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Weight Planner

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Eric Griffin edited this page Jul 26, 2026 · 1 revision

Weight Planner

Most weighting advice is a table: so many kilos for a 5 mm wetsuit, add some for an aluminium tank, add a bit more for salt water. It gets you in the water, and then you spend the first ten minutes of the dive fixing it.

Submersion's Weight Planner starts from that table and then learns. Compose the rig you plan to dive — gear, tanks, water type — and it predicts how much lead you will need and where to put it, based on what you have actually carried on your own dives.

Find it under Planning → Weight Calculator. It is also built into the dive planner as the Gear & Weights card, so a plan carries its own weighting.

Composing a Rig

The rig composer is the top half of the tool:

  • Gear — add individual equipment items, or tap Use set to pull in a whole equipment set at once. Your exposure suit is the single biggest term, so start there.
  • Tanks — add one or more tanks from your presets. Material and size matter: an aluminium cylinder is meaningfully buoyant when it is nearly empty, and that is exactly when you need to hold a stop.
  • Water type — salt or fresh.
  • Body weight (optional) — improves the prediction and is required for the highest confidence rating. Save weight to profile records it as a dated entry, so your weighting history stays meaningful as your body weight changes.

Every change updates the prediction immediately, and a chip shows what that change cost you — +0.5 kg vs previous rig when you swap a 3 mm suit for a 5 mm one. Swapping gear to see the delta is the fastest way to use this tool.

Reading the Prediction

The card at the top shows four things:

Element Meaning
Predicted weight Total lead for this rig
Confidence How much the prediction rests on your data rather than on defaults
Based on N logged dives How many of your dives informed it
Suggested placement How to split the total across belt, integrated pockets, trim, and backplate

Confidence

Rating Roughly means
High confidence At least 10 supporting dives, most of the rig's items seen before, your body weight known, and your history is consistent
Medium confidence At least 3 supporting dives covering about half the rig
Low confidence – estimate Not enough history yet; this is essentially the classic table

Low confidence is not a failure — it is what a new logbook looks like. It climbs as you log dives with weights recorded.

Placement

Placement is predicted from how you have habitually distributed weight on recent dives with the same exposure suit, then rounded to whole increments of your unit (0.5 kg or 1 lb) so the suggestion is something you can actually assemble from real weights.

How this was calculated

Expand the breakdown to see each term and where its number came from:

Source Meaning
measured from your dives Learned from your own weighting history
from your gear specs The buoyancy you entered on that equipment item
default estimate A type-based default, because nothing better is known yet
physics Computed — tank buoyancy when near-empty, the salt/fresh water shift

This breakdown is the honest part of the feature: it shows you exactly which numbers are yours and which are guesses.

How It Learns

The engine is a hybrid, not a lookup table and not pure statistics:

  1. Physics is computed, not learned. Tank near-empty buoyancy and the water-density shift are calculated from cylinder specs and displaced mass.
  2. Those physics terms are subtracted from your history first. What remains is what physics cannot explain — your body, your suit's real buoyancy, your habits.
  3. The remainder is fitted across your dives, pulled toward sensible priors so that a single unusual dive cannot swing the answer.

Two consequences worth knowing:

  • A salt-only history still predicts fresh water sensibly, because the water term is physics rather than something that had to be observed.
  • Recent dives count for more. Older observations fade with a half-life of roughly two years, so a change in fitness, suit, or technique works its way in rather than being outvoted by a decade of old dives.

Teaching It

Three things you record elsewhere make the prediction better:

1. Post-dive weighting feedback

At the bottom of a dive's weights section, How was your weighting? offers Felt right, Overweighted, or Underweighted — and if you pick one of the last two, by about how much.

This is the single highest-value thing you can log. Without it, the model can only learn what you carried, not what you needed — and if you were consistently 2 kg heavy, it will faithfully learn to keep you 2 kg heavy. One tap after the dive fixes that.

2. Recording weight by type

Logging weights as typed entries (belt, integrated, trim, backplate, ankle) rather than a single total is what makes placement prediction possible. A total alone still trains the amount.

3. Equipment attributes

Buoyancy figures and attributes on your gear — a suit's thickness and style, a BCD's type and rated lift, an item's measured buoyancy — feed the priors. An explicitly entered buoyancy value outranks anything derived, which in turn outranks a flat default.

This matters most when you have no history: a brand-new logbook or a just-bought drysuit is predicted entirely from these attributes.

Through the Dive

Below the prediction, Through the dive simulates net buoyancy over a square profile you shape with two sliders (max depth and bottom time). It answers a different question from "how much lead": will this rig actually be holdable?

Readout Tells you
Verdict Whether you would be buoyant, heavy, or neutral at the final stop — the moment that matters, with a near-empty tank
Start of dive / End of dive Net buoyancy at both ends
Buoyancy swing How much you change across the dive as gas leaves the cylinder
Peak lift needed The most your BCD has to hold, flagged if it exceeds your wing's rated lift
Min ditchable weight Whether you could still get positive by dumping what is actually droppable
Fixed weight is not ditchable weight. Backplate and trim weights are excluded from the "you can ditch" figure, because you cannot ditch them in an emergency. That distinction is the whole point of the min-ditchable check.

In the Dive Planner

The Gear & Weights card in a dive plan runs the same prediction against the gear and tanks attached to that plan, and shows the same through-the-dive summary. Use as planned weight snapshots the predicted figure onto the plan, so the plan carries its weighting alongside its depths and gases.

Plans assume salt water. A dive plan has no water-type field, so the Gear & Weights prediction uses the salt-water baseline. For a fresh-water dive, use the standalone Weight Planner, where water type is an explicit control.

After the Dive

A logged dive with a recorded profile gets a Buoyancy section in its detail page: the same simulation run against what actually happened, including a comparison of what you carried against what the model would have suggested, and a what-if sheet for re-running the dive with different lead. That is where "why was the last stop so hard to hold" gets answered.

Limits

Always do a buoyancy check. This is a prediction from your history and from physics, not a measurement of you on the day. Wetsuits compress with age, weight belts get borrowed, and how you breathe on a given dive matters more than any model. Get in the water, hold a normal breath, and verify.
  • Predictions need recorded weights on past dives. Dives logged without weights teach it nothing.
  • Gear it has never seen is predicted from attributes and defaults; the first few dives with a new suit will move the number.
  • The through-the-dive simulation uses a square profile you set with the sliders — a planning approximation, not your actual dive.

See also

Getting Started

Logging Your Dives

Your Dive World

Diver & Gear

Insights & Planning

Setup & Data

Reference

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