How long should a fence post spike be?

The most common sizes of metal spike include: A 600mm (24 inch) spike – this will support fences up to 1.2m (4 feet) high. A 750mm (30 inch) spike – this will support a fence which is 1.8m (6 feet) high. In order to drive the spike into position you will need to use a sledgehammer and/or driving tool.