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 the straight-line angle the rode makes at the anchor, which is what a scope ratio really controls:
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 water depth below the bow
- F is the freeboard, the height of the bow roller above the water
- R is the rode you already have deployed
- T is the tidal rise you want to allow for
- A is the angle of the rode at the anchor, measured up from the seabed
Pick what you want to find at the top of the calculator. In “Rode length I need” mode you enter the depth and freeboard, choose a rode type and a condition, and the tool returns the length to let out, the total depth it used, and the pull angle. Choosing rope and chain adds 2 to the base scope ratio, because rope is light and pulls in a straighter line than heavy chain. In “Check my actual scope” mode you enter the rode you have out and the depth, and the tool returns your real scope ratio with a plain reading of whether it is enough. The advanced options add tidal rise to the depth and use your boat length to estimate the swing circle.
Scope Ratios and the Angle They Produce
The first table is the standard starting point for how much scope to use. Add roughly 2 to the all-chain figure when part of your rode is rope.
| Situation | All chain | Rope + chain |
|---|---|---|
| Lunch stop, calm and attended | 3:1 | 5:1 |
| Overnight, calm and settled | 5:1 | 7:1 |
| Moderate, general standard | 7:1 | 9:1 |
| Heavy weather or poor holding | 10:1 | 12:1 |
The second table is the reason those numbers matter. A scope ratio is really a way of setting the angle the rode makes at the anchor, and most anchors only hold well when that angle is low and the pull is close to horizontal. This table gives the straight-line angle for each ratio and what it means for holding. It assumes a straight rode, so real chain sits even flatter at the anchor thanks to its own weight.
| Scope | Angle at anchor | What it means for holding |
|---|---|---|
| 3:1 | 19.5 deg | Steep pull, close to the limit where many anchors start to break out |
| 4:1 | 14.5 deg | Marginal, holding drops off in gusts |
| 5:1 | 11.5 deg | Workable in calm weather with all chain |
| 6:1 | 9.6 deg | Solid for a quiet night |
| 7:1 | 8.2 deg | The general standard, near full rated holding |
| 8:1 | 7.2 deg | Extra margin for wind shifts and chop |
| 10:1 | 5.7 deg | Heavy weather, pull is nearly horizontal |
Example Problems
Example 1: Find the rode you need.
You are anchoring in 10 ft of water, your bow roller sits 4 ft above the water, and you want the moderate 7:1 standard on all chain. The total depth is 10 + 4 = 14 ft. Multiply by the scope ratio:
L = 7 * 14 = 98 ft of rode. At 7:1 the rode meets the anchor at about 8.2 degrees, a low angle that lets the anchor hold near its rated power.
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. That is fair-weather scope, fine for a calm night on all chain but short of the 7:1 standard, so let out more if wind is forecast.
Frequently Asked Questions
What is a good anchor scope ratio?
For all-chain rode, 7:1 is the general standard and handles most overnight conditions. You can drop to 5:1 in settled calm weather and should go to 10:1 or more in heavy wind or poor holding. A 3:1 ratio is a minimum for a short, attended lunch stop only. If part of your rode is rope, add about 2 to each of these figures, because rope does not sag into the low-angle catenary that chain does.
Do I add the tide and bow height to the water depth?
Yes. Scope is measured from the bow roller to the seabed, not from the waterline, so you add your freeboard to the charted or sounded depth. You also add any tidal rise you expect while anchored. If you set the anchor at low water and figure scope on that depth, a rising tide deepens the water, lowers your real scope, and can lift the anchor. Calculating on the deepest expected depth keeps the anchor set through the whole tide cycle.
Why does all chain need less scope than a rope rode?
Chain is heavy, so it hangs in a curve called the catenary that keeps the pull at the anchor close to horizontal even at lower ratios. Rope is light and stays nearly straight, so for the same length it pulls at a steeper angle. To reach the same holding angle you need more length, which is why a rope and chain rode wants roughly 2 more scope than all chain. Adding a length of chain at the anchor end of a rope rode helps, but it does not fully replace the extra scope.
