Anchor Scope Calculator (How Much Chain and Rope to Let Out)

What this tool does

Scope is the ratio of rode length to the height from the bow roller to the seabed, and it is what keeps the pull on the anchor near horizontal so it digs in instead of lifting. Count the height at the highest tide you expect, not the depth you see when you arrive. Common advice is around 5:1 in calm weather and 7:1 or more when it blows, with more again in a storm; a heavy chain lets you get away with less than rope. The swing radius shows how much room you need from other boats and from the shore.

Formula

rode = scope × (depth + tide + bow height) · horizontal reach = √(rode² − height²) · swing radius = reach + boat length

Variables

Symbol	Meaning	Unit
`dp`	Water depth at low tide	m
`ti`	Rise of the tide expected while you are anchored	m
`fb`	Height of the bow roller above the water	m
`sc`	Scope (rode length : height)	: 1
`lo`	Boat length	m
`RL`	Length of rode (chain and line) to let out	m
`HT`	Height from the bow roller to the seabed at high tide	m
`HD`	Horizontal distance from the bow to the anchor	m
`SW`	Swinging circle radius (to the stern)	m

Worked example

Water depth at low tide5 m
Rise of the tide expected while you are anchored2 m
Height of the bow roller above the water1 m
Scope (rode length : height)5 : 1
Boat length10 m

Length of rode (chain and line) to let out40.0 m
Height from the bow roller to the seabed at high tide8.0 m
Horizontal distance from the bow to the anchor39.2 m
Swinging circle radius (to the stern)49.2 m

Limitations

The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.