Accurately determine the apparent weight of any object when submerged in fluid. Use our professional tool to calculate weight of submerged object for engineering, diving, and construction applications.
The mass of the object when weighed in air (Vacuum/Standard).
Please enter a valid positive mass.
Custom Material
Steel (7850)
Concrete (2400)
Aluminum (2700)
Lead (11340)
Wood – Oak (700)
Density determines the volume displaced. Water density is ~1000 kg/m³.
Density must be positive.
Custom Fluid
Fresh Water (1000)
Sea Water (1025)
Oil / Petroleum (800)
Mercury (13560)
Standard fresh water is 1000 kg/m³. Sea water is typically 1025 kg/m³.
Fluid density must be positive.
Apparent Submerged Weight
4,305.4 N
Object Sinks
True Weight (in Air)
4,905.0 N
Buoyant Force (Upward)
599.6 N
Volume Displaced
0.0637 m³
Formula Used: Wsub = Wair – (ρfluid × Vobj × g)
Force Breakdown for Submerged Object
Parameter
Value
Unit
What is calculate weight of submerged object?
To calculate weight of submerged object is to determine the "apparent weight" of an item when it is completely immersed in a fluid (usually water). Unlike weighing an object in a vacuum or air, weighing an object underwater yields a lower value due to the upward force exerted by the fluid, known as buoyancy.
This calculation is critical for engineers designing offshore structures, scuba divers determining ballast requirements, and crane operators lifting loads from the seabed. Understanding how to calculate weight of submerged object ensures safety and prevents equipment failure by accurately predicting the load on lifting cables or the stability of submerged structures.
A common misconception is that an object's mass changes underwater. In reality, the mass (amount of matter) remains constant; only the weight force effectively measured by a scale decreases due to the counter-acting buoyant force.
Calculate Weight of Submerged Object: Formula and Math
The physics behind the ability to calculate weight of submerged object is governed by Archimedes' Principle. This principle states that the upward buoyant force on a submerged body is equal to the weight of the fluid that the body displaces.
The formula to calculate the apparent weight (Wsub) is:
Wsub = Wair – Fb
Where:
Wair = True weight of the object in air (Mass × Gravity).
Fb = Buoyant Force (Density of Fluid × Volume of Object × Gravity).
Expanding this for practical calculation:
Wsub = (mobj × g) – (ρfluid × (mobj / ρobj) × g)
Variables Explanation
Variables required to calculate weight of submerged object
Variable
Meaning
Typical Unit
Common Range
mobj
Dry Mass of Object
kg
> 0
ρobj
Density of Object
kg/m³
700 (Wood) to 11,340 (Lead)
ρfluid
Density of Fluid
kg/m³
1000 (Fresh) to 1025 (Sea)
g
Gravitational Acceleration
m/s²
~9.81 (Earth standard)
Practical Examples (Real-World Use Cases)
Example 1: Lifting a Concrete Anchor
A construction team needs to calculate weight of submerged object for a concrete block to ensure their crane can lift it from a harbor floor.
Dry Mass: 2,000 kg
Material: Concrete (Density ≈ 2,400 kg/m³)
Fluid: Sea Water (Density ≈ 1,025 kg/m³)
Calculation:
True Weight = 2,000 × 9.81 = 19,620 N
Volume = 2,000 / 2,400 = 0.833 m³
Buoyant Force = 1,025 × 0.833 × 9.81 = 8,377 N
Submerged Weight = 19,620 – 8,377 = 11,243 N (approx 1,146 kg equivalent mass)
Interpretation: The crane only feels a load equivalent to ~1,146 kg, not the full 2,000 kg, while the block is underwater.
Example 2: Scuba Diving Weight Belt
A diver wants to calculate weight of submerged object (a lead weight) to see how much downward force it provides.
Mass: 10 kg
Material: Lead (Density ≈ 11,340 kg/m³)
Fluid: Fresh Water (Density ≈ 1,000 kg/m³)
Calculation:
True Weight = 10 × 9.81 = 98.1 N
Volume = 10 / 11,340 = 0.00088 m³
Buoyant Force = 1,000 × 0.00088 × 9.81 = 8.65 N
Net Downward Force = 98.1 – 8.65 = 89.45 N
Interpretation: Lead is very dense, so it loses very little "weight" underwater, making it excellent for ballast.
How to Use This Calculate Weight of Submerged Object Tool
Follow these steps to effectively calculate weight of submerged object using the tool above:
Enter Dry Mass: Input the mass of the object as measured on a standard scale in air.
Select Object Material: Choose a preset material (like Steel or Concrete) to automatically fill the density, or select "Custom" to enter a specific density value. Accurate density is crucial when you calculate weight of submerged object.
Select Fluid Type: Choose between Fresh Water (pools/lakes) or Sea Water (oceans). You can also enter custom fluid densities for oil or industrial chemical baths.
Review Results: The "Apparent Submerged Weight" is the force required to hold the object stationary underwater.
Reading the Results: If the result is positive, the object sinks and weighs that amount. If the result is negative (indicated in the tool), the object floats, and the value represents the force needed to push it entirely underwater.
Key Factors That Affect Results
When you calculate weight of submerged object, several external factors can influence the final figures:
Fluid Salinity: Sea water is denser (1025 kg/m³) than fresh water (1000 kg/m³). Higher density fluids provide more lift, reducing the submerged weight.
Object Porosity: Porous materials (like certain rocks or wood) may absorb water, increasing their effective mass and density over time, which changes the calculation.
Temperature: Fluid density changes with temperature. Cold water is denser than warm water, slightly increasing buoyancy.
Depth (Compression): For extreme depths, water density increases slightly, and the object's volume might compress. However, for most engineering tasks where you calculate weight of submerged object, fluids are treated as incompressible.
Gravitational Variance: While we use 9.81 m/s², gravity varies slightly by location on Earth. For precision scientific instruments, local gravity must be factored in.
Trapped Air: Any air trapped inside the object (or hollow sections) significantly increases volume without adding mass, drastically reducing submerged weight.
Frequently Asked Questions (FAQ)
1. Does depth affect the submerged weight?
Generally, no. Since water is nearly incompressible, its density remains constant at typical depths. Therefore, the buoyant force remains constant regardless of how deep the object is, provided it is fully submerged.
2. Why do I need to calculate weight of submerged object for cranes?
It is vital for safety. While the static load is lower underwater, the "added mass" effect during rapid acceleration through water can actually put higher dynamic loads on the crane than the dry weight. However, the static hold calculation is the baseline.
3. What if the result is negative?
If you calculate weight of submerged object and get a negative number, the object is positively buoyant (it floats). The number represents the force required to hold it down underwater.
4. Can I use this for non-water fluids?
Yes. Simply change the fluid density input. For example, to calculate weight of submerged object in hydraulic oil, use a density of approx 870 kg/m³.
5. How accurate is the standard 1025 kg/m³ for seawater?
It is an average. Salinity varies by ocean and region (e.g., the Dead Sea is far denser). For precise offshore operations, local water sampling is recommended.
6. Does the shape of the object matter?
For the static weight calculation, no. Only the total volume displaced matters. However, shape drastically affects drag if the object is moving through the water.
7. What is "Specific Gravity"?
Specific gravity is the ratio of an object's density to water. If Specific Gravity > 1, the object sinks. If < 1, it floats. This is a quick way to estimate behavior before you calculate weight of submerged object.
8. Why does a steel ship float if steel sinks?
A ship is not solid steel; it contains a vast amount of air. The average density of the ship (steel + air) is less than water. This calculator assumes a solid object of uniform density.
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