Calculate the anchor rode length you need from depth, freeboard, and scope ratio, or check your actual scope from the rode you have out, in feet or meters.
Anchor Scope Formula
Scope is the ratio of the rode you let out to the vertical distance from your bow roller to the seabed. The calculator works in two directions. To find the rode you need, it multiplies your chosen scope ratio by the total depth:
L = S * (D + F + T)
To check the scope you are actually getting, it divides the rode you have out by the same total depth:
S = R / (D + F + T)
It also reports an ideal straight-line angle for a taut rode. This geometric angle does not model chain catenary, loading or anchor holding:
A = arcsin(1 / S)
Variables:
- L is the length of rode to let out, in the unit you select
- S is the scope ratio, written as S:1
- D is the waterline-to-seabed depth, with any sounder offset corrected
- F is the freeboard, the height of the bow roller above the water
- R is the rode you already have deployed
- T is additional tidal rise not already included in D
- A is the ideal straight-line angle above a horizontal seabed
Choose rode length or actual scope at the top. Enter waterline depth and bow roller height, then choose a numerical target ratio or enter a custom ratio. The actual-scope mode compares your deployed rode with that chosen target; it does not assess holding. Rode type is context only, with no universal +2 adjustment. Additional tidal rise must not duplicate rise already included in depth. Optional boat length adds a rode-plus-boat geometric envelope, not a safe separation distance.
Scope Ratios and the Angle They Produce
The first table illustrates the arithmetic for 10 ft of total vertical distance. These ratios are selectable examples, not recommendations for particular weather or equipment. Choose scope using the anchor and rode guidance and actual conditions.
| Chosen ratio | Rode for 10 ft vertical distance |
|---|---|
| 3:1 | 30 ft |
| 5:1 | 50 ft |
| 7:1 | 70 ft |
| 10:1 | 100 ft |
The second table shows ideal straight-line geometry only. Chain may sag into a catenary, but it can straighten under load. These angles do not predict holding, anchor breakout, weather suitability or safe clearance.
| Scope | Ideal straight-line angle |
|---|---|
| 3:1 | 19.5 deg |
| 4:1 | 14.5 deg |
| 5:1 | 11.5 deg |
| 6:1 | 9.6 deg |
| 7:1 | 8.2 deg |
| 8:1 | 7.2 deg |
| 10:1 | 5.7 deg |
Example Problems
Example 1: Find the rode you need.
For a numerical example, use 10 ft of waterline depth, a bow roller 4 ft above the water, and a chosen ratio of 7:1. Total vertical distance is 10 + 4 = 14 ft. Multiply by the scope ratio:
L = 7 * 14 = 98 ft of rode. The ideal straight-line angle is about 8.2 degrees. This does not establish anchor holding capacity.
Example 2: Check the scope you have.
You already have 90 ft of chain out. The water is 12 ft deep and your freeboard is 3 ft, so the total depth is 15 ft. Divide the rode by the total depth:
S = 90 / 15 = 6:1. This is numerically below a chosen 7:1 target, which would require 105 ft for this depth. The comparison does not decide whether either ratio is suitable.
Frequently Asked Questions
What is a good anchor scope ratio?
Choose a ratio using your anchor and rode manufacturer guidance, bottom, depth, weather, tide and available swing room. The selectable ratios illustrate geometry; no single ratio guarantees holding or suitability for overnight or heavy-weather anchoring. The calculator applies no universal rope-versus-chain correction.
Do I add the tide and bow height to the water depth?
Scope uses vertical distance from bow roller to seabed. Add bow roller height to waterline depth and include additional expected tidal rise. Correct sounder offsets and do not add the same rise twice if depth already represents expected high water. A rising tide reduces scope for fixed rode length. Accounting for it improves the geometry estimate without guaranteeing holding.
How does rode type affect scope planning?
Chain weight can create catenary, while rope has different stretch and weight characteristics. Catenary can diminish as load increases. Equipment-specific guidance may recommend different ratios, but there is no universal +2 correction valid for every ratio and condition. This calculator models neither catenary nor stretch.
