1 kg of iron is significantly heavier than 1 kg of feathers. The weight of both 1 kg of feathers and 1 kg of iron is exactly the same, which is 1 kilogram. However, the volume or amount of space that 1 kg of feathers occupies would be much larger compared to 1 kg of iron, due to the difference in density between the two materials. https://medium.com/@davala936/understanding-tmt-bars-the-backbone-of-modern-construction-0e4d65488c26