Most divers only need one tank filled with compressed air, not pure oxygen. Still, the right number depends on your dive depth, duration, and safety plan. This guide clears up the confusion about scuba tanks, explains when more than one might be necessary, and walks you through planning the right setup for your dive—step by step.

What Do Scuba Divers Really Breathe Underwater?

Compressed Air – The Standard for Most Dives

If you’re diving recreationally, you’ll most likely be using compressed air. It’s the same air we breathe on land—around 21% oxygen and 78% nitrogen—stored at high pressure in a scuba tank on your back.

Nitrox – For Longer Bottom Times

Some divers use Nitrox, a mix with more oxygen (usually 32% or 36%) and less nitrogen. This reduces nitrogen buildup in your body, letting you stay underwater longer and lowering the risk of decompression sickness.

Mixed Gases – Used for Deeper Diving

For deeper dives, gas blends like trimix (oxygen, nitrogen, and helium) or heliox (helium and oxygen) are used to reduce narcosis and oxygen toxicity.

Heliox and Hydrox – For Extreme Depths

Heliox helps prevent nitrogen narcosis during deep dives. In rare cases, professionals may use hydrox (hydrogen and oxygen) for extremely deep or experimental dives.

Regulators – Making Air Breathable Underwater

The air in your tank is under high pressure. A regulator reduces that pressure to match the surrounding water so you can breathe comfortably and safely at depth.

Related Reading: What Is in a Scuba Tank? A Complete Guide for Divers

How Many Tanks Are Typically Needed for One Scuba Dive?

One Tank Is Enough for Most Dives

For recreational diving, one 11-liter aluminum tank (also known as an "80-cubic-foot" tank) is typically enough for a dive lasting 30–60 minutes.

When Divers Use Two Tanks

Some divers use two tanks when diving deeper, staying down longer, or if they know their air consumption is high. It’s also common in technical or cave diving.

Picking the Right Tank Size

Larger tanks provide more air but are heavier. Your ideal tank depends on your depth, time underwater, and breathing rate.

Double Tank vs. Two-Tank Dives

Double tank diving means using two tanks at once during a single dive. A two-tank dive means doing two separate dives in a day, usually with one tank per dive.

Backup and Specialized Tanks

Many divers carry a pony bottle as an emergency air source. Others use tanks with special mixes like Nitrox or Trimix for advanced dives.

How to Calculate Your Air Needs Before a Dive

Start With Your SAC Rate

Your Surface Air Consumption (SAC) rate tells you how much air you use per minute at the surface. It’s key for estimating how much air you’ll need underwater.

Calculate PSI Used Per Minute

To find your SAC rate, divide the PSI used by the dive time, then divide that by the pressure at your average depth.

Related Reading: How Long Does 1 Scuba Tank Last? Everything You Need To Know

Understand Depth Pressure Changes

Water pressure increases with depth. Use the formula: (Depth in feet ÷ 33) + 1 to get pressure in the atmosphere.

Estimate Total Air Needed

Multiply your adjusted SAC rate by your dive time to get total air use. Check if your tank size and pressure cover this amount.

Always Follow the Rule of Thirds

Use one-third of your air for descending, one-third for returning, and save one-third for emergencies. It’s a simple but effective safety rule.

Backup Air Options Divers Use

Bailout Cylinders and Stage Tanks

These smaller tanks have their own regulator and can be worn separately. They serve as full backup air systems for emergencies or planned gas switches.

Spare Air for Quick Emergencies

Spare Air is a compact emergency tank that gives you just enough air to surface or reach your buddy in a crisis.

Sharing Air With a Buddy

Most divers carry an octopus—an extra regulator—for air sharing in case their buddy runs low.

Air Sharing Techniques Matter

Some divers are trained to donate their primary regulator and switch to their backup. Others pass the octopus. It depends on the setup and dive plan.

Why Technical Divers Use Multiple Tanks

Tech divers often carry multiple tanks with different gas blends and backup systems for complex, deep, or decompression dives.

What Gear Setup Do Professional or Experienced Divers Use?

Core Equipment Built for Performance

Advanced divers prioritize high-quality gear: well-fitted suits, a comfortable BCD, and a reliable regulator system.

Powerful Fins and a Clear Mask

Strong fins and a wide-view mask improve navigation, efficiency, and safety—especially in tough currents or deeper dives.

Dive Computers and Navigation Tools

Dive computers track depth, time, and ascent rate. A compass helps with navigation, especially in low visibility.

Safety Gear and Emergency Tools

Pro divers often carry a dive knife, torch, signaling devices, and even slates or underwater communication tools for emergencies.

Rebreathers and Specialized Setups

Rebreathers recycle exhaled air and extend dive time. They’re used in military, cave, or technical diving where long or silent dives are needed.

Common Misconceptions About Oxygen Use in Scuba

Scuba Tanks Are Not Filled With Pure Oxygen

Despite what many think, scuba tanks typically contain compressed air—not pure oxygen—which mirrors the air we breathe on land.

Pure Oxygen Can Be Dangerous at Depth

Oxygen toxicity becomes a risk at deeper depths. Breathing pure oxygen underwater can lead to serious symptoms like nausea or seizures.

Pure Oxygen Is Only Used in Emergencies

Pure oxygen is used in first aid for diving injuries like decompression sickness, not as a standard gas mix during dives.

Nitrox Doesn’t Reduce Air Use

Nitrox reduces nitrogen absorption, allowing longer bottom times. However, it doesn’t change how fast you breathe or how much air you consume.

“Oxygen Tank” Is a Misleading Term

Calling scuba tanks “oxygen tanks” is inaccurate. They typically contain compressed air or Nitrox—not medical-grade oxygen.

Final Thoughts – How Many Tanks Do You Really Need?

One Tank Is Enough for Most Recreational Dives

For beginner and recreational divers, one standard tank is usually enough for a safe and enjoyable dive.

You Might Need More for Longer or Deeper Dives

If your dive is deep or extended, you may need a larger tank or even a second one. It depends on your plan and air consumption.

Backup Tanks Provide Extra Safety

Having a pony bottle or second tank adds safety and peace of mind—especially when diving solo or in remote areas.

Let Your Dive Plan Guide Your Setup

There’s no one-size-fits-all answer. Base your setup on dive depth, time, training level, and your personal safety comfort zone.

Conclusion

 In most cases, one tank filled with compressed air is all you need for a recreational dive. But if you’re going deeper, staying longer, or want more safety, using an extra tank or backup air system is a smart move. Plan ahead, know your limits, and choose the tank setup that fits your dive.