How much anchor rode to let out
Scope = rode ÷ height of the bow above the bottom. The Coast Guard’s rule of thumb is 5 to 7 times; add the boat’s length to see its swing.
The swing radius assumes the rode stretched straight, at low water when you enter a rise: the circle is largest then. The calculator can’t tell whether an anchor will hold: that depends on the anchor, the bottom and the weather.
Let out five to seven times the distance from your bow to the bottom. That is the U.S. Coast Guard’s rule of thumb: add the depth of the water to the height of the bow above the water, where the rode leaves the boat, and multiply. In the Coast Guard’s example, 8 feet of water and 2 feet to the bow cleat make 10 feet, so you put out 50 to 70 feet of anchor line. The calculator above does the same for any depth, scope and rise of the tide, in feet or meters, and shows how far the boat can swing.
Rode length = scope × (water depth + bow height + expected rise of the water) · Swing radius ≈ √(rode² − (water depth + bow height)²) + boat length
How to Anchor a Boat
Choose the spot
Check the depth and make sure that when the boat swings around the anchor with a change of wind or current, it won’t go aground or hit anything, other boats included.
Work out the rode
Measure the depth, add the height of the bow and any rise of the tide you expect, and multiply by the scope you want. Secure the bitter end of the line to a bow cleat before the anchor goes over.
Lower the anchor
Come up to the spot heading into the wind, stop, and lower the anchor to the bottom from the bow. Don’t throw it: the rode can tangle.
Set it
Back away slowly until about three times the depth in rode is out; you will feel the anchor dig in and the boat stop. Then let out the rest of the rode and back down gently to be sure it holds.
Check that you aren’t dragging
Take bearings on two objects ashore, or watch landmarks that line up. If they change, the anchor is dragging and needs to be reset.
Anchor from the bow only. The Coast Guard warns that anchoring from the stern could swamp the boat. The steps follow the Coast Guard Auxiliary and the BoatUS Foundation, listed in the sources below.
How Much Anchor Rode Do You Need?
Scope is the length of the rode divided by the height of the bow above the bottom. The more scope, the lower the angle at which the rode pulls on the anchor. Published rules of thumb:
- 5 to 7 times the depth plus the bow height: the U.S. Coast Guard’s general rule.
- 7:1 recommended, and 5:1 enough for a lightweight anchor on a small boat in good weather: the BoatUS Foundation, whose suggested rode sizes assume a scope of at least 5:1 to 10:1.
- 7 to 1 normally, and 3 or 5 to 1 for a short stop in good weather; carry a rode at least 10 times the depth you usually anchor in: the Coast Guard Auxiliary.
Rode for each scope, by the height from the bow to the bottom:
| Bow to bottom | 5:1 | 7:1 | 10:1 |
|---|---|---|---|
| 10 ft | 50 ft | 70 ft | 100 ft |
| 15 ft | 75 ft | 105 ft | 150 ft |
| 20 ft | 100 ft | 140 ft | 200 ft |
| 25 ft | 125 ft | 175 ft | 250 ft |
| 30 ft | 150 ft | 210 ft | 300 ft |
| 40 ft | 200 ft | 280 ft | 400 ft |
| 50 ft | 250 ft | 350 ft | 500 ft |
In meters:
| Bow to bottom | 5:1 | 7:1 | 10:1 |
|---|---|---|---|
| 3 m | 15 m | 21 m | 30 m |
| 5 m | 25 m | 35 m | 50 m |
| 8 m | 40 m | 56 m | 80 m |
| 10 m | 50 m | 70 m | 100 m |
| 12 m | 60 m | 84 m | 120 m |
| 15 m | 75 m | 105 m | 150 m |
Scope and the Tide
When the water rises, the bow rises with it and the scope you set shrinks. With 70 feet out in 8 feet of water and a bow 2 feet up, the scope is 7:1; if the tide then comes up 3 feet, the bow is 13 feet above the bottom and the scope is down to 5.4:1. To keep 7:1 at high water, put out 91 ft. Enter the expected rise in the calculator: it works out the rode for high water and shows the scope you have before the rise. NOAA publishes tide predictions for U.S. waters, the Tide Calculator estimates the height of the tide between high and low water, and our guide How to Read a Nautical Chart explains how charted depths relate to the tide.
Swing Radius
An anchored boat can swing all the way around its anchor as the wind or current shifts, with its rode stretched out. With the rode straight, the bow can be up to √(rode² − height²) from the anchor, and the stern a boat length farther. The lower the water, the farther the same rode reaches, so when you enter a rise of the tide the calculator measures the swing at low water, the largest circle. With 70 feet of rode and 10 feet from bow to bottom, the bow can reach 69 ft from the anchor; a 20-foot boat sweeps a circle 89 ft in radius, 179 ft across. A chain or a heavy rode sags, so the real circle is usually smaller, but plan for the full one. BoatUS advises against anchoring inside another boat’s swing circle.
Chain and Rode
Many boaters put 6 to 12 feet of chain between the anchor and the line: it adds weight at the anchor, which helps it dig in and makes dragging less likely (Coast Guard Auxiliary). Mark the rode every 10 feet, with tape for example, so you know how much is out. Terms on deck, from the bow to the stern, are in our guide Port vs Starboard, and depths given in fathoms convert with the Fathoms to Feet Converter.
What the Calculator Doesn’t Know
Rode length is only part of anchoring. Whether an anchor holds depends on its type and size, the bottom, the boat and the weather, and none of these is in the calculation. Pick an anchor of the size recommended for your boat, set it, check it, and watch the weather; as the wind picks up, letting out more rode helps the boat stay in place (BoatUS).
Frequently Asked Questions
What is a good anchor scope?
Five to seven times the distance from the bow to the bottom is the Coast Guard’s general rule, and the BoatUS Foundation recommends 7:1. A small boat with a lightweight anchor in good weather can use 5:1.
How much anchor line do I need for 20 feet of water?
Add the height of your bow above the water and multiply. With the bow 3 feet up, the height is 23 feet, so 7:1 scope takes 161 ft of rode and 5:1 takes 115 ft.
Do you measure scope from the water or from the bow?
From the bow. Scope is the rode divided by the height of the bow above the bottom, so add the bow height to the depth before you multiply.
What is the swing radius of an anchored boat?
With the rode stretched straight, it is the horizontal distance from the anchor to the bow plus the length of the boat. The boat can swing all the way around that circle when the wind or current changes, and the circle is largest at low water.
Can you anchor from the stern?
The Coast Guard warns not to: anchoring from the stern could swamp the boat. Anchor from the bow.
How do you know if your anchor is dragging?
Take bearings on two objects ashore, or pick landmarks that line up, and check them from time to time. If they change, the anchor is dragging and needs to be reset.
Sources
- U.S. Coast Guard, A Boater’s Guide to the Federal Requirements for Recreational Boats (revised Nov 2023), p. 40, Anchoring: five to seven times the depth plus the height of the bow; the 8- and 2-foot example; do not anchor from the stern (read Oct 21, 2026).
- BoatUS Foundation, Anchoring & Mooring (online boating course; read Oct 21, 2026): scope as rode over the height of the bow above the bottom, 7:1 recommended, 5:1 for a lightweight anchor on a small boat in good weather, rode sizes that assume 5:1 to 10:1, a full 360-degree swing, more line as the wind picks up, bearings to check the anchor.
- BoatUS, Anchoring Etiquette: Rules of the Rode (December 2017; read Oct 21, 2026): setting at about 5 to 1 measured from the anchor roller, swinging room with the rode stretched out.
- U.S. Coast Guard Auxiliary, Newspaper Article 4: Anchoring (read Oct 21, 2026): 7 to 1 normally, 3 or 5 to 1 for a short stop in good weather, a rode of at least 10 times the usual depth, 6 to 12 feet of chain, marks every 10 feet, setting at about 3 times the depth.
- NOAA, Tides and Currents (read Oct 21, 2026): tide predictions for U.S. waters.