How Much Anchor Chain to Let Out: Scope Explained With Examples

"How much chain should I let out?" is the most common question in any anchorage. The quick answer is "five times the depth", but that leaves out which depth to multiply and when five times is not enough. Here is the full calculation, with three worked examples.

The classic mistake: multiplying the depth only

Scope compares the rode you let out with the height from the bow roller to the bottom, not with the number on the depth sounder. That height has three parts:

• Depth on the sounder right now (check whether it reads from the surface or from the transducer).
• Tide still to rise until the next high water. In New England or the Pacific Northwest that can be 10 ft or more.
• Bow height above the water: about 3 ft on a 33 ft sailboat.

Anchor in 20 ft at low tide, multiply only those 20 ft, and when the tide comes in your real scope can drop to 3:1. Usually at night, when the wind likes to shift.

Which scope: 3:1, 5:1 or 7:1

Situation	Scope
Short daytime stop, calm weather, all chain, someone aboard	3:1 minimum
Normal anchoring, all chain	5:1
Chain and rope, fresh breeze, poor holding or overnight	7:1 or more

Chain is heavy and hangs in a curve (the catenary) that absorbs snatch loads and keeps the pull on the anchor horizontal. Rope weighs almost nothing, so you need more of it to get the same angle.

Three worked examples

1. Calm afternoon in a sheltered cove

Depth: 15 ft. Tide: negligible. Bow: 3 ft. Total height: 18 ft. All chain at 5:1: 18 × 5 = 90 ft.

2. You anchor at low tide

Depth: 12 ft. Another 9 ft of tide to come. Bow: 3 ft. Total height: 24 ft. At 5:1: 24 × 5 = 120 ft. Multiply the sounder alone (12 × 5 = 60 ft) and at high water you are at 2.5:1.

3. Overnight with 25 knots forecast

Depth: 22 ft. Bow: 3 ft. Total height: 25 ft. At 7:1: 175 ft. If you carry 150 ft of chain, find shallower water (in 15 ft you need 126 ft) or add rope and a snubber.

Will you fit? The swing radius

The more rode you let out, the bigger the circle your boat swings through when the wind shifts. The radius is roughly rode out plus boat length: with 120 ft of chain on a 33 ft boat, you need about 150 ft clear around the anchor.

Watch your neighbours: a boat on a rope rode swings further and more slowly than one on chain. If you cannot fit with the scope you need, move rather than let out less.

How to know how much chain is out

• Chain markers every 25 or 30 ft, with coloured tags or paint. The most reliable method.
• Chain counter on the windlass, if you have one. Check it against the markers now and then.
• Windlass timing: time how long it takes to pay out 30 ft at the dock and use that as a guide.

Make sure it is set (and stays set)

With the rode out, back down gently, then with more power. Line up two transits ashore: if they do not move, the anchor is set. For the night, mark the anchor position and set an alarm with a radius equal to the rode out plus boat length, with a few feet of margin for GPS error.

For the rest of the manoeuvre, read our guide on how to anchor step by step and the anchor chain guide.