Dive Weight & No-Fly Calculator
A starting weight to get in the water with, the in-water check that actually settles it, and how long before you can fly.
This gives you a starting point to get in the water with, not a final number. The only correct weighting is one you verify by floating — and the check below is the part that actually matters.
The weight check
The only correct weighting is the one you verify in the water. With an almost empty cylinder, a deflated BCD and a normal breath held, you should float at eye level; breathe out fully and you should sink slowly. Do it at the END of a dive, not the start — get it right with a full tank and you will be several pounds light when it matters, which is exactly when you need to hold a safety stop.
Why over-weighting causes everything else
- Over-weighting is the most common fault in recreational diving and it causes almost everything else. Excess lead means an inflated BCD to compensate, which means more drag, worse trim, harder finning and a consumption rate that looks like a fitness problem and is not.
- Weight has to be checked again whenever anything changes — a different suit, a different cylinder, fresh water instead of salt, or a few months away from diving.
- Distribute it rather than piling it on a belt. Trim weight on the tank band or in trim pockets is what gets you horizontal, and horizontal is what makes finning efficient.
- Steel cylinders are negative throughout the dive and aluminium ones go positive as they empty. Switching between them without re-checking your weighting is a genuine surprise at the safety stop.
Flying after diving
Driving over a mountain pass counts. Cabin altitude in an airliner is typically 6,000 to 8,000 feet, and plenty of roads out of dive destinations go higher than that — the pressure change is what matters, not whether you left the ground.
Published minimum surface intervals
| Case | Before flying | Notes |
|---|---|---|
| A single no-decompression dive | 12 h | The published minimum. Twelve hours is a floor rather than a target, and there is no benefit to cutting it fine. |
| Multiple dives, or several days of diving | 18 h | The case that covers most dive trips. Note this is from the LAST dive, not from the last day. |
| Any dive requiring decompression stops | 24 h | And treat 24 hours as a minimum rather than a plan. Decompression obligations and altitude are a poor combination. |
How to use
- Enter body weight, exposure suit, water type and cylinder material for a starting figure.
- Then do the in-water check — that is the number that counts.
- Use the no-fly section to work out when you are clear to travel.
Frequently asked questions
How much weight do I need?
The calculator gives a starting point from your suit, the water and the cylinder, but the only correct answer is the one you verify. With an almost empty cylinder, a deflated BCD and a normal breath held, you should float at eye level and sink slowly when you breathe out.
Why check with an empty tank?
Because an aluminium cylinder swings about four pounds positive as it empties, and you have to be able to hold a safety stop at the END of the dive. Weight yourself correctly with a full tank and you will be several pounds light exactly when it matters most.
Why is over-weighting such a problem?
It causes almost everything else. Excess lead means an inflated BCD to compensate, which means more drag, worse trim, harder finning, and a consumption rate that looks like a fitness problem and is not. It is the most common fault in recreational diving.
How long after diving before I can fly?
The published minimums are 12 hours after a single no-decompression dive, 18 after multiple dives or multiple days, and 24 after any decompression dive. Those are floors rather than targets, and there is no benefit to cutting them fine.
Does driving over a mountain pass count?
Yes. Cabin altitude in an airliner is typically 6,000 to 8,000 feet, and plenty of roads out of dive destinations go higher. The pressure change is what matters, not whether you left the ground.
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