Runway isn't just a cash word. Anything finite — disk, seats, database connections, warehouse space, a team's hours — has a ceiling, and demand rarely stands still. The catchThe runway is how many periods until growing usage reaches the ceiling. Under compound growth it's ln(capacity ÷ usage) ÷ ln(1 + growth) — far shorter than the flat (capacity − usage) ÷ increment estimate, because each period's growth builds on the last. is that when demand compounds, the wall arrives far sooner than a flat back-of-envelope suggests.
Using 420 of 1,000 GB (42%). At a flat pace it'd be 17 months — compounding costs you 6 months.
The solid line is your demand; the faint dashed line is the same starting pace under the flat model, and the amber line is your 80% alert. Where demand meets the red ceiling is the day you're out of room — and compounding bends the curve up to it sooner than the straight line.
| Threshold | Usage | Reached in |
|---|---|---|
| 50% | 500 GB | 2.3months |
| 80% | 800 GB | 8.4months |
| 90% | 900 GB | 9.9months |
| 95% | 950 GB | 11months |
| 100%the wall | 1,000 GB | 11months |
You cross 80% — the usual "time to act" line — and the wall is only 2.9 months later. Compounding bunches the thresholds at the end, so the warning buys you far less than it looks.