Subsea calculator
ROV buoyancy and ballast calculator
Work out whether a vehicle floats, sinks or hovers from its mass and displaced volume — and how much syntactic foam it takes to reach the trim you want.
Estimation tool, not design authority. This calculator gives a first-pass figure from the stated method and the assumptions listed below the result. It does not replace engineering sign-off, and it does not model everything a real build has to account for — see “Assumptions and limits” for what is left out. For a figure you can build against, send us the requirement.
Result
Why it matters
Buoyancy is a running total, not a final check.
Every component adds both mass and displaced volume, and the two do not move together. A sealed aluminium housing adds a lot of both; a solid steel bracket adds mass and almost no useful volume; foam adds volume and very little mass. A platform that is designed without tracking the running total arrives at trials nose-down, or unable to hold depth, and gets fixed with lead bolted on the night before.
Aim slightly positive. A few hundred grams to a kilogram or two of positive buoyancy means a power or thruster failure floats the vehicle for recovery instead of losing it — without the thrusters spending their authority fighting upward drift for the whole dive.
How it is calculated
Archimedes, then foam.
The upward force is the weight of water displaced: displaced volume × water density. Net buoyancy is that minus the vehicle's mass in air. Positive floats, negative sinks.
Foam lift is the difference between water density and foam density: a 0.40 g/cm³ foam in seawater lifts 0.625 kg per litre, because each litre displaces 1.025 kg of water while weighing 0.4 kg itself. Denser deep-rated grades lift less per litre, which is the trade you make for surviving depth.
Displaced volume means the external volume of everything below the waterline — frame members, housings, thrusters, foam and cabling — not the internal volume of the housings. Measuring it by water displacement in a tank is more reliable than adding up CAD volumes and forgetting the tether.
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