Concentration measures the amount of one substance (the solute) contained within a unit measure of a mixture (the solution).
For example, a concentration of 2.7 millilitres per cubic metre would mean that 2.7 mL of the solute are contained in every cubic metre of the solution. Note that, in the calculator, the fluid ounces [fl.oz] are (UK) or (US) to match the gallon.
It can also be expressed as mass per unit mass. (mg/kg etc.)
Another common form for concentration is to give it in parts per million [ppm]; parts per thousand or ‘per mil’ [‰]; or parts per hundred or ‘per cent’ [%]. The advantage of these three is that, since they contain no units, the relative ’strengths’ of two mixtures can be seen at a glance.
A concentration cannot equal or exceed 100% (or 1000 000 ppm) since that would mean that there was as much as or more of the substance being held than there was of the space to hold it!
A concentration can also be given in terms of mass per unit volume or volume per unit mass. In both of these cases there is no general way of changing to ppm or % since it depends upon the densities of the ingredients. Though it worth noting that mass/unit volume is the definition of DENSITY.

Two very practical and commonly used measures of small volumes are the drop and the teaspoon. These are ill-defined but, for the purposes of the calculator
1 drop = 0.05 (or 1/20 th) of a millilitre
and, if needed for practical work, take
1 teaspoon = 5 millilitres or 100 drops.