TPC is tonnes per centimetre immersion: the mass that must be added or removed to change the mean draft by one centimetre. It comes from the ship's hydrostatic tables at a given draft and reflects the waterplane area.

How is the ballast mass found?

Multiply the required draft change in centimetres by the TPC. If you need to sink the ship 30 cm and the TPC is 25 tonnes, you need 30 times 25, which is 750 tonnes of ballast added.

Why divide mass by density for volume?

Mass equals volume times density. To find the volume of water to pump into the tanks, divide the required mass by the water density. Salt water at 1.025 gives slightly less volume per tonne than fresh water at 1.000.

Does TPC change with draft?

Yes, slightly, because the waterplane area changes as the hull immerses. For a modest draft change you can use a single TPC at the mean working draft. For large changes, recompute with TPC at the new draft for accuracy.

Does this plan trim across tanks?

This tool sizes the total ballast needed for the target mean draft. Distributing it between forward, aft, and wing tanks to achieve the desired trim and heel is a separate step using each tank's longitudinal centre and capacity.

Ballast Calculation Tool

After discharging cargo a ship rides too high to handle safely, so deck officers take on ballast water to restore an adequate sailing draft. This tool sizes that ballast: it turns the gap between your present and target draft into a tonnage using the vessel’s TPC, then converts that mass to a pumping volume.

How it works

The required mass follows directly from the draft change and the tonnes-per- centimetre immersion, and volume follows from density:

draft change (cm) = (target draft − current draft) × 100
ballast mass (t)  = draft change (cm) × TPC
ballast volume (m³) = ballast mass / water density

A positive draft change means you must add ballast to sink the ship; a negative result means you are already deeper than target and would need to deballast. Because mass equals volume times density, dividing the required mass by the ballast water density gives the cubic metres to pump aboard.

Example and notes

A ship sitting at 6.20 m mean draft after discharge that needs 7.00 m to sail has a draft change of 80 cm. With a TPC of 25 tonnes, that is 80 × 25 = 2,000 tonnes of ballast. In dock water at 1.010, that mass occupies 2,000 / 1.010 ≈ 1,980 m³. For large draft changes, re-read TPC at the new draft rather than the old one, and remember this gives the total quantity only — splitting it across tanks to trim the vessel correctly is the next planning step.