpendicular

Ballast Tank LCG from Aft (m)

Centre of gravity of ballast tank(s) from aft

Target Trim (m) (aft − fore, positive = stern-heavy)

Current Trim (m) (aft − fore)

Fill in vessel parameters and click Calculate

Ballast Requirement

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Ballast Required (t)

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Target Displacement (t)

Draft Summary

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Fore Draft (m)

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Mean Draft (m)

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Aft Draft (m)

Weight Breakdown

Component	Mass (t)	% of Target Disp.

Summary

Enter hull displacement, cargo weight, and draft targets to calculate how much ballast is needed to achieve your desired draft and trim.

How it works

Enter the vessel's light ship displacement and the total cargo weight currently aboard.
Enter TPC (Tonnes Per Centimetre Immersion) from the hydrostatic tables at the current loading condition.
Enter the target mean draft and, optionally, a target trim (aft draft minus fore draft).
Enter LBP (Length Between Perpendiculars), LCF position, and the longitudinal center of gravity (LCG) for the ballast tanks.
Click Calculate. The tool determines the ballast mass required to reach the target mean draft and evaluates the trim produced at the entered tank position.

Use cases

Determine ballast water needed before a light-ship ocean passage.
Plan ballast loading to achieve even keel or a specified trim by stern.
Verify that ballast capacity is sufficient to meet minimum draft requirements for propeller immersion.
Support deck officers during pre-departure stability checks.
Assist naval architecture students with practical trim and draft problems.
Estimate ballast adjustment when cargo distribution changes during a voyage.

Frequently Asked Questions

Last updated: 2026-09-19 · Reviewed by Nham Vu