How it works
How to read a forecast, and its confidence band
A single line pretending to know the future is worse than useless. The band around the line is the honest part. Here is what it means and how to use it.
Arjun Varma, Co-founder and CTO · April 2, 2026 · 6 minute read
Every forecast is wrong in the small details. The question is whether it is useful, and a forecast becomes useful the moment it is honest about how sure it is. That honesty is the shaded band around the line.
The line is the most likely path for your balance. The band is the range it could reasonably land in. Near today the band is narrow, because tomorrow is easy to predict. The further out it goes, the wider the band, because a month from now genuinely has more room to move.
Where the band comes from
It is built from how much your day-to-day spending actually wobbles. Steady spender, tight band. Variable income and lumpy weeks, wider band. It widens with the square root of time, which is the standard way uncertainty grows when each day adds a bit more unknown to the last.
None of this is a black box. The numbers are computed in code you could check, and a separate language model only writes the explanation around them. It is never handed a blank cheque to make up a figure.
How to actually use it
- Look at the lowest point first. That is the day to plan around, not the end balance.
- If the lower edge of the band touches zero, treat it as a tight week even if the line does not.
- Add more history when you can. More transactions make the band tighter and the recurring detection more reliable.
- Re-check after anything unusual. A big one-off changes the picture, and the forecast updates the moment you add it.
A narrow band you can trust beats a confident number you cannot. We would rather show you honest uncertainty than a tidy lie.
That is why Tellwise always shows the range, and always says, in plain words next to it, that a forecast is an estimate and it can be wrong. The band is not a hedge. It is the most useful part of the picture.