Anchor Rode, Scope & Swing Calculator
Enter the water depth, expected tide rise, bow height, planned scope, and rode aboard to get a practical anchoring plan. The result shows required rode, chain and rope to pay out, available scope, maximum usable depth, and a conservative swing circle.
Required rode (ft)154
- High-water vertical distance (ft)
- 22
- Chain to deploy (ft)
- 30
- Rope to deploy (ft)
- 124
- Rode surplus or shortfall
- 46 ft surplus
- Scope available if all rode is deployed
- 9.09:1
- Maximum current anchoring depth (ft)
- 21.57
- Maximum anchor-to-bow reach (ft)
- 152.42
- Conservative swing radius (ft)
- 182.42
- Conservative swing-circle diameter (ft)
- 364.84
Required rode is rounded up for display; reach and swing geometry use the unrounded length.
The swing estimate treats the rode as a taut straight line and does not model catenary, yaw, current, or neighboring boats.
How to use this calculator
- Choose feet or meters, then enter the current depth at the spot where the anchor will lie.
- Add the expected tide rise and the bow roller height above the waterline.
- Choose a scope ratio for the conditions and enter the deployable rode and chain aboard.
- Enter boat length overall, then compare the required rode, shortfall, depth limit, and swing-circle estimate.
How the rode and swing numbers are calculated
Anchor scope is based on the vertical distance from the anchor on the bottom to the point where the rode leaves the boat, not only on the number shown by the depth sounder. This calculator adds the current water depth, the expected tide rise, and the bow roller height before applying the selected scope.
The main formula is:
D = d + t + b
L = s × D
- d is current water depth.
- t is the expected rise in water level while anchored.
- b is bow roller height above the water.
- s is the selected scope number, such as 5, 7, or 10.
- L is the required rode length.
The displayed required rode is rounded upward so the plan does not call for paying out less than the selected scope. Dependent calculations use the unrounded value, so the swing geometry and margin are not distorted by display rounding.
For deployment, usable chain is capped at the total rode aboard. The chain to deploy is the lesser of usable chain, required rode, and total rode. Rope to deploy is the remaining required rode after the chain portion. Rode margin is total rode aboard minus required rode, reported as a surplus, zero margin, or shortfall.
Maximum depth and swing room
The calculator also reverses the scope calculation. Maximum current anchoring depth is A ÷ s - t - b, where A is total rode aboard. If that result is zero, the rode aboard cannot deliver the selected scope after tide rise and bow height are included.
For clearance, the tool treats the required rode as a straight taut line. The horizontal anchor-to-bow reach is sqrt(L × L - D × D). Conservative swing radius then adds boat length overall, and swing-circle diameter is twice that radius. This is a planning circle, not a guarantee that every boat nearby will swing the same way.
What moves the result most
Depth and tide rise move every result because they change the vertical distance that scope is based on. Scope ratio has a large direct effect: moving from 5:1 to 7:1 increases required rode by 40%. Bow height matters most on smaller boats and in shallow water, where a few feet or meters can be a meaningful share of the vertical distance.
What this calculator leaves out
Holding depends on anchor design and size, bottom type, wind, waves, current, vessel windage, the set, and how the load changes as the boat sails around the anchor. The swing estimate does not model rode catenary, snubber stretch, yawing, GPS antenna offset, stern gear, or another vessel's different response to wind and current. Use local tide predictions and leave extra room when the forecast, bottom, or anchorage is uncertain.
Worked example
Suppose the current depth is 15 ft, the tide may rise 3 ft, and the bow roller is 4 ft above the water. The high-water vertical distance is 15 + 3 + 4 = 22 ft. At 7:1 scope, required rode is 7 × 22 = 154 ft.
With 200 ft of rode aboard and 30 ft of chain, the plan pays out 30 ft of chain and 124 ft of rope. The rode margin is 46 ft surplus, and using all 200 ft would give 200 ÷ 22 = 9.09:1 available scope. The maximum current depth at 7:1 is 200 ÷ 7 - 3 - 4 = 21.57 ft.
The straight-line anchor-to-bow reach is sqrt(154 × 154 - 22 × 22) = 152.42 ft. For a 30 ft boat, the conservative swing radius is 182.42 ft and the swing-circle diameter is 364.84 ft.
Common questions
Is anchor scope measured from water depth or from the bow roller?
Scope is measured from the anchor on the bottom to the point where the rode leaves the boat. That means the vertical distance should include water depth, expected tide rise, and bow roller height above the waterline.
How does a rising tide change the amount of rode needed?
A rising tide increases the vertical distance, so the same amount of rode gives less scope later. Add the expected tide rise before multiplying by the scope ratio, and include extra margin when weather can push water above the predicted tide.
Should I use 5:1, 7:1, or 10:1 scope?
A lower scope can be suitable for a brief calm stop with good holding and close watch. Many crews use around 7:1 as a normal overnight planning figure, while rough weather, poor holding, or more elastic rope rode can justify more. Follow the boat maker, anchor maker, local rules, and conditions at the anchorage.
How much chain and rope should I pay out?
This calculator deploys chain first, up to the required rode or the usable chain aboard, then treats the remainder as rope. If the required rode exceeds the total rode aboard, the result shows a shortfall instead of calling the plan adequate.
How deep can I anchor with the rode aboard?
Maximum current depth is total rode divided by the selected scope, minus the expected tide rise and bow roller height. If that number is zero, the rode aboard cannot provide the selected scope after the water rises and the bow height is counted.
How much swinging room does an anchored boat need?
A conservative first estimate is the horizontal anchor-to-bow reach plus the boat length, doubled for the full circle diameter. Real boats can swing differently because of windage, current, keel shape, rode type, and yawing, so leave extra space in a crowded anchorage.