Anchor Scope Calculator
Enter water depth, tidal rise, and bow height to get the total rode length to let out at your chosen scope ratio. Accounts for tidal range so the anchor doesn't drag at high water. Nothing uploaded.
Anchorage Details
Advanced options
Rode Length
All-chain rode requires less scope due to catenary action. Mixed chain-and-rope rode requires more scope. In rough conditions or with a rope-only rode, use 7:1 or more.
Learn more: anchor scope, rode length, and swinging room
What counts as depth
Scope is measured against the whole vertical distance the rode has to span, not the number printed on the chart. This calculator adds charted depth, expected tidal rise, and the height of the bow roller above the water, because the rode runs from the seabed up to the boat rather than up to the surface.
Anchor in 5 m charted with a 1 m tide and a 1 m bow and the working depth is 7 m, about 23 ft. At 7:1 that comes to 49 m of rode, roughly 161 ft; the same depth at 5:1 would be 35 m.
Chain and rope get different pass marks
The verdict line uses 4:1 as the floor for all-chain rode and 7:1 for rope. Chain is heavy enough to sag into a curve that keeps the pull at the anchor closer to horizontal, so it holds with less out. Rope is light and lies straighter under load, so it needs the extra length to pull at the same angle.
This is what catches people out on the 5:1 preset. It is labelled standard because it is the common all-round number, but pick it with rope rode and the tool still calls it too short, since 5 sits below the 7:1 rope floor.
Reading the swinging circle figure
The swinging circle radius shown here is simply the full rode length. That is deliberately pessimistic: with 49 m out in 7 m of water the bow can only actually reach about 48.5 m from the anchor, because part of that rode is going upwards rather than outwards.
It also stops at the bow. The stern swings wider still, so add your boat's length before judging clearance from a neighbour, a mooring, or a shoal, and bear in mind every other boat in the anchorage is sweeping a circle of its own.
FAQ
Why does bow height go into the depth figure?
The rode is made off at the bow, not at the waterline, so freeboard forward is real distance it has to cover. Leaving it out understates the rode you need, and the error grows on boats with a high bow.
Why is 5:1 flagged as too short?
Only on rope. The floor built into this tool is 7:1 for rope and 4:1 for chain, so a 5:1 setting passes on all-chain rode and fails on rope.
Is the minimum ratio enough in bad weather?
Treat it as a floor, not a target. These ratios assume reasonable conditions and decent holding, and building wind, swell working into the anchorage, or a soft bottom all call for more rode than the minimum.