Anchor Scope Calculator
Calculate how much anchor chain or rope to let out based on water depth, boat freeboard, and weather conditions.
Stay safe at anchor.
What is anchor scope?
Scope is the ratio of anchor rode (chain or rope) paid out to the total depth from the bow roller to the seabed. Total depth = water depth at the anchoring point + height of the bow above the waterline (freeboard).
Scope ratio = Rode length ÷ (Water depth + Freeboard) Rode length = Scope ratio × (Water depth + Freeboard)
Why scope matters
An anchor works best when the pull on it is as horizontal as possible — pulling along the seabed rather than upward. More scope means a lower angle of pull, which makes the anchor dig in harder and hold more securely. Too little scope results in an upward pull that breaks out the anchor under load.
Recommended scope ratios
The required scope depends on the ground tackle type and weather conditions:
- Chain only, calm conditions: 3:1 minimum, 4:1 recommended. Chain is heavy and its weight creates a natural catenary (curve), maintaining a low pull angle with less scope than rope.
- Chain only, moderate conditions: 5:1 to 6:1
- Rope or rope+chain, calm: 5:1 minimum
- Rope or rope+chain, moderate: 7:1 to 8:1
- Storm anchoring (any rode): 10:1 or more
Swinging circle
With more scope out, your boat swings in a wider circle around the anchor. In crowded anchorages, more scope means more risk of collision with other vessels. Balance scope against swinging room available.
A good practice: let out the minimum safe scope for conditions, and increase it if conditions deteriorate. Never anchor on less than 3:1 in any conditions.