A concentration is an amount of something per unit volume. We typically report
protein concentrations in milligrams per milliliter(mg/ml), although it is sometimes
convenient to use micrograms/microliter (µg/µl) or perhaps even µg/ml (for very small
concentrations). For an unknown, we divide amount of substance (from the standard
curve) by the volume of sample used in the assay. Note that this volume is not the assay
volume, nor is it the diluted sample volume. Divide by the volume of undiluted sample
that you placed in the assay tube.