How do you calculate the air volume needed to fill a lift bag in salt water?

Two steps: first convert the object's weight in kilograms to the equivalent water displacement by dividing by 1.03. This gives the total volume of salt water the object needs to displace to become neutrally buoyant. Then subtract the volume the object is already displacing. The result is the minimum air volume the lift bag needs to provide. For example: 175 kg object, 45 litres volume — 175 ÷ 1.03 = 169.9 litres needed; 169.9 − 45 = 124.9 litres of air required.

How do you calculate the air volume needed for a lift bag in fresh water?

In fresh water the calculation is simpler because density is 1 kg per litre — no division by 1.03 is needed. Subtract the object's existing volume directly from its weight in kilograms. For example: 175 kg object, 45 litres volume — 175 − 45 = 130 litres of air required. The method is identical to salt water except the division step is removed.

What is the most common mistake on lift bag questions in the PADI physics exam?

The most common mistake is forgetting to subtract the volume already displaced by the object. Many candidates calculate the total displacement volume correctly but do not remove the object's existing volume. The lift bag only needs to make up the shortfall — not provide the full displacement from zero. Always complete both steps: find total displacement needed, then subtract existing displacement.

Lift Bag Questions

Physics — Topics

Pressure Depth Calcs Density Partial Pressure Buoyancy Lift Bags Fresh Water General Knowledge

Practice Questions ← Physics Hub ← Theory Hub

Lift bag calculations tutorial — PADI IDC physics two-step method

Will Welbourn introduces the two-step method for calculating lift bag volume in the PADI IDC physics exam.

What Lift Bag Questions Test

Everything you need to solve a lift bag question follows directly from what you already know about buoyancy calculations. The only new piece is this: an object sitting on the bottom is already displacing some water through its own physical volume. The lifting device does not need to do all the work — it only needs to make up the shortfall.

The core concept The object has a volume. That volume is already displacing water and generating some upward force. The lifting device fills the gap between the upward force the object already provides and the total upward force needed to make it neutrally buoyant.

The Memory Aid — KG to L, L to KG

You learned this in the buoyancy section. It applies directly here and it is the single most useful tool for avoiding the most common mistakes on lifting device questions.

KG → L: Divide | L → KG: Multiply L has 1 letter. KG has 2 letters. The bigger unit has more letters — so going from the bigger unit to the smaller one means dividing, and going the other way means multiplying. When converting kg to litres: divide by 1.03 (salt water) or carry the number straight across (fresh water). When converting litres to kg: multiply by 1.03.

Lift bag questions always ask for an answer in litres. The object's weight is in kg. That means you are always converting kg → L — which means you always divide. You should never be dividing a volume in litres, and you should never be multiplying a weight in kg. If you find yourself doing either of those, you are working in the wrong direction.

The Two-Step Method

Apply these two steps in order on every lifting device question — salt water or fresh water, simple or complex.

Identify the water type, then find the neutral volume. Calculate the total litres of displacement needed to make the object neutrally buoyant. Salt water: divide the weight (kg) by 1.03. Fresh water: the neutral volume equals the weight — no conversion needed.
Subtract the object's own volume. The object already displaces some water. Subtract its volume (litres) from the neutral volume. The result is the minimum displacement the lifting device must provide.

Fresh Water — The Simple Case

In fresh water, 1 litre of water weighs exactly 1 kilogram. The weight in kg and the neutral volume in litres are the same number. There is nothing to convert. Step 1 requires no calculation at all — carry the weight straight across as litres, then subtract the object's volume. Fresh water density and its effect on buoyancy calculations are covered in more detail on the fresh water physics page.

Fresh water Neutral volume (litres) = weight (kg). Subtract the object's own volume. That is the answer. No division, no multiplication — the simplest calculation in the physics exam.

Salt Water — Divide First, Then Subtract

In salt water, 1 litre of water weighs 1.03 kg. The weight is in kg and the answer must be in litres. The memory aid confirms the approach: kg → L means divide. Divide the weight by 1.03 to get the neutral volume in litres, then subtract the object's own volume.

Worked Example — Question 1 (Salt Water)

An object weighs 175 kg and has a volume of 45 litres. It is in salt water. What is the minimum amount of water that needs to be displaced using a lifting device to bring the object to the surface?

Lift bag example question 1 — salt water, 175 kg object — PADI IDC physics

Will Welbourn works through a salt water lift bag calculation using a 175 kg, 45 litre object.

Worked Example 1 — 175 kg object, 45 L volume, salt water

Salt water. Weight is in kg, answer must be in litres. KG → L: divide. 175 ÷ 1.03 = 169.9 L (neutral volume)
Subtract the object's own volume → 169.9 − 45 = 124.9 L

Minimum displacement required: approximately 125 L

Worked Example — Question 2 (Salt Water)

An object weighs 325 kg and has a volume of 160 litres. It is in salt water. What is the minimum amount of water that needs to be displaced using a lifting device to bring the object to the surface?

Lift bag example question 2 — salt water, 325 kg object — PADI IDC physics

Will Welbourn works through a salt water lift bag calculation using a 325 kg, 160 litre object.

Worked Example 2 — 325 kg object, 160 L volume, salt water

Salt water. KG → L: divide. 325 ÷ 1.03 = 315.5 L (neutral volume)
Subtract the object's own volume → 315.5 − 160 = 155.5 L

Minimum displacement required: approximately 156 L

Worked Example — Question 3 (Fresh Water)

An object weighs 175 kg and has a volume of 45 litres. It is in fresh water. What is the minimum amount of water that needs to be displaced using a lifting device to bring the object to the surface?

This is the same object as Question 1, but in fresh water. The answer is different — and this time the calculation is simpler, not harder.

Lift bag example question 3 — fresh water, 175 kg object — PADI IDC physics

Will Welbourn works through a fresh water lift bag calculation — same object as Question 1, different answer.

Worked Example 3 — 175 kg object, 45 L volume, fresh water

Fresh water. Neutral volume equals the weight — 175 L. No conversion needed.
Subtract the object's own volume → 175 − 45 = 130 L

Minimum displacement required: 130 L

Same object, different water type, different answer Q1 (salt water): 125 L. Q3 (fresh water): 130 L. The fresh water answer is larger because fresh water is less dense — each litre of displacement provides less upward force, so more total displacement is needed. Always identify the water type before starting Step 1.

Common Exam Mistakes

Now that the method is clear, it is worth knowing exactly how students go wrong — so you can recognise if you are heading in the wrong direction before you commit to an answer.

The two most common mistakes Mistake 1: Multiplying the volume (litres) by 1.03. This converts litres to kg — the wrong direction. The answer needs to be in litres, not kg.
Mistake 2: Multiplying the weight (kg) by 1.03. This makes the kg number bigger, not smaller, and does not convert it to litres at all.
Both mistakes produce a number in kg when the answer must be in litres. The memory aid catches both: the answer is in litres, the weight is in kg, so you are going KG → L — divide.

Practice Quiz — Lift Bag Calculations

Physics — Topics

Pressure Depth Calcs Density Partial Pressure Buoyancy Lift Bags Fresh Water General Knowledge

Practice Questions ← Physics Hub ← Theory Hub