This is the hallmark of both Mvelopes and YNAB. Keeping tabs on what you actually spend will help you not only stay within the current month’s budget but also adjust your assumptions for next month’s. This was the service Mint so uniquely supplied when it launched back in 2006. As Olen points out, this can be tricky for some, but less so if you earn an income via a regular W-2. You need to have a guess as to how much you’ll earn to know how much you can spend.