Anchor Scope Calculator
Anchor scope is the ratio of total rode length to the vertical distance from bow to seabed (water depth plus freeboard). Greater scope reduces the upward pull angle on the anchor, dramatically improving holding power. The US Coast Guard recommends a minimum 7:1 scope for overnight anchoring. Enter your water depth and bow freeboard to calculate the rode length required at 5:1, 7:1, and 10:1 scope.
Anchor scope formula
Rode length = Scope ratio × (Water depth + Freeboard + Tidal rise)
The scope ratio is the key variable. A 7:1 ratio means the rode is 7 times the total vertical distance. Increasing scope decreases the angle of pull on the anchor shank, which is critical because most anchors rely on a near-horizontal pull to set and hold properly.
Scope ratio guidance
- 5:1: Minimum for calm, protected anchorages with limited swinging room. Suitable only for short-duration stops in settled weather.
- 7:1: Standard recommended minimum for overnight anchoring in normal conditions (US Coast Guard guidance).
- 10:1: Recommended for heavy weather, exposed anchorages, or when using a rope rode with limited chain.
Anchor scope calculator: frequently asked questions
What is anchor scope?
Scope is the ratio of anchor rode (chain plus rope) length to the vertical distance from the bow to the seabed (water depth plus freeboard). A 7:1 scope means the total rode is 7 times the depth-plus-freeboard distance. More scope lowers the anchor pull angle, improving holding power.
What scope ratio is recommended for overnight anchoring?
The US Coast Guard and most seamanship guides recommend a minimum of 7:1 scope in normal conditions. In heavy weather, 10:1 scope is advisable. In calm, protected anchorages with limited swinging room, 5:1 may be acceptable for short stops but is not ideal.
How do I measure freeboard for scope calculation?
Freeboard is the height of your bow chock or anchor roller above the waterline. For most sailboats this is 1 to 2 metres (3 to 6 feet). For powerboats with high bows it can be 2 to 3 metres (6 to 10 feet). Add freeboard to water depth to get the total vertical distance for the scope calculation.
Does all-chain rode require less scope than rope rode?
Yes. All-chain rode is heavier and sags in a catenary curve, which absorbs shock loads and keeps the anchor pull angle low even at shorter scope. Many sources suggest 5:1 scope is adequate for all-chain in calm conditions. Rope rode or chain-rope combinations require more scope to achieve the same low pull angle.
Should I account for tidal rise when calculating scope?
Yes. If anchoring in a tidal area, add the expected tidal rise to the current water depth before calculating scope. Failing to account for tide can leave you underscoped at high tide, reducing anchor holding power significantly.
Official sources
- US Coast Guard: Anchoring your vessel safely.
- US Coast Guard Boating Safety: USCG Boating Safety Division.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.