How Often Should a Household Reconcile Its Books? Error Accumulation and the Optimal Interval Between Checks

Households that keep any kind of ledger — a notebook, a spreadsheet, an app reviewed by eye — face a scheduling question nobody answers for them: how often to reconcile it against reality. Errors enter at a stated rate of 1.14 a month (95 transactions, 1.2% error rate), and an error's chance of causing harm — a missed bill, a decision taken on a wrong balance — grows the longer it survives. This paper simulates intervals from twice a month to once a year over twenty-year horizons, pricing reconciliation time at USD 7.50 a session. Total annual cost (harm plus time) is USD 191.53 at twice-monthly reconciliation, USD 101.47 monthly, USD 61.92 every two months, USD 52.17 quarterly, and USD 68.00 annually. The minimum sits at the 101.47-dollar monthly point in the tabulated set: monthly reconciliation is the optimum under the stated rates, with quarterly the value choice for households whose time is dearer. The study is simulated, not measured. SIMULATED, NOT MEASURED.