The engine formula
Impact days = −Purchase / |Monthly net flow| × 365.2425 / 12
With $130,000 spending, $100,000 income, and a $60,000 purchase, monthly burn is $30,000. The purchase costs about 60.9 days of runway—the equivalent of two months of burn.
With positive cash flow, the interpretation changes: the purchase does not create a depletion date in this constant model; it delays accumulation by the time required to rebuild the amount.
When the metric helps
- put a large purchase on the same scale as runway;
- interrupt an impulse and compare alternatives;
- show why one price affects two households differently;
- connect everyday spending with a stated reserve goal.
Where it misleads
At zero net flow, division has no finite result. With a tiny surplus, the day count can become extreme. The metric does not measure utility, health, work necessity, future raises, financing terms, or the emotional value of a purchase. Do not turn it into a guilt score.
Ask alongside it: Does the purchase reduce emergency cash? Does it create a recurring cost? Can it wait without causing harm? Is the remaining reserve still above the chosen floor?
This is a Cash Years interpretation of cash-flow arithmetic, not an industry financial standard. It informs a comparison; it does not decide what is valuable to you.