To calculate how long your breathing gas will last at a certain depth, you need to know your:

To determine how long your breathing gas will last at a certain depth, it is essential to know your Surface Air Consumption (SAC) rate. The SAC rate is a measure of how much air you consume while diving, typically expressed in liters or cubic feet per minute at the surface. This is critical because it allows you to calculate how much gas you will use over time based on the depth and the corresponding pressure.

At greater depths, the pressure increases, and consequently, the volume of gas in your tank will compress. The SAC rate helps to convert this consumption rate to the conditions at the depth you are diving, enabling you to estimate how long your air supply will last. To make precise calculations, you can use your SAC rate to assess the equivalent gas consumption at depth, typically through a formula that factors in depth and tank capacity.

While the tank pressure, average depth, and even a buddy's SAC rate can provide supportive information and may be relevant in diving planning or risk management, they are not as directly integral to calculating your personal gas consumption duration under the water.