Anchor Scope Calculator – Rode Length & Swing Circle
How much anchor rode should I let out? Multiply your selected scope ratio by water depth plus expected tide rise plus bow height: rode length = scope × (depth + rise + bow height). For 20 ft depth, 3 ft rise and 3 ft bow height at 7:1, that is 182 ft of rode.
Scope is only one part of safe anchoringThis calculator does not choose a safe scope, anchor, chain, or anchorage. Use vessel and anchor manufacturer guidance, local knowledge, weather and tide forecasts, holding-ground information, and adequate room. Monitor the boat for dragging.
Anchoring measurements
Rode to deploy
Enter the anchoring measurements and calculate.
Live anchor scope comparison
Reference examples, not universal recommendations. Required rode uses the maximum vertical distance. Optional swing radii use each row's exact required rode at the current water level; actual rounded payout may need more room.
Anchor scope and swing circle diagrams
Schematic diagrams; proportions are simplified for legibility. Labels update with your inputs. Reach is calculated at the current water level, which gives more reach than the entered higher level for the same rode. Water falling below the entered depth is not modelled.
Anchor rode and scope formula
Scope compares the complete rode paid out—including chain and rope—with the vertical distance from the seabed to the bow attachment point:
Actual scope = deployed rode ÷ (depth + bow height); at high water add expected rise to the denominator.
For 26 ft of maximum vertical distance at 7:1 scope, the exact calculation is 182 ft; with 5 ft marks, the rounded payout is 185 ft.
Depth, tide, and bow height
Do not multiply water depth alone when the bow fitting is above the surface. If water may rise while the boat is anchored, also calculate at the greatest expected level.
Keep references consistent: If depth comes from a chart datum, enter the maximum tide height above that datum. If it is a current sounder reading, enter only the additional rise expected after the reading.
How chain is handled
Anchor rode includes both chain and rope. The optional allocation applies chain first to the rounded payout in forward mode, or the entered deployed length in reverse mode. It reports rope after chain and any unused chain. Guided all-chain mode uses chain for the entire length; its available-rode input means usable chain.
It does not recommend chain length or diameter. Anchor type and size, vessel displacement, windage, fittings, seabed, chafe, shock loads, corrosion, and windlass compatibility all affect ground-tackle selection.
Swing-clearance estimate
The horizontal anchor-to-bow reach uses a straight, taut-line triangle:
Horizontal reach ≈ √(rode² − vertical distance²)
Forward mode uses rounded payout; reverse mode uses entered deployed rode. Reach uses current depth plus bow height, before the entered rise. Adding full boat length gives a conservative allowance within this simplified model for an anchor-to-stern radius. Real rode forms a catenary, the vessel yaws and sheers, the anchor may move, and nearby boats may swing differently, so this is not a safe-clearance guarantee.
How to use this calculator
Choose Rode needed from scope to plan payout, or Actual scope from rode deployed to check rode already paid out.
Select feet or meters. Enter surface-to-seabed depth at the anchor, the greatest expected rise above that level, and bow attachment height above the water.
For a rode plan, select a reference ratio, guided reference or custom ratio and enter your marking interval. For an actual-scope check, enter only the rode paid out between bow and anchor.
Expand Optional: chain allocation, available rode & boat length to compare usable rode carried and estimate allocation and swing radius.
Select Calculate to review the result. Forward mode shows exact required rode and rounded payout; reverse mode shows current and high-water scope. Verify scope, usable length and clearance for the actual anchorage.
Worked anchor scope examples
Illustrative selected ratios; not safety recommendations
Scenario
Depth + rise + bow
Scope or deployed rode
Calculation and answer
Imperial plan
20 + 3 + 3 ft
7:1; 5 ft marks
26 × 7 = 182 ft exact; 185 ft payout. With 30 ft chain: 152 ft rope exact, 155 ft rope at the mark. With 200 ft available: 18 ft exact margin, 15 ft after rounding.
Metric plan
5 + 1.5 + 1 m
5:1; 5 m marks
7.5 × 5 = 37.5 m exact; 40 m payout.
Reverse: rising tide
5 + 2 + 1 m
42 m deployed
Now: 42 ÷ 6 = 7:1. High water: 42 ÷ 8 = 5.25:1 with the same rode.
Shortfall to mark
20 + 3 + 3 ft
7:1; 183 ft available
182 ft exact fits, but the 185 ft mark exceeds available rode by 2 ft.
Frequently asked questions
What exactly is anchor scope?
Scope is total rode paid out divided by the vertical distance from the seabed at the anchor to the boat's rode attachment point. A 7:1 scope means seven units of rode for each one unit of vertical distance.
Should I include tide and bow height?
Yes. Add bow roller, chock or cleat height above water and the greatest expected rise above the entered depth. If using chart-datum depth, enter tide height above that datum; if using current surface depth, enter only the additional rise. The reverse result labelled current refers to your entered depth reference.
Where should I measure depth if my transducer is below the waterline?
Use surface-to-seabed depth at the anchor. If the sounder displays depth below the transducer, add its depth below the waterline. If it displays depth below the keel, add draft. Do not add either offset again when the instrument already displays surface depth. Check the sounder settings and manual.
How much rode should I carry versus deploy?
Carried rode is inventory; deployed rode is the portion paid out between bow and anchor. Plan inventory for expected maximum depths, selected scope and a usable reserve; a single payout calculation does not determine what to carry for an entire trip. BoatUS Foundation advises carrying extra line and chain. Enter usable inventory under Optional and paid-out length in reverse mode.
Does the chain count in total rode?
Yes. Rode includes both chain and rope. For 182 ft total with 30 ft of chain, the exact rope allocation is 152 ft. Rounding payout to 185 ft increases that rope allocation to 155 ft. This tool does not size the anchor, chain, rope or fittings.
RYA illustrates four times maximum high-water depth for chain and six for chain and warp. Guided daytime mode uses those ratios with bow height included. Overnight uses 7:1 for either type as a comparison with the upper USCG general rule; deteriorating conditions uses the rope-rode 10:1 example linked beside the controls. These situation mappings are illustrative, not a source-approved safety assessment. Chain weight is not a guarantee of holding in stronger wind.
How should I mark rope or chain?
Measure rode from the anchor connection and use a repeatable colour or tag pattern at known intervals; keep the key visible at the bow. Use materials compatible with the rope, chain and windlass, inspect marks for wear, and check the length actually outside the bow roller. BoatUS describes measured rode markings. The calculator rounds up to whole marking intervals; verify the marks, secure attachment and usable length before payout.
Why do boats with different rode types swing differently?
Chain weight and sag, rope stretch, and the amount deployed affect movement as wind and current change. BoatUS explains chain weight and nylon shock absorption. Hull shape and windage also matter. Neighbouring boats may not move together, even with similar calculated scope; allow extra clearance.
Is more scope always better?
More scope generally lowers the angle of pull, but increases the area the boat can occupy. Check hazards, other boats, depth changes and local restrictions before paying out more rode. A calculated circle is not proof of safe clearance.
How accurate is the swing circle?
The triangle formula is exact only for an inextensible straight rode over level seabed. Adding full boat length is a conservative allowance within that model. Actual sag, stretch, yaw, changes in depth, and anchor movement are not modelled. The diagrams and circle are estimates, not safe separation boundaries.
Are my anchoring measurements tracked?
Calculations run locally in your browser. The calculator does not add your measurements to analytics events.
Limits and safety disclaimer
The calculator does not assess holding ground, anchor setting, dragging, wind or wave loads, current, fetch, shelter, anchor type, or vessel windage.
It does not size or inspect the anchor, chain, rope, shackles, swivel, windlass, bow roller, cleats, snubber, bridle, or attachment points.
Depth can change across the swing circle. Verify charted hazards, under-keel clearance, tide, shoreline, restricted areas, cables, pipelines, and room for every plausible wind and current direction.
The straight-line clearance geometry is not a catenary or dynamic-load model and does not establish a safe separation distance.
Safety disclaimer: This is an educational planning estimate, not anchoring, navigation, weather, or engineering advice. Follow official requirements, notices, charts, vessel documentation, manufacturer instructions, and qualified local guidance. Keep an anchor watch and be prepared to act if the vessel drags.
Methodology and sources
Maintained by: Starlight Robotics for Starlight Tools. Updated: . No qualified maritime review is claimed.
Method: Rode and scope use arithmetic from your measurements and selected ratio. Rounded payout uses the next whole marking interval (or the existing mark when exact). Triangle geometry assumes a straight, unstretched rode; full boat length adds a conservative allowance within that model. Presets are reference comparisons and cannot determine safe anchoring conditions.