Boating Calculators
Anchor scope.
Enter the depth, the bow height and any tide rise to allow for, pick a scope, and see how much rode to pay out and how far the boat can swing. Runs in your browser; nothing is sent or stored.
Inputs
Result
Fill the form and press Calculate.
Rode to let out = (water depth + bow height + tide rise) x scope. In 10 ft of water with the bow roller 4 ft above the water, 7:1 scope is (10 + 4) x 7 = 98 ft of rode; the moderate-conditions 4:1 is 56 ft.
Worked example
Depth 10 ft, bow 4 ft above the water, no tide: height from bow to bottom = 14 ft. At 7:1 the rode is 14 x 7 = 98 ft. Straight-line horizontal reach is about 97 ft, so a 30 ft boat swings in a circle about 127 ft in radius.
How it works
Rode = (depth + bow height + tide rise) x scope. Scope is the ratio of rode length to the water depth measured from the highest point, usually the bow roller or chock, to the seabed, allowing for the highest expected tide. A large enough scope keeps the pull on the anchor as horizontal as possible, so it is less likely to break out and drag. A common reference gives 4:1 for moderate conditions and up to twice that in rougher conditions; where there is room to swing, more is better. The 5:1, 7:1 and 10:1 choices are common rules of thumb, not published standards.
The swing radius is a straight-line estimate (a chain sags, so the true reach is a little less) and assumes no other boats; leave extra room. Check holding ground, the forecast, and local anchoring rules. This is a planning aid, not navigation advice.
Sources opened 2026-10-08: Anchor (section: Scope) - Wikipedia; Anchoring Scope - PredictWind glossary. Planning aid only, not professional, engineering or safety advice. Your vehicle, trailer, tire, battery and boat manuals and the weight labels on your equipment come first; when in doubt, weigh it or ask a qualified professional.
FAQ
What is anchor scope?
Scope is the ratio of the length of anchor rode let out to the depth of water measured from the bow roller to the seabed, allowing for the highest expected tide.
How much scope should I use?
A common reference gives 4:1 in moderate conditions and up to twice that in rougher conditions. Many boaters use 7:1 as a rule of thumb when there is room. More is better where you have swinging room.
Do I measure depth from the water surface?
No. Measure from the bow roller or chock, so add the height of the bow above the water to the depth, plus any tide rise you expect.
Does the tide matter?
Yes. If the water will be deeper later, the same rode gives less scope, so calculate for the highest expected tide.
How big is the swing circle?
Roughly the horizontal reach of the rode plus the boat length. The result panel gives a straight-line estimate; keep clear of other boats and shallows.