Dive Weight Calculator

Inputs

Example Data Table

Examples are illustrative. Real needs vary by body composition, gear configuration, and technique.

Formula Used

The calculator provides a starting estimate by combining a baseline buoyancy fraction with gear adjustments:

The “gas swing” shown is an approximate buoyancy change from full to reserve. You should be neutral near reserve pressure with minimal air in the BCD.

How to Use This Calculator

1. Choose input units and enter your body weight.
2. Select a body type that best matches your buoyancy tendency.
3. Pick your water type, then choose your exposure suit.
4. Select your cylinder, or enter custom reserve buoyancy.
5. Add fixed negative gear and any buoyant accessories.
6. Press Calculate to view results above this form.
7. Download CSV or PDF to save the estimate in your log.
8. Perform an in-water weight check near your reserve pressure.

Professional Article

1) Why correct weighting matters

Proper weighting reduces effort, improves trim, and stabilizes ascents. Excess lead requires extra BCD gas, increasing drag and making buoyancy “bouncy.” Too little lead can prevent a safe stop at reserve. This calculator offers a practical starting estimate.

2) Understanding buoyancy components

Total buoyancy comes from body composition, equipment materials, trapped gas, and water density. Positive items (neoprene, buoyant accessories) must be balanced by lead. Negative items (steel backplates, lights, reels, steel cylinders) reduce required lead. Small changes accumulate across the kit. Log your final configuration so future dives start closer to ideal each time.

3) Freshwater versus saltwater density

Saltwater is denser than freshwater, so it provides more upward force for the same displaced volume. Many divers need about 1–3% more ballast when moving from freshwater to the ocean, depending on suit thickness and equipment. Brackish water usually sits between.

4) Exposure suit buoyancy and compression

Neoprene contains gas bubbles that add buoyancy near the surface. Thicker suits generally require more lead: common starting additions are about 1–2 kg for thin suits, ~3 kg for mid-thickness, and 5 kg or more for very thick suits. At depth, neoprene compresses and buoyancy drops.

5) Cylinder type and reserve behavior

Aluminum cylinders often become more buoyant as they empty, while many steel cylinders remain negative. Weighting should target neutrality near your reserve pressure. This tool includes a reserve-buoyancy factor so you can plan for the final minutes of the dive, not only the start.

6) Gas swing during the dive

Breathing gas has mass. As cylinder pressure drops, a few kilograms of gas can leave the tank on a single dive. That change is the “gas swing,” explaining why you feel heavier at the beginning and lighter near the end. Reserve neutrality keeps stops calm.

7) Ditchable weight and trim weight

Not all ballast should be ditchable. Many setups split lead into a ditchable portion (belt or integrated pockets) and a fixed trim portion (backplate or trim pockets) to keep posture horizontal. A useful starting split is about 60% ditchable and 40% trim, then refine after checks.

8) A practical buoyancy check workflow

In calm water, with a near-reserve cylinder and your normal exposure protection, float at eye level while holding a normal breath and keeping the BCD empty. When you exhale, you should slowly sink. If you cannot, add small increments and repeat.

FAQs

1. How accurate is the lead estimate?

It is a structured starting point based on typical buoyancy patterns. Your breathing control, exposure suit fit, cylinder model, and accessory placement can shift results. Always confirm by performing an in-water buoyancy check near your reserve pressure.

2. Why does saltwater usually require more weight?

Saltwater has higher density, so it exerts greater buoyant force on the same volume. That extra lift often means you add ballast compared with freshwater. The difference is commonly noticeable when switching environments with the same suit and gear.

3. What does “reserve buoyancy” for a cylinder mean?

It is how floaty or sinky the cylinder behaves near the end of the dive. Aluminum tanks can become positive as they empty, while many steel tanks stay negative. Weighting for reserve neutrality helps you hold stops without overinflating the BCD.

4. How do I use the custom suit input?

If you know how much lead your suit typically requires, enter that amount in kilograms under Custom suit. Choose “Custom” in the suit list so the calculator uses your value instead of the generic thickness assumptions.

5. How should I split ditchable and trim weight?

A common starting approach is roughly 60% ditchable and 40% fixed trim, then adjust for comfort and posture. Keep emergency-ditch needs in mind, and avoid placing all weight where it forces feet-down trim.

6. Why does the tool show a gas swing value?

Gas has mass, so you become lighter as you breathe it down. The gas swing reminds you that start-of-dive weighting should still allow end-of-dive neutrality. Stable stops usually happen when you plan around reserve pressure.

7. What rounding should I choose?

Pick a step that matches the blocks you actually carry. Many divers use 0.5 kg or 1 kg in metric, or 2 lb in imperial. Smaller steps allow finer tuning during the buoyancy check.