Zero-Baseline Budget Act of 2021 This bill changes the assumptions that the Congressional Budget Office uses to calculate the baseline for discretionary spending. (A baseline is a projection of federal spending and receipts during a fiscal year under current law.) The bill changes the assumptions used for the discretionary spending baseline to exclude emergency and supplemental spending; eliminate adjustments required under current law for inflation, expiring housing contracts, social insurance administrative expenses, pay adjustments, and changes to other personnel benefits; and prohibit other adjustments for inflation or any other factor.