# Forecasts

> A Cost Explorer forecast projects your recent average daily spending over a future period of up to 90 days.

A forecast estimates what a project is going to spend over a future period if it keeps spending at its
recent rate. Cost Explorer averages the daily cost over a lookback period, 7 days unless you choose
otherwise, and projects that average forward.

For example, if a project spent 245 over the last 7 days, its average is 35 a day, and the forecast
for the next 30 days is 1050.

A forecast is a straight-line estimate, so:

* It does not know about resources you are about to create or delete.
* It does not follow trends: a project whose spending is growing is under-forecast.
* It does not account for price changes or seasonal patterns.
* It forecasts zero for a project with no spending in the lookback period.

## Choosing a lookback

A longer lookback smooths out one-off spikes, such as a large test that ran for a day. A shorter
one reacts faster when you add or remove resources. The lookback can be from 1 to 90 days.

## Forecasts and budgets

To be alerted when a month is on course to cost more than you expect, use a
[budget with a forecasted threshold](/budgets/concepts/thresholds) instead
of checking forecasts yourself. Budgets projects the spending so far in the current window to its
end.