// physics › Density & Pressure
Find density from mass and volume (or solve for mass or volume), pick a material, and see the weight and whether it is safe to lift.
ρ = m / V
Density is how heavy something is for its size. Imagine two boxes the same size: one full of cotton, one full of iron. The iron box is far heavier — it is denser, because much more stuff (mass) is packed into the same space.
Because each material has its own density. Fill the same cup with cotton, water, or gold and you get wildly different masses — the gold is about 19 times heavier than the water, and the water far heavier than the cotton. Pick a material in the calculator and it fills in that material's density for you.
Because mass is what your back actually has to lift. Safety experts (NIOSH) say about 23 kg is the most a single person should lift in good conditions. A small block of iron or gold can already pass that, while a big bundle of cotton stays light — the calculator shows the weight and whether it is in the safe range.
Sure. Take a block of iron (density 7860 kg/m³) that is 0.001 cubic metres in size. Mass = density × volume = 7860 × 0.001 = 7.86 kg. Its weight is about 7.86 × 9.81 = 77 newtons — heavy, but still safely liftable. Make it ten times bigger and it would be 78.6 kg, far too heavy to lift alone.
It comes down to density compared with water (about 1000 kg/m³). Anything less dense floats; anything denser sinks. Cotton and wood float; iron and gold sink. The calculator notes which side of water your material is on.
Use the 'Solve for' box to pick mass or volume. Mass = density × volume, and volume = mass ÷ density. If you pick a material, its density is filled in automatically, so you just need the other value.