HesapHanem

Anchor Scope Calculator

Enter depth, bow roller height and scope ratio to find the rode length to deploy.

How it is calculated

The tool works out how much rode (chain or line) to pay out. It first finds the vertical rise, rise = depth + freeboard, where depth is the water depth in metres and freeboard is the height in metres of the bow roller above the water. The rode length is then rode = ratio × rise, in metres, and the same length is also shown in feet (m × 3.2808). The ratio is the first number of a scope such as 7:1, so enter 7 for 7:1. Depth and freeboard cannot be negative, and the ratio must be above zero; otherwise nothing is shown. All values are rounded to one decimal. The tool takes the depth you enter as it is: it does not add a tidal rise, a wind or wave allowance, or a safety margin, so enter the depth you expect at the highest water.

Worked example

You anchor in 5 m of water and the bow roller is 1 m above the surface, aiming for a 7:1 scope. Step 1: the vertical rise is 5 + 1 = 6 m. Step 2: the rode length is 7 × 6, which the tool shows as 42.0 m. Step 3: in feet, 42 × 3.2808 gives 137.8 ft. If your chain is marked every 10 m, you would let out a little over four marks.

Try this example →

Common mistakes

  • Using the water depth alone. Scope is measured from the bow roller, so a 5 m depth with a 1 m bow gives a 6 m rise, and the rode comes out 7 m longer at 7:1 than depth alone would suggest.
  • Entering the depth in feet or fathoms. The depth and bow height fields are in metres, and only the result is also given in feet.
  • Typing the ratio as 1:7 or as a fraction. Enter just the first number: 7 for 7:1. A value such as 0.14 would give a very short rode.
  • Measuring the bow height from the deck or from the waterline marks of another boat. It is the distance from the water surface up to the bow roller, where the rode leaves the boat.
  • Using the depth at low tide. If the tide will rise during the night, enter the depth at high water, or the rode will be too short when the water is deepest.

About this calculator

Scope is the ratio of anchor rode length let out to the total vertical distance from the seabed to the bow roller — that vertical distance being water depth plus bow height, not just depth alone. A common rule of thumb is 5:1 to 7:1 for all-chain rode in calm conditions and 7:1 to 10:1 for rope/chain combinations or heavier weather, since more scope flattens the pull angle on the anchor shank and lets it dig in and hold rather than dragging or breaking out.

The calculator takes water depth, bow roller height above the waterline, and your target scope ratio, and multiplies their sum by that ratio to give the rode length to deploy. Recreational boaters and sailors use it before setting anchor for the night to make sure they pay out enough rode to hold securely without over- or under-estimating by eye.

Was this helpful?

You can pick up to 3 reactions.

Comments (1)

  • Editorial team

    The result is a length to pay out, and it is also roughly the farthest your boat can drift from the anchor, so use it to check the swing circle against neighbouring boats and shallows. With the default 6 m depth, 1.5 m freeboard and 7:1 scope the rode is 52.5 m. Marking your chain or line at set intervals makes it easy to pay out the right amount. What scope do you use in different conditions?

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators