About Anchor Rode Calculator (Scope & Swing Radius)
The anchor rode calculator sizes how much anchor line and chain (rode) to let out and how big a circle the boat will swing through. The customary rule from standard seamanship references — Chapman Piloting & Seamanship and the US Coast Guard Auxiliary's Boating Skills and Seamanship course — is a scope ratio: rode length as a multiple of the vertical distance from the bow chock down to the seabed. That vertical distance is more than the charted depth: it is the water depth at the spot, plus the bow freeboard (the height of the roller or chock above the waterline), plus a tide allowance for however much the water will rise while you are anchored.
The presets encode the customary practice figures: 7:1 is the general-purpose recommendation for overnight stays or moderate conditions, 5:1 is a common fair-weather minimum for a calm daytime stop with someone aboard, and 10:1 or more is storm practice. These are guidance from seamanship practice, not regulation — holding ground, anchor type, and crowding all move the answer. The swing circle uses the honest taut-rode geometry: the horizontal reach of a straight rode plus the boat's length overall. A real rode sags in a catenary, so the actual circle is somewhat smaller — the calculator's number is the safe upper bound to clear other boats and hazards. An optional check grades the rode you actually carry as adequate or short against the selected condition.
How It Works
- Enter the water depth where the anchor will lie, your bow freeboard (deck or roller height above the waterline — the rode is measured from there, not from the surface), and a tide allowance for the rise to high water while anchored. The three add up to the vertical distance the scope multiplies.
- Pick the condition: calm daytime stop (5:1), overnight or moderate conditions (7:1 — the general-purpose recommendation), storm (10:1), or enter a custom scope between 2:1 and 15:1.
- Enter the boat's length overall. The required rode is scope × vertical distance; the swing radius is the horizontal projection of that rode pulled taut — √(rode² − vertical²) — plus the LOA, and the swing diameter is twice that.
- Optionally enter the rode you actually have on board (or plan to let out). The calculator shows your actual scope (your rode ÷ vertical distance) and badges it adequate when it meets the selected condition's recommendation or short when it does not, and draws the swing circle for your rode instead.
- The top-view sketch shows the anchor, the taut rode's horizontal reach to the bow, the boat length, and the resulting swing circle — the clearance you need from other boats, moorings, and shoals in every direction.
Worked Example
A 30 ft boat anchors overnight in 20 ft of water. The bow roller sits 4 ft above the waterline and the tide will rise another 2 ft, so the vertical distance is 20 + 4 + 2 = 26 ft. At the overnight recommendation of 7:1 the required rode is 7 × 26 = 182 ft. Pulled taut, that rode reaches √(182² − 26²) = 180.1 ft horizontally from the anchor, so the swing radius is 180.1 + 30 = 210.1 ft and the boat needs a clear circle about 420 ft across. For a brief calm-day lunch stop, 5:1 would need only 130 ft of rode; if only 100 ft of rode were aboard, the actual scope would be 100 ÷ 26 = 3.8:1 — short even of the day figure.
Formulas
- Vertical distance
V = depth + freeboard + tide- Required rode (customary scope practice)
rode = scope × V- Swing radius (taut rode)
R = √(max(0, rode² − V²)) + LOA; D = 2R- Actual scope check
actual scope = myRode ÷ V
Standards & References
- Chapman Piloting & Seamanship — customary scope practice (7:1 general, 5:1 minimum fair weather, 10:1 heavy)
- USCG Auxiliary, Boating Skills and Seamanship — scope measured on depth + bow height above water
- Practice guidance, not regulation — holding ground, anchor type, and crowding modify it
Frequently Asked Questions
What is anchor scope and why does more scope hold better?
Scope is the ratio of rode paid out to the vertical distance from your bow chock to the seabed. More scope flattens the angle at which the rode pulls on the anchor: at 7:1 a taut rode meets the bottom at only about 8 degrees, so the pull is nearly horizontal and the anchor's flukes dig in. At short scope the rode pulls upward, which levers the shank up and breaks the anchor out — exactly what you do on purpose when you retrieve it straight up-and-down.
How much anchor line should I let out?
Customary US practice (Chapman Piloting, USCG Auxiliary courses) is 7:1 for normal conditions — seven feet of rode for every foot of depth-plus-freeboard-plus-tide. A calm daytime stop with someone aboard is commonly held to 5:1 as a minimum, and storm practice is 10:1 or more. In 20 ft of water with a 4 ft bow and 2 ft of tide to come, that means 182 ft overnight, 130 ft for lunch, and 260 ft riding out a blow.
Why do I add freeboard and tide to the water depth?
Because the scope triangle runs from the seabed up to the point the rode leaves the boat — the bow roller or chock — not to the water surface. A 4 ft bow on 10 ft of water is 14 ft of vertical distance, a 40% difference the depth sounder never shows. Tide matters the same way: anchor at low water with 5 ft of rise coming and your carefully set 7:1 quietly shrinks toward 5:1 at high water as the vertical leg grows.
How big is my swing circle at anchor?
The upper bound is the horizontal reach of the rode pulled taut, √(rode² − vertical²), plus your boat's length — with 182 ft out over a 26 ft vertical distance on a 30 ft boat, about a 210 ft radius, a 420 ft circle. The real circle is somewhat smaller because the rode sags in a catenary rather than pulling straight, but planning on the taut figure keeps you clear of neighbors when the wind pipes up and straightens the rode. Remember boats on different rode types (all-chain vs rope) swing differently in the same anchorage.
Do I need chain on my anchor rode?
Common practice for small craft on a rope rode is to fit roughly a boat length of chain at the anchor end. The chain's weight holds the pull on the shank flat along the bottom, resists chafe on rock and shell, and adds catenary that absorbs surge loads before they reach the anchor. This calculator treats the rode as a single length — the chain-lead figure is practice guidance for making up the rode, not part of the scope arithmetic.
Is the 7:1 scope rule a legal requirement?
No. Scope ratios are seamanship practice taught by Chapman Piloting, the USCG Auxiliary, BoatUS, and similar references — not regulation. They assume decent holding ground and an appropriately sized anchor. Poor holding (soft mud, grass, rock), a crowded anchorage that limits swing, all-chain rodes, or modern high-holding-power anchors all justify departing from the presets, which is what the custom scope input is for.