What is anchor scope?

Scope is the ratio of rode deployed to the vertical distance from your bow roller down to the seabed. A 5 to 1 scope in 6 metres of working depth means 30 metres of chain. More scope gives a flatter pull on the anchor, which helps it dig in and hold.

How much scope should I use?

A common rule is 5 to 1 for all chain in settled conditions and 7 to 1 or more when it blows. Rope rode needs more, often 7 to 1 up to 10 to 1, because it lacks the heavy catenary that lets chain hold at lower scope. This tool recommends a ratio from your rode type and conditions.

Why add the tidal range to the depth?

Your rode must still reach the seabed at high water, when the water is deepest. If you size it only for the depth at low water it can snub short as the tide rises and pluck the anchor out. Always calculate the working depth at the highest water expected during your stay.

Why does the bow roller height matter?

Scope is measured from the bow roller, not the waterline, so the height of the roller above the water is part of the vertical distance the rode must span. On a high-freeboard vessel this can add a metre or two and noticeably increases the rode needed.

What is the swing radius for?

Once anchored the boat can swing in a circle as the wind and tide change. The swing radius is roughly the horizontal distance from the anchor to the bow, plus the boat length. Use it to keep clear of other boats, moorings, and shallows when choosing your spot.

Anchor Scope & Chain Calculator

Letting out the right amount of rode is what makes an anchor hold. Too little scope and the pull lifts the anchor out; too much and you swing into your neighbours. This calculator works out the recommended scope and the exact rode length to deploy from the depth, tide, and conditions, and estimates the circle you will swing in.

How it works

Scope is measured from the bow roller down to the seabed at high water, so the calculation first builds the working depth:

working depth = charted depth (LW) + tidal rise + bow roller height
rode length   = scope ratio × working depth
swing radius  ≈ √(rode length² − working depth²)

The recommended scope ratio comes from the rode type and the expected weather: all-chain rode holds at lower scope because its catenary keeps the pull horizontal, while nylon rope rode needs more scope to achieve the same low pull angle. You can override the ratio if your own anchor guidance differs.

Example and notes

In 6 m of charted depth with a 2 m tidal rise and a 1.5 m bow roller, the working depth is 9.5 m. At a moderate-conditions 5:1 all-chain scope that is about 48 m of chain to deploy and roughly a 46 m swing radius. If a gale is forecast, stepping up to 8:1 means 76 m of chain. Always pay out for the deepest water of the stay, add extra scope in poor holding or surge, and reset if the anchor drags. These figures are planning guidance, not a substitute for good seamanship.