Anchor Rode Length Calculator (Scope)

How it works

Rode = (depth + bow height + tidal rise) × scope; scope 4–5:1 all chain, 7:1 mixed rode, 10:1 rope or storm; swing radius ≈ √(rode² − height²) + LOA

An anchor holds by digging in, and it digs in only when pulled nearly horizontally, so the length of rode paid out relative to the height from bow roller to seabed — the scope — is the single most important anchoring number. All-chain rode is heavy and hangs in a catenary that absorbs shocks and keeps the pull flat, so 5:1 suffices in normal conditions; rope is light and needs 7:1, and everyone should go to 10:1 when the wind gets up. Depth must include the rise of tide while you are anchored and the height of the bow above the water, both routinely forgotten. More scope means a bigger swinging circle, which is why crowded anchorages fill with boats on short chain that then drag at 2 a.m.

Worked example

6 m of water plus 1.2 m of bow and 1.5 m of tide to come is 8.7 m; on a mixed rode overnight at 7:1 you need about 61 m, and a 10 m boat will swing in a circle of roughly 70 m radius.

Frequently asked questions