Gauge Hole

Drilling Terminology

When a wellbore is drilled, it should be the same size as the bit that drilled it, this is what is considered gauge hole (or in-gauge if you include tolerance).,

In some cases where rock strength is weak or easily washed away, hole size can become much bigger than the bit size, this would be considered over-gauge and needs to be considered for cement calculations as the hole volume can be significantly greater.

Adversely, if drilling through tough formation, the bit can wear down and become smaller causing the drilled hole to be under-gauge. This can be problematic if there are full size components in the string which could become stuck. Once at surface the bit is measured with a gauge ring to ensure the hole is of correct size, if the bit is under gauge then a wiper trip will be necessary to rectify the situation.