So what's the deal with NDL and the 120 rule

To most recreational divers, the No Decompression Limit (NDL) is the line in the sand between being and not being in a state of deco. You know, those long tedious safety stops that you need to endure while off-gassing Nitrogen.

Simply put NDL is the maximum limit in time that you can stay at a given depth without having to decompress (off-gas Nitrogen) on the way up.; yes those tedious rides to the surface.

Fortunately unlike most technical things about recreational diving (remember those RDP tables)., working out your NDL is relatively straightforward. Enter, stage right the 120 rule:

120 - depth in feet = NDL.

For example. at 33 feet the NDL would be 87 minutes.

So the longest you can stay on air at 33 feet is 87 minutes without going into deco.

What's that I hear you say, - what if you are a metric person.? Sadly this does not work quite well for metric folks. Only kidding OF COURSE IT DOES. 33 feet happens to be 10 meters. so every meter is 3.3 feet approximately.

If you are a metric person then all you need to really do is multiply in your head the depth in meters by 3.3 and then subtract the result (feet) and you are back on track.

OR

You can do it the lazy way and build yourself a calculator on your phone that does all the heavy lifting.

What then happens if you want to plunge to 40 meters? Well, that's when yo need the good old RDP tables. to explain why you should not go there while certified as a Recreational diver unless in an emergency. But if you insist, be prepared for all sorts of fancy stops on the way up and spoil the rest of your day..