Reckon up Definition 1 of 2Pro Tip 1/3Mathematical Context Often used where numbers are involved and a precise total is needed. Before we finalize the budget, we need to reckon up all expenses. Pro Tip 2/3Not Guesswork Indicates a calculated and deliberate process, not a rough estimate. The accountant will reckon up the year-end financials accurately. Pro Tip 3/3Often Financial Commonly appears in discussions about money or expenses. Let's reckon up the bill and see how much each of us owes.