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Buoyancy Calculations — PADI IDC Physics

Buoyancy questions decoded — PADI IDC and Divemaster physics exam

Will Welbourn introduces the key to decoding buoyancy questions in the PADI IDC and Divemaster exam — identifying the unit of measurement in the answer options and converting everything else to match using the kg-to-litres method.

Buoyancy is the most frequently failed topic in the PADI IDC physics exam — not because the concept is difficult, but because there are two distinct question types that look similar on the exam paper and each requires a different method. Part 1 covers describing buoyancy state: given an object's weight and volume, is it positive, negative, or neutral? It also covers what happens when an object moves between salt and fresh water — the question cluster that generates more uncertainty than any other in the exam. Part 2 covers finding an unknown weight or volume when one of those values is missing from the question.

The one number that drives all buoyancy calculations 1 litre of salt water weighs 1.03 kg. 1 litre of fresh water weighs 1 kg. That small difference — 0.03 kg per litre — is what makes objects behave differently in the two water types, and it underpins every calculation on this page.
If you only take one thing from this page — take this Students struggle with buoyancy calculation questions because of all the words. The trick is to ignore the words and look at the answer options first.

Are the answers in litres? Find the kg figure in the question. Divide it by 1.03.
Are the answers in kg? Find the litres figure in the question. Multiply it by 1.03.
Is it fresh water? Weight and volume are the same number. No calculation needed.

KG → L    Divide   |   L → KG    Multiply

Once you have converted the numbers to the same unit as the answer, the calculation almost solves itself. Everything else on this page builds on top of this one rule.
Salt water vs fresh water buoyancy — the fresh water is more negative mental model

Will Welbourn explains the salt/fresh water buoyancy transition using the "fresh water is more negative" mental model — a practical alternative approach for answering cannot-be-determined questions in the PADI IDC and Divemaster exam.

Part 1 — Describing Buoyancy State

Buoyancy state questions give you some combination of an object's weight, volume, and water type and ask you to describe what will happen to it. There are four things that determine what happens to any object in water, and if you know three of them, you can almost always find the fourth.

The four factors
  • The weight of the object (its downward force)
  • The volume (displacement) of the object
  • The type of water it is in (salt or fresh)
  • The current buoyancy state of the object (positive, negative, or neutral)

Moving between water types — what we can and cannot determine

Before reading on, think through each of these scenarios. For each one, decide whether you can say with certainty what will happen, or whether the answer is "cannot be determined." Then read the explanation below.

  • An object is neutrally buoyant in salt water. You move it to fresh water. What happens?
  • An object is positively buoyant in salt water. You move it to fresh water. What happens?
  • An object is negatively buoyant in salt water. You move it to fresh water. What happens?
  • An object is neutrally buoyant in fresh water. You move it to salt water. What happens?
  • An object is positively buoyant in fresh water. You move it to salt water. What happens?
  • An object is negatively buoyant in fresh water. You move it to salt water. What happens?
Part 1 — describing buoyancy state when moving between salt and fresh water

Will Welbourn explains what we can and cannot say about an object's buoyancy when it moves between salt and fresh water — PADI IDC and Divemaster physics exam study notes.

The logic behind each scenario comes down to one fact: salt water provides more upward force than fresh water (1.03 kg per litre vs 1 kg per litre). With that in mind:

Starting state Starting water Moving to Result
Neutral Salt water Fresh water Will sink (negative) — less upward force
Neutral Fresh water Salt water Will float (positive) — more upward force
Positive Salt water Fresh water Cannot be determined
Negative Salt water Fresh water Will definitely sink — less upward force, already sinking
Positive Fresh water Salt water Will definitely float — more upward force, already floating
Negative Fresh water Salt water Cannot be determined
The "cannot be determined" trap The two "cannot be determined" cases are the ones the exam tests most: positive in salt water moving to fresh, and negative in fresh water moving to salt. In both cases, moving to the other water type works against the object — but we don't know by how much. If a positive object has a huge buoyancy margin, it will still float in fresh water. If it was only just positive, the drop in upward force might tip it to negative. Without knowing the exact kg of positive or negative buoyancy, there is no way to say. Only neutral objects give us a certain answer when the water type changes.

Calculating buoyancy state — the 2-step method

When you have an object's weight and volume, calculating its buoyancy state is a two-step process. The first step finds the upward force. The second step finds the difference between the upward and downward forces — and the sign of that difference tells you the buoyancy state.

The 2-step method — Part 1 Step 1 — Upward force: volume × water constant (1.03 for salt, 1 for fresh)
Step 2 — Buoyancy: upward force − weight

Result is positive → positively buoyant (floats)
Result is negative → negatively buoyant (sinks)
Result is zero → neutrally buoyant
Buoyancy calculations Part 1 — calculating an object's buoyancy state

Will Welbourn explains how to calculate an object's buoyancy state using the upward force and downward force diagram method — PADI IDC and Divemaster physics exam study notes.

Part 1 — Worked examples

Example Q1 — is it positive, negative, or neutral? An object has a weight of 209 kg and a volume of 200 litres. It is in salt water. What is its buoyancy state?
  1. Upward force: 200 × 1.03 = 206 kg
  2. Buoyancy: 206 − 209 = −3 kg
The object is negatively buoyant by 3 kg. It sinks.
Buoyancy calculations Part 1 example question 1 — PADI IDC physics exam

Will Welbourn works through buoyancy example question 1 — finding the buoyancy state of an object given its weight and volume in salt water.

Example Q2 — is it positive, negative, or neutral? An object has a weight of 51 kg and a volume of 50 litres. It is in salt water. What is its buoyancy state?
  1. Upward force: 50 × 1.03 = 51.5 kg
  2. Buoyancy: 51.5 − 51 = +0.5 kg
The object is positively buoyant by 0.5 kg. It floats.
Buoyancy calculations Part 1 example question 2 — PADI IDC physics exam

Will Welbourn works through buoyancy example question 2 — a positive buoyancy result where the upward force narrowly exceeds the object's weight.

Example Q3 — is it positive, negative, or neutral? An object has a weight of 309 kg and a volume of 300 litres. It is in salt water. What is its buoyancy state?
  1. Upward force: 300 × 1.03 = 309 kg
  2. Buoyancy: 309 − 309 = 0 kg
The object is neutrally buoyant. Upward and downward forces are exactly equal.
Exam trap — the numbers that seem too close to call Examples Q2 and Q3 are designed to test this. In Q2, the volume (50 L) and weight (51 kg) look almost identical — students who skip the multiplication and just compare the two numbers directly will get the direction right by luck but miss the actual buoyancy figure. In Q3, the numbers look like they should produce a round result — and they do, but only after multiplying by 1.03 first. Always complete the multiplication step before drawing any conclusion.
Confident with Part 1? If you are comfortable with buoyancy state and the salt/fresh water transition rules, the first five questions in the practice quiz cover exactly what you have just studied. Come back for Part 2 when you are ready — or if questions 6 to 10 catch you out.

Jump to the practice quiz ↓

Part 2 — Finding an Unknown Weight or Volume

Part 2 questions give you either the weight or the volume of an object — but not both — and ask you to find the missing value. These are the questions that produce the most wrong answers in the PADI IDC physics exam. Before reading the method, look at these four question types and think about what is different between them:

  • An object with a volume of 150 L is neutrally buoyant in fresh water. What is its weight?
  • An object with a volume of 150 L is neutrally buoyant in salt water. What is its weight?
  • An object with a weight of 150 kg is neutrally buoyant in salt water. What is its volume?
  • An object with a volume of 75 L is negatively buoyant by 20 kg in salt water. What is its weight?
Before you read any further — look at the answer options This is the rule from the top of the page, applied directly to the questions above. For each one, look at what the question is asking for — is the answer in kg or in litres? Then find the number in the question that is in the other unit, and convert it:

Answer in litres, salt water → find the kg → divide by 1.03
Answer in kg, salt water → find the litres → multiply by 1.03
Fresh water → weight = volume, same number

For the first three questions above, that one step gives you the answer directly. Question 4 has an extra piece of information (the 20 kg offset) which adjusts the result — but the same starting rule applies.

The first three questions look nearly identical. The fourth introduces a new layer. The method for each is different, and choosing the wrong one is the most common source of errors in this section.

The key insight — neutral means upward force equals downward force

Every Part 2 calculation starts from the same place: the upward force. Once you know the upward force, you can find either the weight or the volume depending on what the question is asking.

Neutral means the two forces are equal If an object is neutrally buoyant, its upward force and its downward force are exactly the same. That is what neutral means. This is the bridge that lets you calculate the missing value:

• Know the volume → find the upward force → that equals the weight
• Know the weight → that equals the upward force → reverse to find the volume

The fresh water shortcut

In fresh water, 1 litre weighs exactly 1 kg. That means for a neutrally buoyant object in fresh water, weight and volume are always the same number. No multiplication required.

Fresh water shortcut — neutral objects only Neutral in fresh water → weight (kg) = volume (litres). Write down the number you have and you already have the answer.
Buoyancy calculations Part 2 example question 1 — neutral in fresh water

Will Welbourn works through Part 2 example question 1 — finding the weight of an object that is neutrally buoyant in fresh water, where no calculation is needed.

Salt water — finding weight from volume

In salt water, 1 litre weighs 1.03 kg. To find the upward force, multiply volume by 1.03. Since the object is neutral, upward force equals weight — so that multiplication gives you the weight directly.

Salt water — volume to weight (neutral object) Weight = volume × 1.03

Memory aid: KG has two letters. L has one. Going from L to KG is going from fewer letters to more — you are getting bigger, so you multiply.
Buoyancy calculations Part 2 example question 2 — neutral in salt water, find weight

Will Welbourn works through Part 2 example question 2 — finding the weight of an object that is neutrally buoyant in salt water.

Salt water — finding volume from weight

If you know the weight and need the volume, you reverse the process. The weight equals the upward force (neutral), and upward force = volume × 1.03, so volume = weight ÷ 1.03.

Salt water — weight to volume (neutral object) Volume = weight ÷ 1.03

Memory aid: KG has two letters. L has one. Going from KG to L is going from more letters to fewer — you are getting smaller, so you divide.
Buoyancy calculations Part 2 example question 3 — neutral in salt water, find volume

Will Welbourn works through Part 2 example question 3 — finding the volume of an object that is neutrally buoyant in salt water.

When the object is not neutral — the offset method

The questions above all involved neutral objects, where upward force and downward force are equal by definition. The trickier Part 2 questions tell you the object is positive or negative by a known number of kilograms. That known offset is the additional step between the upward force and the weight.

The offset method — non-neutral objects Negative by X kg means the downward force exceeds the upward force by X kg.
• Finding weight: upward force + X = weight
• Finding volume: weight − X = upward force → then ÷ 1.03

Positive by X kg means the upward force exceeds the downward force by X kg.
• Finding weight: upward force − X = weight
• Finding volume: weight + X = upward force → then ÷ 1.03
Which direction does the offset push? The offset always adjusts between the upward force and the weight. If the object is negative, its weight is heavier than the water it displaces — add the offset to the upward force to reach the weight. If the object is positive, its weight is lighter than the water it displaces — subtract the offset from the upward force to reach the weight. Getting this backwards is the single most common error in Part 2 questions.
Buoyancy calculations Part 2 example question 4 — negatively buoyant, find weight

Will Welbourn works through Part 2 example question 4 — finding the weight of an object with a known negative buoyancy offset in salt water.

Example Q4 — negative offset, find weight An object has a volume of 75 L and is negatively buoyant by 20 kg in salt water. What is its weight?
  1. Upward force: 75 × 1.03 = 77.25 kg
  2. Negative by 20 kg → weight = upward force + 20: 77.25 + 20 = 97.25 kg
Weight = 97.25 kg. Answer C.
Buoyancy calculations Part 2 example question 5 — positively buoyant, find weight

Will Welbourn works through Part 2 example question 5 — finding the weight of an object with a known positive buoyancy offset in salt water.

Example Q5 — positive offset, find weight An object has a volume of 75 L and is positively buoyant by 20 kg in salt water. What is its weight?
  1. Upward force: 75 × 1.03 = 77.25 kg
  2. Positive by 20 kg → weight = upward force − 20: 77.25 − 20 = 57.25 kg
Weight = 57.25 kg. Answer C.
Buoyancy calculations Part 2 example question 6 — negatively buoyant, find volume

Will Welbourn works through Part 2 example question 6 — finding the volume of an object with a known negative buoyancy offset in salt water.

Example Q6 — negative offset, find volume An object weighs 75 kg and is negatively buoyant by 20 kg in salt water. What is its volume?
  1. Negative by 20 kg → upward force = weight − 20: 75 − 20 = 55 kg
  2. Volume = upward force ÷ 1.03: 55 ÷ 1.03 = 53.4 L
Volume = 53.4 L. Answer B.
Buoyancy calculations Part 2 example question 7 — positively buoyant, find volume

Will Welbourn works through Part 2 example question 7 — the final example in this section, finding the volume of an object with a known positive buoyancy offset in salt water.

Example Q7 — positive offset, find volume An object weighs 75 kg and is positively buoyant by 20 kg in salt water. What is its volume?
  1. Positive by 20 kg → upward force = weight + 20: 75 + 20 = 95 kg
  2. Volume = upward force ÷ 1.03: 95 ÷ 1.03 = 92.2 L
Volume = 92.2 L. Answer B.

Putting it all together — subscriber question

The following question involves two separate calculations in sequence. It tests whether you can hold the result of the first calculation and use it as the starting point for the second. This is the most complex question type in Part 2 — work through the video first, then check your working against the example below.

Buoyancy Part 2 — compound true/false subscriber question, PADI IDC physics exam

Will Welbourn works through a compound buoyancy true/false question — calculating initial buoyancy state, then applying an additional weight to find whether the final statement is correct.

Example Q8 — compound true/false (subscriber question) An object weighs 82 kg in a freshwater lake and displaces 93 litres of water. You would need to add 15 kg of weight to make it 4 kg negatively buoyant. True or false?
  1. Upward force (fresh water): 93 × 1 = 93 kg
  2. Current buoyancy: 93 − 82 = +11 kg (positively buoyant by 11 kg)
  3. New downward force after adding 15 kg: 82 + 15 = 97 kg
  4. New buoyancy: 93 − 97 = −4 kg (negatively buoyant by 4 kg)
True. Adding 15 kg produces exactly −4 kg buoyancy.
True/false questions — read the full statement before calculating This question type tests whether you arrive at the stated conclusion through your own working, not whether you trust the statement. Work out the answer independently first — only then check whether what the question claims is correct. Candidates who jump to confirming the numbers given lose marks when those numbers turn out to be wrong.

Practice — Buoyancy

Questions 1–5 cover Part 1 (buoyancy state and salt/fresh water transitions). Questions 6–10 cover Part 2 (finding unknown weight or volume). If you jumped here early and questions 6–10 caught you out, go back to Part 2 and work through the teaching notes and examples before trying again.

Physics — Topics

Pressure Depth Calcs Density Partial Pressure Buoyancy Lift Bags Fresh Water General Knowledge
Practice Questions ← Physics Hub ← Theory Hub