Anchor Scope Calculator
Anchor scope calculator. Works out the rode (anchor line) length to let out from water depth, bow height and tide range at a chosen scope ratio. Educational.
Anchor Scope Calculator
How to use the anchor scope calculator
Enter the water depth
Use the depth where you'll anchor, from your sounder or chart. If you anchor at low water, use the depth then; if mid-tide, account for the rise separately.
Add the bow height
Measure the height of the bow roller (where the rode leaves the boat) above the water. This adds to the vertical distance the rode must span.
Add the rise of tide
Enter how much the tide will rise while you're anchored. Anchoring at low water and forgetting the flood is a classic way to end up dangerously short of scope.
Pick a scope ratio and read the rode
Choose a ratio for the conditions — more in wind and waves. The result is the total rode to let out. Adjust for your ground tackle, holding, and swing room.
Anchor scope — why a low angle holds
Scope is the ratio that sets the pull
An anchor holds best when it is pulled nearly horizontally along the seabed, so that its flukes dig in rather than being levered out. Scope is the ratio of the length of rode (the anchor line and chain) you let out to the vertical distance from the bow to the seabed. The greater the scope, the flatter the angle at which the rode meets the anchor, and the better it holds. The vertical distance isn't just the charted depth: it's the water depth, plus the height of the bow above the water (where the rode leaves the boat), plus any rise of tide expected while you're at anchor. Multiply that total height by your chosen scope ratio and you have the rode to deploy. A common starting point is 5:1 for an all-chain rode in settled conditions and 7:1 for a rope-and-chain rode or when it's breezy; in heavy weather sailors go to 10:1 or more. So in 20 feet of water, with a 4-foot bow height and a 6-foot tide, the total height is 30 feet, and at 7:1 you'd let out about 210 feet of rode.
Chain and rope behave differently, which is why the ratios differ. A heavy chain rode sags into a curve (a catenary) under its own weight, and that curve keeps the pull on the anchor low even at a shorter scope, which is why all-chain setups can use less. A lighter rope rode stays straighter, so it needs more scope to achieve the same low angle at the anchor — hence the higher 7:1 guidance. Wind and waves increase the load and lift the rode, effectively reducing the angle's benefit, so the worse the conditions, the more scope you pay out.
"An anchor digs in when it's pulled flat, not lifted. Scope buys that low angle — and the total height it multiplies is depth plus bow height plus the tide you haven't seen yet."
Scope is necessary, not sufficient
Getting the scope right is essential, but it is only part of anchoring safely, and this calculator deliberately addresses just the rode length. The number it gives assumes you have the swinging room for it: a boat on more rode sweeps a larger circle as the wind and tide shift, so you must check that the radius — roughly the rode length plus the boat — clears other boats, moorings, shallows, and hazards through a full swing. Holding also depends on the seabed: sand and mud generally hold well, while weed, rock, and shingle can be poor regardless of scope, and the anchor type must suit the bottom. The rode must be set properly by backing down to dig the anchor in, and watched for dragging. And the tide figure matters enormously — anchoring at low water with no allowance for the flood can leave you badly under-scoped a few hours later, while a big ebb can leave a deep-draught boat aground. Treat the rode length here as general guidance to be combined with seamanship: local conditions, your vessel and ground tackle, the holding, the weather, and prevailing standards all bear on a safe anchorage, and the responsibility for the decision is the skipper's.
10 Facts About Anchor Scope
Scope = rode length ÷ vertical height to the seabed.
More scope = a flatter pull that holds better.
Height = depth + bow height + rise of tide.
5:1 all-chain settled; 7:1 rope/chain or breezy.
Heavy weather: 10:1 or more.
Chain sags into a catenary, keeping the angle low.
Rope stays straighter, so it needs more scope.
Forgetting the flood tide leaves you under-scoped.
More rode means a bigger swing circle.
Holding also depends on the seabed and set.
Frequently asked questions
Scope is the ratio of the length of anchor rode you let out to the vertical distance from your bow to the seabed. A scope of 7:1 means seven units of rode for every unit of that height. More scope makes the rode pull the anchor at a flatter angle, which helps it dig in and hold. It's the single most important variable you control when anchoring, alongside choosing good holding ground.
Multiply the total height — water depth plus bow height above the water plus the expected rise of tide — by your chosen scope ratio. For example, 20 feet of water, a 4-foot bow height, and a 6-foot tide give 30 feet of height; at 7:1 that's about 210 feet of rode. Use a lower ratio (5:1) for all-chain in calm conditions and more (7:1 to 10:1) for rope rode, wind, or waves. The calculator does the arithmetic for your inputs.
Because scope is based on the vertical distance from where the rode leaves the boat to the seabed — not just the charted depth. The bow roller sits above the water, adding to that height, and the tide will rise while you're anchored, adding more. Leaving them out makes you let out too little rode. The tide especially catches people out: anchor at low water with no allowance and you can be dangerously under-scoped after the flood comes in.
An all-chain rode is heavy and sags into a curve called a catenary. That curve keeps the pull at the anchor closer to horizontal even at a shorter scope, and the weight also absorbs snatch loads. A rope rode is much lighter and stays comparatively straight, so it needs more length to achieve the same low angle at the anchor. That's why guidance suggests roughly 5:1 for all chain and 7:1 or more for rope-and-chain combinations.
More — often 10:1 or whatever you safely can, given swing room. Strong wind and waves greatly increase the load on the ground tackle and tend to lift the rode, raising the angle at the anchor and reducing holding. Paying out extra scope flattens the angle again and lets the chain's catenary absorb the shock loads. Many sailors also add a snubber and set a second anchor in severe conditions. The right amount depends on the holding, the room available, and seamanship.
Roughly the length of rode you've let out plus your boat's length, as a radius around the anchor — because the boat sweeps a circle as wind and tide shift. Before settling, check that this circle clears other anchored boats (who may be on different scope and swing differently), moorings, shoals, and hazards through a full 360°. More scope holds better but needs more room, so in a crowded anchorage there's a genuine trade-off that the rode length alone doesn't capture.
Yes. Even perfect scope won't help in poor holding. Sand and firm mud generally hold well; weed, rock, shingle, and very soft mud can be unreliable, and the anchor type should suit the bottom. After laying the rode you should set the anchor by gently backing down to dig it in, then confirm it isn't dragging. Scope, holding ground, a properly set anchor, and adequate swing room all have to come together — the calculator handles only the rode length.
Use it as general guidance for how much rode to let out. The final decision must account for your vessel and ground tackle, the holding, the weather, swing room, the tide, and prevailing seamanship standards (such as RYA, USCG, or AMSA guidance). The calculator gives a rode length, not a guarantee of a safe anchorage. The responsibility for anchoring safely rests with the skipper.
Yes. Switch the units toggle to metres and enter the depth, bow height, and tide in metres; the rode length is then given in metres. Because scope is a pure ratio, the calculation is identical in either system — only the units of the inputs and the answer change. Use whichever your charts and depth sounder display, and keep all three height inputs in the same unit.
No. The values you enter are processed entirely in your browser. Nothing is sent to a server, stored, or shared, and no account is required. The calculation runs on your device only.
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