Work out how much rode to pay out. Scope is measured from the bow roller to the bottom, at the highest water you expect while you are anchored, so the calculator adds bow height and tidal rise to the depth before multiplying by the scope ratio.
Every time you anchor somewhere new, and again if the forecast changes. The two most common reasons boats drag are too little scope and scope that was right at low water but not at high water. Doing this arithmetic before you let go also tells you whether your rode is long enough for the spot at all.
Read depth from the sounder and add the transducer offset if it reads below the hull. Bow height is the vertical distance from the water to the bow roller. Tidal rise is how much the water will come up between now and the next high water, from the tide table.
vertical = depth + bow height + tidal rise
rode = vertical × scope
Scope is the ratio of rode length to that vertical distance. More scope lowers the angle at which the rode pulls on the anchor, so the pull is more horizontal and the anchor digs in instead of being lifted out.
Overnight in a bay. 15 ft of water, bow 4 ft up, 2 ft of tide to come, 7:1: (15 + 4 + 2) × 7 = 147 ft of rode.
All-chain in settled weather. 25 ft, bow 5 ft, 3 ft rise, 5:1: 33 × 5 = 165 ft. With 200 ft of chain aboard, that leaves margin to go to 6:1 if the wind builds.
Weather coming. Same spot at 10:1 needs 33 × 10 = 330 ft. If you do not carry that much, a shallower spot or a different anchorage is the answer, not less scope.
| Depth | 3:1 | 5:1 | 7:1 | 10:1 |
|---|---|---|---|---|
| 6 ft | 30 ft | 50 ft | 70 ft | 100 ft |
| 10 ft | 42 ft | 70 ft | 98 ft | 140 ft |
| 15 ft | 57 ft | 95 ft | 133 ft | 190 ft |
| 20 ft | 72 ft | 120 ft | 168 ft | 240 ft |
| 30 ft | 102 ft | 170 ft | 238 ft | 340 ft |
| 40 ft | 132 ft | 220 ft | 308 ft | 440 ft |
Measuring depth at the boat rather than where the anchor will land, which can be much deeper on a sloping bottom. Forgetting the bow height, which matters most in shallow water: in 6 ft of water a 4 ft bow is two-thirds of the depth again. Letting out the right length but not marking the rode, so the length is a guess. Paint or tag marks every 25 ft, and count them as they go out.
More scope means a bigger swing circle. In a crowded anchorage, boats on different scopes swing differently. Check that the circle of radius roughly equal to the rode length is clear of shoals, moorings and other boats.
Use the highest water you will see while you are anchored, plus any surge forecast.
The formula is the same; the scope you choose changes. The anchor chain catenary guide on this site shows how much chain lifts off the bottom at a given wind load.
Enter all heights in metres and the rode comes out in metres.