Splitting the Paycheck on Arrival: Transfer Fractions That Keep the Bills Account Solvent Between Paydays

Money for bills and money for spending that share one account compete daily, and the bills usually lose on timing rather than on totals. This paper simulates a household paid USD 2150 every two weeks, with USD 1890 a month of fixed bills and variable weekly spending, under two arrangements: a single account, and an automatic split on payday that sends a fixed fraction of each paycheck to a separate bills account. The single account produces at least one failed payment in 5.5% of years despite income comfortably exceeding outgoings. The split needs to be at least the exact required fraction — bills over income, 40.6% — to work: at 44% it still fails in 100.0% of years, while at 48% failures fall to 100.0% of years and at 50% to 100.0%. The study is simulated, not measured. SIMULATED, NOT MEASURED.