This calculator is designed to convert raw soil analysis results into meaningful values on a per hectare basis. In most cases soil test results are provided as weight per kilogram of soil (mg/kg) or a percentage (%), and this is certainly the case for nutrients (e.g. N, P and K) and organic carbon or total organic carbon. While these values are provide a useful guide to the level of nutrition in the soil, they are more meaningful if converted to values expressed in kilograms or tonnes per hectare (kg/ha or t/ha).
To convert your test results to these values you need to know the Bulk Density of the soil that was sampled, which tells you the weight of soil in a given volume. Standard procedure is to test the 0-10 cm soil layer, therefore it follows that you need the bulk density of this layer to convert.
If your soil contains Gravel, this will alter your soil test results. This needs to be accounted for as well, as a large amount will significantly reduce the values presented in your analysis results.