Anchor scope calculator
How much rode to let out, and how wide you’ll swing. It adds the tide still to come and your bow-roller height, the two things the “just use 7:1” rule of thumb quietly ignores.
Charted depth now, or at low water
Look up on our tide charts
Waterline to your bow roller
Overall length (LOA)
The water you need to yourself so you don’t hit anything as the wind shifts.
Rode = scope × (depth + tidal rise + bow-roller height). Swing radius uses the horizontal reach √(rode² − depth²) plus your boat length, an estimate of calm rotation, not a promise you’ll clear neighbors or hazards. Holding depends on bottom type, anchor, and conditions.
Not sure of the tide? Look up the rise at your anchorage on our free tide charts.
How much scope should you use?
Scope is the ratio of rode you let out to the vertical distance from the seabed up to your bow roller. More scope means a flatter pull on the anchor, which is how it holds, a steep pull lifts the anchor out. The right ratio depends mostly on your rode:
- All-chain rode, about 5:1. The chain’s weight keeps the pull horizontal, so you can hold on less.
- Rope and chain, about 7:1. The classic all-round figure for an overnight stop in settled conditions.
- Calm lunch stop, 3:1 minimum. Fine for a short stay in flat water while you’re aboard and watching.
- Heavy weather, 10:1 or more. When it’s blowing, more rode and a bigger swing circle.
Two mistakes catch people out. First, measure to your bow roller, not the waterline, and add your freeboard to the depth. Second, use the depth at high water: anchor at low tide without allowing for the rise and you’ll be short-scoped when it floods. This calculator does both for you.
Frequently asked
How much anchor scope do I need?+
There's no single right number, it depends on your rode and the conditions. A common rule of thumb is 7:1 for a mixed rope-and-chain rode in settled conditions. All-chain rode holds at a shorter 5:1 because the chain's weight keeps the pull horizontal. 3:1 is a bare minimum for a lunch stop in calm water, and 10:1 or more is a storm figure. This calculator lets you compare ratios instantly.
Do I measure scope from the water or from the bow?+
From your bow roller, not the waterline. Scope is the ratio of rode paid out to the vertical distance from the seabed up to your bow roller, so you add your bow-roller height (freeboard) to the water depth. Forgetting the freeboard is the classic mistake that leaves you short-scoped, especially on a high-bowed boat in shallow water.
How does the tide change my scope?+
Always calculate for high water. If you anchor at low tide and let out rode for the depth you see now, the rising tide increases the depth under you and shrinks your effective scope, you can end up dragging at high water. Add the tidal rise still to come to your depth before you compute rode.
What is the swing circle and why does it matter?+
Once anchored, your boat rotates around the anchor as the wind and current shift. The swing circle is the water you need to yourself to rotate without hitting another boat, a shoal, or a dock. It's roughly your horizontal rode reach plus your boat length. Boats on different scope and rode types swing differently, so leave margin.
Will this scope guarantee my anchor holds?+
No. Scope is only one factor. Holding also depends on your anchor type and size, the bottom (sand, mud, grass, rock), how well it's set, and the wind and current. Treat the result as a planning estimate, set the anchor properly, and keep an anchor watch.
