Anchor Scope Calculator

Work out the minimum anchor rode to let out for a chosen scope, allowing for the tide and your bow roller height. The diagram and pull angle show why more scope holds better.

Anchor Scope Calculator

Work out the minimum anchor rode to deploy for a given scope, allowing for tide and bow roller height.

m

Depth at your chosen spot from the sounder or chart.

m

Expected rise above the current level, up to high water during your stay.

m

Freeboard: height of the bow roller above the waterline.

Rule of thumb: all-chain 4–5:1, rope rode 7:1, heavy weather 10:1.

1 m 1 m 5 m 7 m 35 m bow roller seabed anchor

Minimum rode length

35 m

5:1 scope

Total depth

7 m

5 + 1 + 1 m

Pull angle at the anchor ≈ 11.5°. Scope 5:1 — moderate scope — settled conditions.

How it works

Scope is the ratio between the length of rode you let out and the vertical height from the seabed up to the bow roller — the point where the rode leaves the boat.

minimum rode = scope × (water depth + tide increase + bow roller height)

A longer rode pulls more horizontally on the anchor, which helps it dig in and hold. That is why more scope — a lower pull angle at the anchor — gives better holding.

Limitations & good practice
  • Scope is a starting point, not a guarantee. Holding also depends on anchor type and size, the seabed (sand, mud, weed or rock), and the wind and swell load on the boat.
  • Plan for the deepest water and the load. Use the depth at your spot plus the highest tide during your stay. In stronger wind or for an overnight stay, increase the ratio (7:1 to 10:1).
  • Chain and rope behave differently. All-chain rode sags into a catenary that lowers the pull angle, so it needs less scope; rope rode has little catenary and needs more.
  • Check your swinging room. The boat can swing in a circle roughly the rode length plus the boat length around the anchor — make sure that circle is clear of other boats and hazards.
  • The pull angle assumes a straight rode. Real chain hangs in a curve, so the angle at the anchor is usually lower (better) than the straight-line figure shown here.

Deploy generously when there is room, set the anchor by backing down on it, take a transit ashore to watch for dragging, and add a snubber to an all-chain rode.