Scope is the single biggest factor in whether your anchor holds. Get it right and the hook digs in and stays put; get it wrong and even the best anchor will drag. Here is how much rode to let out, and why.
What is anchor scope?
Anchor scope is the ratio between the length of rode (chain or rope) you let out and the depth of water, measured from your bow roller down to the seabed. If you are in 5 metres of water and let out 25 metres of rode, you are anchored at 5:1 scope.
Scope matters because of the angle it creates at the anchor. The lower and more horizontal the pull, the better the anchor digs in and resists breaking out. A short scope pulls upward on the anchor and lifts it out of the bottom; a generous scope pulls almost horizontally and drives it deeper.
How much scope do you really need?
This is where sailors disagree — often loudly. There is no single correct number, and the textbook 7:1 everyone quotes dates from the days of rope rodes. Most modern cruisers on all-chain use considerably less, because the weight of the chain does much of the work of keeping the pull horizontal.
A realistic way to think about it:
- All-chain, settled conditions — around 5:1 is the usual standard, and plenty of experienced sailors happily anchor at 3:1 to 4:1 in a flat calm with good holding, or where a crowded anchorage forces it.
- Rope (or mostly rope) — more, closer to 7:1, because rope lacks the weight that flattens the pull.
- As the wind builds — ease out more: 7:1, and 10:1 or beyond in a real blow. Seven-to-one is a heavy-weather number, not an everyday one.
Some skippers skip ratios altogether and use a length-plus-constant formula, which suits all-chain because a fixed ratio gives too little rode in shallow water. A popular one is twice the depth plus 15 metres: in 4 m that is 23 m, in 10 m that is 35 m — generous where it is shallow, sensible where it is deep. Others use three times depth, or simply a minimum they never go below. All of these are defensible.
The honest takeaway: think about your rode type, the conditions, the holding and the room you have — not one magic ratio. When in doubt, let out more; in a crowded anchorage, match how the boats around you are lying. And always account for tide — the depth you anchor in at low water is not the depth you will swing to at high water.
How to measure depth correctly
The most common scope mistake is using the number on the depth sounder and stopping there. Your scope calculation needs the total height from the seabed to your bow roller, at the highest water you will see before you next reset. So:
- Start with the charted or sounded depth beneath you.
- Add the height of your bow roller above the water (freeboard) — often 1 to 2 metres.
- Add the expected rise of tide before the next low, so you do not end up short-scoped at high water.
Multiply that total by your chosen ratio to get the rode to let out.
5 m sounded depth + 1.5 m bow height + 2 m tidal rise = 8.5 m total. At a 5:1 chain baseline, let out about 43 m. In a flat calm you might use 3–4:1 (26–34 m); as the wind builds, ease out toward 7:1 (60 m) or more. Twice-the-depth-plus-15 gives about 32 m — right in the middle.
Scope table by depth
A quick reference for all-chain rode, from a flat calm to a strong wind. Depths include a typical 1.5 m bow height — add your tidal rise before you read off a row, and give mostly-rope rode roughly 40% more.
| Total depth | Calm 3:1 | Standard 5:1 | Strong wind 7:1 |
|---|---|---|---|
| 3 m | 9 m | 15 m | 21 m |
| 5 m | 15 m | 25 m | 35 m |
| 8 m | 24 m | 40 m | 56 m |
| 10 m | 30 m | 50 m | 70 m |
| 15 m | 45 m | 75 m | 105 m |
Chain vs rope
Both work; they just behave differently.
- All-chain holds at lower scope thanks to its weight and catenary (the sag that keeps the pull horizontal), and shrugs off chafe on rock or coral. It is heavy and needs a snubber to absorb shock loads.
- Rope with a chain leader is lighter and cheaper, and stretches to absorb gusts, but it needs more scope and must be protected from chafe at the bow and on the bottom.
Whichever you run, more scope generally means better holding — but in settled conditions you rarely need the old textbook 7:1, especially on all-chain. Balance holding against the swing room you actually have.
Scope and your alarm radius
Scope and your anchor alarm radius are two sides of the same coin. The more rode you let out, the larger the circle your boat can swing through — so your alarm radius has to grow to match, or it will false-alarm every time the wind shifts. A good rule: set your radius to the rode you deployed plus your boat length plus a small GPS margin. If you shorten scope in a crowded spot, tighten the radius to suit and keep a closer eye on the watch.
Anchor with confidence tonight
Get your scope right, then let HoldFast keep the watch. Free to download — premium is $4.99/year or $9.99 once for Lifetime.
Download on theApp StoreFrequently asked questions
It is genuinely debated. A common baseline is about 5:1 on all-chain rode in settled conditions, and many cruisers use 3:1 to 4:1 in a flat calm, increasing to 7:1 or more as the wind builds. Rope needs more than chain because it lacks the weight that keeps the pull horizontal. The old 7:1 rule is usually more than an all-chain boat needs in normal conditions. Some skippers skip ratios and use a formula such as twice the depth plus 15 metres. There is no single right number.
Measure the total vertical distance from your bow roller to the seabed, not just the depth shown on your sounder. Add the height of the bow above the water and add the expected rise of tide before the next low, so your scope stays adequate when the water is at its deepest.
All-chain rode holds better at lower scope because its weight keeps the pull on the anchor more horizontal, and it resists chafe on the bottom. Rope with a chain leader is lighter and cheaper but needs more scope and a snubber. Whichever you use, more scope almost always improves holding.
Up to a point, yes. More scope lowers the angle of pull on the anchor so it digs in rather than breaking out. The trade off is a larger swinging circle, so in a crowded anchorage you balance holding against the room you have. Set your anchor alarm radius to match the scope you actually deploy.