How deep should pergola posts be set in the ground?

Bury approximately one-third of the total post length, or a minimum of 600mm, whichever is greater. For a typical hole 300mm in diameter and 600mm deep around a 100x100mm post you will need about 4 x 20kg bags of Postcrete per post: the hole holds roughly 0.036 cubic metres of fill once the post is in place, and each 20kg bag sets around 0.009 cubic metres (about 9 litres). Round up and allow 10% spare.

What is the best timber for a pergola?

Pressure-treated softwood (UC4 rated for ground contact) is the most common and cost-effective choice in the UK. Oak is a premium option that weathers beautifully. Cedar is naturally rot-resistant but expensive. All posts in ground contact must be UC4 treated.

What height should a pergola be?

A standard pergola height is 2.4m (8 feet) to the underside of the beams, which gives comfortable headroom. Under permitted development in England the height limit is 2.5m if any part is within 2m of a boundary; further than 2m from a boundary you can go up to 3m for a flat or mono-pitch roof, or up to 4m for a dual-pitched roof. Always allow for the beam and rafter depth on top of your headroom figure.

Do I need planning permission for a pergola?

In most cases, a garden pergola falls under permitted development in the UK provided it is no higher than 2.5m if within 2m of a boundary, does not cover more than 50% of the garden, and is not in front of the house. Always check with your local planning authority.

WoodCalcs

Pergola Calculator — Timber Quantities for Pergola Build

Calculate timber quantities for pergola posts, beams, and rafters.

Pergola Length

Direction beams run

Pergola Width

Direction rafters span

Height (above ground)

Above-ground clearance to the underside of the beams (burial depth added automatically)

Post SizeSawn post section; use pressure-treated UC4 timber for ground contact

Rafter Spacing

Centre-to-centre spacing of rafters along the length (typically 400-600mm)

Timber Price per Metre (optional)

Enter your supplier price per metre

How We Calculate This

This calculator determines the timber quantities for a standard rectangular pergola.

Component calculations

Posts: 4 if pergola length ≤ 3000mm, 6 if pergola length > 3000mm, x (Height + 600mm burial depth)

Beams: 2 x (Pergola length + 300mm overhang)

Rafters: ceil(Length / Spacing) + 1 x (Width + 300mm overhang)

A total of 300mm overhang (150mm per end) is added to beam and rafter lengths for a traditional look; this 300mm total is the value used in the calculation and can be changed under advanced options. Post length includes 600mm for burial in the ground with concrete.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.