In rain-affected ODIs, a minimum of 20 overs per side is required for the match to have an official result.
Details
If rain or other interruptions reduce the number of overs available, a minimum of 20 overs per side must be bowled for the match to constitute a valid result.
If fewer than 20 overs per side are bowled, the match is abandoned as 'No Result' and any points are shared.
In T20Is, the minimum is 5 overs per side for a result.
The target for the team batting second is recalculated using the DLS method when overs are lost. The par score at any point in the second innings determines who is ahead if play cannot resume.
Historical Evolution
The minimum over requirement has been part of LOI cricket for decades. The number has been adjusted over time — it was originally higher before being settled at 20 for ODIs and 5 for T20Is.
Some have argued the minimum should be lower to avoid matches being abandoned when significant cricket has been played.
Current Rule
ODIs: 20 overs minimum per side. T20Is: 5 overs minimum per side. Result determined by DLS if overs are reduced.