Density calculator
Density is mass divided by volume, which is the easiest formula on this site. The interesting part is what it gets compared to. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Water is 1.000 g/cm³ at one temperature
Relative density — specific gravity — is a ratio against water, and the number everybody uses for water is exactly 1.000 g/cm³. That was once the definition of the gram: the mass of a cubic centimetre of water.
Water is densest at about 3.98 °C, not at freezing. That anomaly is why ice floats and why lakes freeze from the surface downwards instead of solidifying from the bottom, which is a reasonably important fact about the habitability of the planet.
By 25 °C water is down to about 997 kg/m³. So anything within about 0.3% of water can float or sink depending on how warm the day is, and a relative density quoted to four decimal places without a temperature beside it is quoting three decimals it has not earned.
Density is not weight
A tonne of feathers and a tonne of lead weigh the same and this page will happily tell you both. What differs is the volume: the feathers need about four hundred times more room.
The confusion is worth naming because the phrase "heavier than water" almost always means "denser than water", and the two are different claims. A steel ship is enormously heavier than any quantity of water you are likely to meet, and floats.