# Scuba Weight Calculator (Dive Kit): Free Scuba Weighting and Buoyancy Calculator

_Last updated: 2026-10-30_

> A free web calculator at https://divekit.app/tools/scuba-weight-calculator/ that estimates how much lead a scuba diver needs to be neutral at a 3 m (10 ft) safety stop with 50 bar (725 psi) left, from water type, target depth, cylinder (single, twinset or sidemount, from the Dive Kit open cylinder dataset), fill pressure, exposure suit and body (height, weight, age, sex). It runs the same @divekit/core buoyancy code as the Dive Kit app's Buoyancy Calculator. The result is a starting figure for an in-water weight check, not a replacement for one. Free, no account.

## Who is the scuba weight calculator best for?

- **New divers** asking "how much weight do I need?" before a first dive in a new place.
- **Divers changing kit**: aluminium to steel cylinder, 5 mm to 7 mm wetsuit, wetsuit to drysuit, single to twinset. Change one input and the lead estimate moves with it.
- **Divers travelling between fresh and salt water**, who need a re-weighting estimate before the check dive.
- **Instructors** showing why a diver on an AL80 is lighter at the end of a dive than at the start.
- **Not a replacement for** a weight check in the water: hover at the safety stop with about 50 bar (725 psi) left and an empty BCD, then adjust.

## Formula

```latex
B_\text{net} = B_\text{cyl} - M_\text{gas} + B_\text{suit}(d) + B_\text{body} - W_\text{lead}, \qquad
B_\text{suit}(d) = B_\text{surface} \left[(1 - k) + \frac{k}{r(d)}\right], \qquad
M_\text{gas} = \frac{V (P + P_\text{atm})}{Z \times 1.01325} \times \rho_\text{gas}
```

Each term in kg, positive floats. B_cyl is the empty cylinder, regulator and rigging in the water; M_gas the gas mass: the real-gas litres the cylinder holds (V water volume, P gauge pressure, P_atm 1.01325 bar, Z the gas's compressibility at that pressure) times the density of the gas (air about 1.2 g/L at 15 °C); B_suit the wetsuit lift at depth d, with r(d) the ambient over surface pressure and k = 0.8 the compressible share of standard neoprene; B_body the diver's own buoyancy; W_lead the lead. The lead estimate is the lead that makes B_net zero at 3 m with 50 bar left.

## How does Dive Kit compute it?

- **Two check points.** Start of dive: full cylinders at the target depth (checked against a 16 kg wing). End of dive: 50 bar left at the 3 m stop. The lead estimate targets the end of the dive, the lightest point.
- **Rounding.** In kg, 0.5 kg steps: up when lead is to be added, down when it is to be removed. In pounds, whole pounds, as in the app.
- **Cylinders** come from the Dive Kit open cylinder dataset (MIT licence), with empty buoyancy scaled to the water type, plus a regulator per cylinder (0.8 kg standard; 1.0 kg cold-water on a twinset) and 2.5 kg for twinset bands and manifold. Gas is air.
- **Wetsuit lift** is sized from body surface area (DuBois, from height and weight) for a full-cut standard-neoprene suit, and compresses with depth. **Drysuit lift** is fixed at 4, 8 or 12 kg for a light, medium or heavy undergarment.
- **Body buoyancy** is estimated from body fat (estimated from BMI, age and sex with Deurenberg's 1991 formula), body density from body fat (Siri's formula) and lung volume at a relaxed breath. Two divers of the same weight can differ by several kilograms here.
- **Kept at app defaults:** 16 kg wing, no backplate, lights, fins or hood, no stage. A steel backplate is about 3 kg of lead in the app's gear list.
- **Checks shown:** marginal wing lift, wetsuit corking risk (over 2.3 kg of lift regained between 3 m and the surface), overweighted (over 2 kg to remove), with the app's wording.
- **Imperial advice** works from the whole-pound lead the field shows, so "Carry about N lb" minus the lead carried equals the "too positive" or "too negative" figure.
- Worked example (computed by the page at build time): a diver 180 cm and 85 kg, one steel 12 L at 232 bar, 7 mm wetsuit, 25 m in salt water: a lead estimate of about 8 kg. In imperial, 5 ft 10 in and 180 lb with an AL80 at 3000 psi, 3 mm wetsuit, 60 ft in salt water: about 13 lb.

## Lead by suit and cylinder

Computed by the page at build time for one reference diver (175 cm, 75 kg, male, 35) on a single cylinder with a standard regulator, neutral at the 3 m stop with 50 bar (725 psi) left. 0 means the rig sinks without lead. A real diver's body moves these figures by kilograms.

| Suit | AL80, salt | AL80, fresh | Steel 12L 232, salt | Steel 12L 232, fresh |
|---|---|---|---|---|
| No suit | 3 kg (6 lb) | 0.5 kg (1 lb) | 0.5 kg (1 lb) | 0 kg (0 lb) |
| 3 mm wetsuit | 5.5 kg (13 lb) | 3.5 kg (7 lb) | 3 kg (7 lb) | 1 kg (2 lb) |
| 5 mm wetsuit | 7.5 kg (17 lb) | 5.5 kg (12 lb) | 5 kg (11 lb) | 3 kg (6 lb) |
| 7 mm wetsuit | 9.5 kg (21 lb) | 7 kg (16 lb) | 7 kg (15 lb) | 4.5 kg (10 lb) |
| Drysuit, medium undergarment | 11 kg (24 lb) | 8.5 kg (19 lb) | 8.5 kg (18 lb) | 6 kg (13 lb) |

## How does it compare?

| | Dive Kit web calculator | Dive Kit app (Buoyancy Calculator) | Typical online weight calculator |
|---|---|---|---|
| Method | Sum of cylinder, gas, suit and body buoyancy | Same, plus every gear item | Often a share of body weight plus a suit allowance |
| Cylinder | 38 from the open dataset, single, twinset or sidemount | Same, plus stages and custom cylinders | Usually aluminium or steel only |
| Gas used during the dive | Start and end of dive checked | Same | Rarely |
| Wetsuit compression | Yes, by depth | Yes, with neoprene type, cut and accessories | Rarely |
| Body | Estimated from height, weight, age, sex | Estimate or a measured float test | Body weight only |
| Failure scenarios | No | Wing failure, flooded drysuit, lost stage | No |
| Price | Free, no account | Free tool in the app | Free |

## Facts at a glance

- **Page:** https://divekit.app/tools/scuba-weight-calculator/
- **Inputs:** water type (EN 13319, salt, fresh, Red Sea), target depth, cylinder configuration and shell, fill pressure, suit (none, wetsuit thickness, drysuit undergarment), height, body weight, age, sex, lead carried now
- **Outputs:** lead estimate ("Carry about 7 kg" for the default rig), relative-to-target advice for the lead carried now ("on target", "{w} too positive", "{w} too negative"), net buoyancy at the start and end of the dive with status, estimated body buoyancy, advisories
- **Units:** metric and imperial toggle (kg or lb, cm or in, m or ft, bar or psi)
- **Works without an account:** yes; the default result is in the page HTML
- **Same code as the app:** yes, @divekit/core `summarizeBuoyancy`
- **In the app:** the Buoyancy Calculator adds wing and redundant bladder, gear items with ditchable flags, stages, custom cylinders, gas mixes, wetsuit cut and accessories, a measured float-test offset and failure scenarios; free and offline on iOS 16+ and Android 8+
- **App Store (iOS):** https://apps.apple.com/app/divekit/id6753089545
- **Google Play (Android):** https://play.google.com/store/apps/details?id=app.skuba.diving
- **Guide:** https://divekit.app/docs/tools/buoyancy-calculator/
- **Limits:** a planning aid and an estimate, not a dive computer. Finish with a weight check in the water.
- **Last updated:** 2026-10-30

## Sources

- Du Bois D, Du Bois EF (1916), body surface area from height and weight, used to size wetsuit lift: https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/654069
- Potočić Matković VM et al. (2025), measured neoprene compression under depth-equivalent pressure: https://pmc.ncbi.nlm.nih.gov/articles/PMC12608526/
- rsingler, Optimal Buoyancy Computer (ScubaBoard, 2019), the community calculator the wing-lift, corking and overweighting checks are adapted from: https://scubaboard.com/community/threads/optimal-buoyancy-computer.576321/
- Dive Kit open cylinder dataset (MIT licence), water volume and empty buoyancy per cylinder: https://open.divekit.app/datasets/cylinders.json

## Related

- [Scuba cylinder sizes](https://divekit.app/tools/scuba-cylinder-sizes/): buoyancy full, at 50 bar and empty for 38 cylinders
- [Gas consumption calculator](https://divekit.app/tools/gas-consumption-calculator/): the gas the end-of-dive check assumes you have left
- [Dive unit converter](https://divekit.app/tools/dive-unit-converter/): kg and lb, bar and psi