Fence posts hold up everything else: the panel can be quality, the fixings can be quality, but a rotten post drops the lot. The post specification depends on the fence height. 75x75mm posts suit fences up to 1.5m; 100x100mm is the household standard for 1.8m fence; 125x125mm is for 2m+ or storm-exposed sites. Aosom UK, Buy Sheds Direct and Cheaper-Online.co.uk between them cover £2-£218 across material types. Pressure-treated softwood is the standard 15-year post; untreated rots at ground contact in 3-5 years. Concrete posts last 30+ years, are 2-3x heavier to install, but never rot, the right pick for permanent boundaries. Steel and aluminium posts (powder-coated) sit between in cost and labour. Post depth: a 1.8m fence post sits 600-700mm in the ground in concrete; without that depth, the fence shifts in storms. Post hole depth and concrete-mix volume per hole are the two things to check before ordering.