Free Anchor Swinging Circle Calculator

Calculate your boat's anchor swinging radius, diameter, swing area, and anchoring scope. Compare a conservative swing-circle estimate with a geometric estimate based on your water depth, bow roller height, rode length, and boat length.

Imperial (ft) Metric (m)
Depth in Feet (ft).
Height from water to bow roller in Feet (ft).
Total chain/line paid out in Feet (ft).
Boat length in Feet (ft).
Conservative uses rode length + boat length. Geometric uses the horizontal component of the rode.

How to Calculate an Anchor Swinging Circle

An anchored boat can move around its anchor as wind, current, and tide change direction. The area through which the boat can move is commonly described as its anchor swinging circle or swing circle.

What Inputs Do You Need?

This calculator uses four measurements: water depth, bow roller height, total rode paid out, and boat length overall (LOA).

Standard Swing Radius Formula

A simple conservative estimate adds the length of rode paid out to the boat's length:

Swing Radius = Rode Length + Boat Length

This method is simple and intentionally conservative because it treats the full rode length as part of the horizontal swing radius.

Geometric Swing Radius Formula

The geometric method calculates the horizontal component of a straight, taut rode. First calculate the effective vertical distance:

Effective Depth = Water Depth + Bow Roller Height

Then use the Pythagorean theorem:

Horizontal Rode = √(Rode² − Effective Depth²)

Finally:

Swing Radius = Horizontal Rode + Boat Length

How Is Anchoring Scope Calculated?

Scope compares the amount of rode deployed with the effective vertical depth:

Scope = Rode Length ÷ Effective Depth

For example, 100 ft of rode in an effective depth of 20 ft gives a scope of 5:1.

How Is Swing Circle Area Calculated?

Once the swing radius is known, the approximate circular area is:

Swing Area = π × Radius²

Why Calculate at High Tide?

If the tide is expected to rise, the effective water depth changes. Calculate using the water depth expected during the anchoring period rather than assuming that the depth at the moment of anchoring will remain constant.

Is the Swing Circle Exact?

No. This calculator provides a geometric estimate. Real anchor rodes do not always form a perfectly straight line. Chain weight, rope, wind, current, anchor position, seabed conditions, and boat characteristics can all change the actual position of the boat.

For this reason, treat the calculated circle as a planning estimate and allow additional clearance from hazards, shallow water, fixed objects, and neighboring boats.