The Rule of 120 in Scuba Diving: Understanding Depth, Bottom Time, and Dive Safety
Scuba diving in the Red Sea is an unforgettable experience. From the colourful coral gardens of Ras Mohammed to the spectacular reefs of the Strait of Tiran and the famous wreck of the SS Thistlegorm, Sharm El Sheikh offers incredible opportunities for divers of all experience levels.
But enjoying these underwater environments starts with understanding how to dive safely.
One diving guideline you may come across is the Rule of 120. It is a traditional rule of thumb that relates dive depth to an approximate bottom time. Although it can help divers understand the relationship between depth and time, it is important to know what this rule can and cannot tell you.
In this guide, we explain how the Rule of 120 works, why it should never replace proper dive planning, how Nitrox affects your dive limits, and which safety practices every recreational diver should understand.
What Is the Rule of 120 in Scuba Diving?
The Rule of 120 is a traditional recreational diving guideline that uses a simple calculation to relate depth in feet to approximate bottom time in minutes.
The formula is:
120 − Maximum Depth (in feet) = Approximate Bottom Time (in minutes)
For example, if you plan to dive to 60 feet, the calculation is:
120 − 60 = 60 minutes.
Here are some examples:
Maximum depth | Approximate time |
40 feet (12 metres) | 80 minutes |
50 feet (15 metres) | 70 minutes |
60 feet (18 metres) | 60 minutes |
66 feet (20 metres) | 54 minutes |
80 feet (24 metres) | 40 minutes |
100 feet (30 metres) | 20 minutes |
These figures demonstrate the traditional rule, not actual safe bottom-time limits.
Is the Rule of 120 a reliable way to plan a dive?
No. This is one of the most important things to understand.
The Rule of 120 is a simplified historical guideline. It does not account for all the factors that determine a diver's no-decompression limit.
Actual dive limits depend on factors such as:
Maximum depth and the complete dive profile.
Time spent at different depths.
Residual nitrogen from previous dives.
Breathing gas composition.
Individual factors and environmental conditions.
A dive computer or recognised dive-planning tables should always be used to establish appropriate dive limits.
Never use the Rule of 120 to decide how long you can safely remain underwater. It is not a substitute for formal training, dive tables or a properly configured dive computer.
Understanding No-Decompression Limits (NDL)
To understand safe dive planning, it helps to know what a no-decompression limit actually means.
When you breathe compressed air underwater, your body absorbs nitrogen from the breathing gas. As depth increases, the pressure increases, and nitrogen absorption becomes more significant.
The no-decompression limit (NDL) is the maximum planned time at a particular depth that allows a diver to ascend according to the applicable decompression model without requiring mandatory decompression stops.
As a general principle:
Deeper dives generally have shorter no-decompression limits.
Longer dives increase nitrogen absorption.
Repetitive dives must account for residual nitrogen from previous dives.
Multilevel dive profiles can have different limits from dives conducted at a constant depth.
Modern dive computers continuously track depth and time and calculate remaining no-decompression time according to their algorithms and settings.
However, a dive computer cannot guarantee that decompression sickness will not occur.
Divers should always plan conservatively, follow their training, and avoid treating the displayed NDL as a target to reach.
How Deep Can You Dive with 32% Nitrox?
Enriched Air Nitrox, commonly called Nitrox or EANx, is a breathing gas containing more oxygen and less nitrogen than ordinary air.
One of the most popular recreational mixtures is EAN32, which contains 32% oxygen.
Because Nitrox contains less nitrogen than air, it can reduce nitrogen absorption and extend no-decompression limits, particularly during repetitive dives at suitable depths.
However, Nitrox has an important limitation: oxygen toxicity.
What is the Maximum Operating Depth (MOD) of EAN32?
The Maximum Operating Depth (MOD) is the maximum depth at which a particular breathing gas can be used within the selected oxygen partial-pressure limit.
For recreational diving, a commonly used maximum oxygen partial pressure for the working part of a dive is 1.4 bar.
The calculation is:
MOD = [(Maximum PO₂ ÷ Oxygen Fraction) − 1] × 10
For EAN32:
MOD = [(1.4 ÷ 0.32) − 1] × 10
The result is approximately 33.75 metres.
The conventional MOD is therefore rounded down to 33 metres.
This is a gas-specific oxygen limit, not a recommendation that every diver should dive to that depth.
Does Nitrox allow you to stay underwater longer?
Nitrox can provide longer no-decompression limits compared with air at the same depth, but it does not automatically mean that every dive can be longer.
Your actual dive duration still depends on:
Your depth and dive profile.
Your remaining gas supply.
Your dive computer's settings.
Your oxygen exposure.
Your training and certification.
Before using Nitrox, divers must receive appropriate training, analyse the actual oxygen content of their cylinder, confirm the MOD, and configure their dive computer correctly.
Important: Never assume that a cylinder contains 32% oxygen simply because it is labelled as Nitrox. Always analyse and verify the actual gas mixture before diving.
SSI's Enriched Air Nitrox training covers gas analysis, oxygen exposure and appropriate dive planning.
Essential Rules for Safe Dive Planning
Understanding the Rule of 120 is only a small part of becoming a responsible diver. Proper planning involves much more than calculating depth and time.
Here are some important practices to follow before and during recreational dives.
1. Plan Your Dive Before Entering the Water
Discuss the dive with your buddy or guide before entering the water.
Agree on:
Maximum planned depth.
Expected dive duration.
Gas management and turn pressures.
Entry and exit procedures.
Communication signals.
Emergency procedures.
Conditions such as currents and visibility.
Make sure everyone understands the plan and stays within their training and experience.
2. Use a Properly Configured Dive Computer
A dive computer is one of the most important tools for recreational diving.
It tracks your depth and time and calculates decompression information throughout the dive.
Before entering the water:
Check that your computer is working correctly.
Confirm the correct gas setting.
Check the battery.
Understand its alarms and warnings.
Review your previous dive information when planning repetitive dives.
If you use Nitrox, make sure the oxygen percentage is set correctly.
Remember that your computer is a planning and monitoring tool, not a guarantee against decompression sickness.
3. Control Your Ascent Rate
A controlled ascent is essential for reducing decompression stress.
Avoid rushing towards the surface, and follow the ascent-rate guidance provided by your training agency and dive computer.
An ascent rate around 9–10 metres per minute is commonly used as a general reference, but the appropriate rate depends on the applicable training and computer guidance.
Pay particular attention to buoyancy control during the final part of your ascent.
4. Perform a Safety Stop When Appropriate
A safety stop is a standard recreational diving practice.
After many dives deeper than 10 metres, a stop of approximately three minutes at around 5 metres is recommended when conditions and gas supply permit.
A safety stop helps provide additional time for nitrogen elimination and encourages a controlled ascent.
However, a safety stop does not replace mandatory decompression stops, and it cannot guarantee that decompression sickness will be avoided.
Always follow your dive computer and training procedures.
5. Stay Hydrated and Rested
Diving in warm destinations such as Sharm El Sheikh can mean spending long periods in the sun, on boats and in the water.
Dehydration and fatigue are undesirable conditions when diving.
To prepare properly:
Drink enough water throughout the day.
Get sufficient sleep.
Avoid diving when unwell or excessively tired.
Take appropriate breaks between dives.
Avoid strenuous activity when it is not appropriate for your dive profile.
Good preparation supports safer and more comfortable diving.
6. Always Dive Within Your Training and Experience
A beautiful reef or famous wreck should never tempt you to exceed your certification or experience.
Consider the following before choosing a dive:
Your certification level.
Your recent diving experience.
Your buoyancy and air-consumption skills.
Current and visibility conditions.
The depth and complexity of the dive site.
For example, a diver who has never experienced strong currents should not start with a challenging drift dive simply because it is a popular excursion.
Choose dive sites that match your ability, and ask a qualified professional when you are unsure.
How Long Should You Wait Before Flying After Diving?
Flying too soon after scuba diving increases decompression stress because cabin pressure at altitude is lower than atmospheric pressure at sea level.
Divers should therefore allow sufficient surface time after their final dive before boarding a commercial flight.
Divers Alert Network (DAN) provides the following suggested minimum surface intervals for recreational divers without decompression sickness symptoms:
Dive profile | Suggested minimum interval |
One no-decompression dive | 12 hours |
Multiple dives in one day | 18 hours |
Diving over multiple days | 18 hours |
Dives requiring decompression stops | Substantially longer than 18 hours |
These are recommendations, not guarantees of safety. Longer intervals further reduce decompression risk.
For divers enjoying a multi-day diving holiday in Sharm El Sheikh, allowing at least 24 hours between the final dive and a flight is a sensible conservative approach whenever possible.
Divers who have symptoms that could indicate decompression sickness should seek urgent medical advice and must not fly without appropriate medical clearance.
Why Does Dive Planning Matter in Sharm El Sheikh?
Sharm El Sheikh offers a wide variety of diving environments, from calm shallow reefs to deeper walls, currents and historic wrecks.
Popular locations include:
Ras Mohammed: Famous for its spectacular coral reefs, Shark Reef and Yolanda Reef.
Strait of Tiran: Known for its impressive reef walls, marine life and possible currents.
SS Thistlegorm: A historic wreck that requires appropriate training and experience.
Local House Reef: Excellent opportunities for relaxed shore dives, skill development, and underwater exploration.
Every dive site has different conditions, and the best dive plan depends on the experience of the divers, weather, currents, and other environmental factors.
A dive that is suitable for an experienced diver may not be the right choice for someone who has just completed their first certification.
This is why choosing the right dive site and having a professional briefing are important parts of every diving experience.
Dive the Red Sea with Red Sea Marine
At Red Sea Marine Diving Center, we believe that great diving experiences start with proper preparation, professional guidance, and attention to individual needs.
Located at Ghazala Beach in Naama Bay, Sharm El Sheikh, we offer diving experiences for beginners, certified divers, and experienced underwater explorers.
Our services include:
SSI scuba diving courses from beginner to professional levels.
Guided boat diving trips around Sharm El Sheikh.
Guided shore dives on local reefs.
Enriched Air Nitrox training and diving options.
Specialty courses to develop your diving skills.
Diving adventures to famous Red Sea sites.
We take the time to understand your experience, discuss your preferences, and help select suitable dive sites.
Whether you are discovering scuba diving for the first time or returning to explore more of the Red Sea, our aim is to make your experience enjoyable, personal and professionally organised.
Ready to discover the Red Sea?
Explore our SSI Diving Courses or contact Red Sea Marine to plan your next diving adventure in Sharm El Sheikh.
Rule of 120 scuba diving
Frequently Asked Questions About the Rule of 120
Is the Rule of 120 an official scuba diving safety rule?
No. It is a traditional rule of thumb that relates depth in feet to approximate bottom time in minutes. It is not an approved substitute for dive tables, dive computers, or recognised dive-planning procedures.
What is the Rule of 120 formula?
The traditional formula is 120 minus the maximum depth in feet, giving an approximate bottom time in minutes. For example, at 60 feet, the calculation gives 60 minutes. This result must not be interpreted as a safe no-decompression limit.
What is the maximum depth for EAN32 Nitrox?
Using a maximum oxygen partial pressure of 1.4 bar, the calculated MOD for EAN32 is approximately 33 metres. The diver must also respect certification, training, dive computer settings, and other depth limitations.
Does Nitrox reduce the risk of decompression sickness?
Nitrox reduces nitrogen exposure compared with air at the same depth, which can extend no-decompression limits. However, it does not eliminate decompression sickness risk. Proper training and conservative dive planning remain essential.
Is a safety stop necessary after every dive?
A three-minute safety stop at approximately 5 metres is common recreational practice after many dives deeper than 10 metres. It is not a replacement for mandatory decompression stops. Follow your dive computer and training procedures.
How long should I wait before flying after diving?
DAN suggests at least 12 hours after a single no-decompression dive and at least 18 hours after repetitive or multi-day no-decompression diving. Longer intervals are more conservative. A 24-hour interval after a diving holiday is a sensible approach whenever possible.
Can beginners use the Rule of 120 to plan their first dive?
No. Beginners and introductory divers should follow their instructor's guidance and the specific limits of their training or introductory diving programme. The Rule of 120 should not be used to plan an introductory dive.
Final thought: Good diving is not about staying underwater for as long as possible or reaching the greatest depth. It is about making sensible decisions, understanding your limits, and enjoying every dive safely.
The Red Sea has plenty to offer. Take your time, plan carefully, and enjoy every moment underwater.