The household’s future

The screen starts from today’s net worth, the monthly saving (the month’s surplus if you type nothing) and what the household costs. It answers how long until you reach an amount, how much you will have in the chosen term and how long the wealth lasts with a monthly withdrawal — in three real-return scenarios.

On the "Can I buy it?" tab, enter the price, the down payment, the system (SAC or Price), the yearly interest and the term in months, then click Simulate the purchase. Tenka shows three scenarios: as you entered, in the other amortisation system and waiting 12 months (the down payment grows with what the household can save without touching yearly bills or goals). For each: first and last instalment, total interest, how much income is committed, how much you can spend per day afterwards and the household score afterwards.

Under SAC the amortisation is constant (balance ÷ term) and interest falls on a shrinking balance — the instalment starts higher and decreases, and total interest is lower. Under Price the instalment is fixed: at first almost all of it is interest and the amortisation grows over time. The yearly rate becomes monthly by compound equivalence, (1 + a)^(1/12) − 1: 11.5% a year is 0.91% a month, not 0.96%. Compare the "Total interest" column of the first two scenarios.

Freedom number = the household’s yearly cost ÷ the safe withdrawal rate (4% a year, parameter FAMILIA.TAXA_SEGURA). With that invested, the household lives off the returns without eating the principal, on historical average. With no fixed cost registered, it does not show.

Fits: income committed after the purchase (fixed cost + the first instalment, divided by income) stays within the household ceiling (parameter FAMILIA.COMPROMETIMENTO_MAX, 70% by default) and money is still left over in the month. Tight: it exceeds the ceiling by up to 10 points, or the monthly surplus turns negative. Does not fit: it exceeds the ceiling by more than 10 points, or the down payment is larger than today’s balance. With no income registered as a revenue rule, Tenka asks for the income before answering.

Because past returns do not guarantee the future: the screen uses monthly compound interest on a fixed real return (above inflation) and shows three scenarios so you see the range, not an exact number. Change the saving, term and withdrawal freely — nothing is saved.