PADI IDC Theory - Physics

Test yourself with these questions

PADI IDC Theory Physics

Ok, let’s take things one step at a time.
Here we’ll talk about THREE major topics that come up time and time again on PADI Physics exams.

1. Calculating Pressure

2. The Effects of Pressure

3. Displacement

In each of the areas, I’ll give a few tips, and show you a video.

Then you’ll get a chance to answer some questions.

Each of the videos are from my Online Preparation Course. 

You can watch the video first, or you can steam straight into the questions. 

Good luck! 

1. How to calculate pressures underwater

Every 10 metres or 33 feet of seawater, Gauge pressure changes by 1 atm.
Total, or Absolute pressure includes both Gauge pressure and Atmospheric pressure.
E.G. Pressure at 10 metres or 33 feet = 1 amt of Gauge pressure and 1 atm for the atmospheric pressure. 1+1=2 atm.

Why this is important

You need to be able to calculate pressures in seawater to be able to answer all those quesions like:
“what size will a balloon become if you take it down to 30 metres/99 feet? 

Test Questions - PADI IDC Physics - pressures underwater

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Extra information

For every one metre in sea water, pressure changes by 0.1 atm. Example: at 23 metres pressure is: 23 X 0.1 = 2.3 atm for gauge, +1 = 3.3 for absolute. 
For every foot of sea water, pressure changes by 0.0303 atm. Example at 76 feet pressure is 76 X 0.0303 = 2.3 atm for gauge + 1 = 3.3 for absolute
Fresh Water
In fresh water, it’s much the same, but for metric use 0.097 for every metre, and in imperial use 0.0294 for every foot.Effe

2. How to calculate - effects of pressure

As you get deeper, some things get smaller (e.g volumes) and some things get larger (e.g.density, breathing rates, gas percentages) 

Use your practical diving experience to work out whether the answer will be larger or smaller. Then either multiply or divide the number by the absolute pressure.

Why this is important

Usually PADI Physics exam questions have more questions about the effect of pressure than any other topic.

All questions related to depth can be answered using the methods in my video: 

  • Volume of balloons
  • Breathing rates
  • Partial pressures of gasses
  • How much air needs to pumped from the surface

Test Questions - PADI IDC Physics - effect of pressure

3. How to calculate PADI displacement questions


In fresh water, it’s often as simple as taking one number (litres of displaced water) from another (weight in kilos of the object). 

In salt water, first divide the weight (kilos) by 1.03, and then go ahead as with fresh.


Multiply the cubic feet of displaced water by 62.4 in fresh water, or 64 in sea water, then deduct that number from the weight (pounds) of the object. Divide that number by either 62.4 or 64 depending on freshwater or seawater to get the answer in cubic feet.

Why this is important

Back in ancient Greek times, Archimedes said: “a body immersed in a fluid experiences an upthrust equal to the weight of the fluid displaced.”

But he didn’t need to take PADI IDC Exams on diving physics!

The good news is that it’s SO much easier than some people think! Take a look at the video before you attempt the questions. 

Test Questions - PADI IDC Physics - displacement

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How did you get on?

Don’t worry if you got a bit confused with these questions. They’ll become clearer with practice. 

The more that you practice, the more you’ll understand and the more confident you’ll get. 

Here is a lot more help with PADI IDC Dive Theory

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  • 5  Theory Exams each with 12 questions
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Dive Theory for PADI Instructors

PADI Dive Theory - Physics

I’ve met loads of people who tell me that they’re hopeless when it comes to Physics.  

I’m hoping that these videos and test IDC questions have helped. 

Last modified: 26th February 2024
Author: Steve Prior