Technical Debt (TD) captures the consequences of sub-optimal decisions made during the development of a software product. If not managed properly TD can be detrimental to the product in the long-term. To manage TD it needs to be quantified (measured). Many approaches to quantify TD have been proposed, but evaluating them is difficult. We propose TDQM — the Technical Debt Quantification Model, that captures the important concepts in TD quantification and their relationship. TDQM has been used to compare and evaluate proposed quantification approaches. TDQM will also be used to advance our knowledge about TD quantification.