Anchor scope calculator
Enter the water depth, the height of your bow roller above the water and how much the tide will still rise. Choose the scope you want and the calculator tells you how much chain or rope to let out, and how wide a circle the boat may swing in.
Runs in your browser: nothing you enter is sent anywhere.
How to use
- Choose metres or feet and enter the water depth, the bow roller height above the water and the tide still to come.
- To find the rode, pick a scope; to check what you have out, choose “Scope” and enter the rode length.
- Enter the boat length to get the swing radius.
How it works
- height = depth + bow height + rise of tide;
- rode = scope × height, or scope = rode ÷ height;
- horizontal reach = √(rode² − height²); swing radius = reach + boat length;
- rode angle = arcsin(height ÷ rode), for a rode pulled straight.
Example
5 m of water, a bow roller 1 m up and 1 m of tide to come, scope 5:1, a 10 m boat:
- height 5 + 1 + 1 = 7 m;
- rode 5 × 7 = 35 m;
- reach √(35² − 7²) = 34.3 m; swing radius 44.3 m;
- angle arcsin(7 ÷ 35) = 11.5°.
In feet: 15 ft of water, a 3 ft bow and 3 ft of tide at 7:1 need 147 ft of rode.
Tips and common mistakes
- Mark your chain or rope every 5 m or 25 ft so you know how much is out.
- Check the swing circle against nearby boats, which may lie to a different scope or rode.
- Set the anchor with the engine in reverse before you rely on the scope.
Frequently asked questions
What is anchor scope?
The ratio of rode let out to the vertical distance from the seabed to the bow roller. Scope is measured to the bow, not to the water surface, and at high tide, not at the moment you drop the anchor.
What scope should I use?
A common rule is about 7:1 for rope with a short chain leader and 5:1 for all-chain in settled weather, with more scope in strong wind or poor holding. Your anchor maker's and local advice come first.
Why include the bow height and the tide?
Five times the depth alone gives too little rode: the bow is above the water, and when the tide rises the same rode gives a smaller scope. In 5 m of water with a 1 m bow and 1 m of tide, 5:1 is 35 m, not 25 m.
How is the swing radius worked out?
Treating the rode as a straight line, its horizontal reach is √(rode² − height²). Add the boat length for the circle the stern could sweep. A real rode sags, so this is the most room you need.