Anchor Rode Scope Calculator
How much rode to pay out, including bow height and tide — the two things people leave out and then wonder why they dragged.
Check the rode you actually have
How much scope
| Scope | When | Notes |
|---|---|---|
| under 4:1 | Not enough | Below about 4:1 the pull on the anchor is upward rather than along the bottom, and it will not set or hold. This is not a lunch-stop compromise, it is a dragging anchor. |
| 4–5:1 | Minimum, settled conditions | Workable for an attended stop in calm weather with an all-chain rode. Not something to sleep on. |
| 5–7:1 | Standard overnight | The everyday number for a night at anchor in reasonable conditions. If in doubt, this is the answer. |
| 7–10:1 | Heavy weather | More scope means a flatter pull and more shock absorption. Deploying it early is far easier than deploying it at two in the morning. |
| 10:1 and up | Storm | Everything you have, plus a snubber and a second anchor if the holding allows. At this point swinging room, not scope, is usually the limit. |
Scope is about the angle, not the length
An anchor holds by digging in, and it only digs in if the pull on it is roughly horizontal. Short scope pulls upward and levers the anchor out of the bottom no matter how good the anchor or how heavy the boat. That is the whole reason the ratio exists, and it is why more rode in shallow water is not overkill — it is the only way to get the angle right.
Two things people leave out of the sum, both of which quietly shorten their real scope. The bow roller is above the water, sometimes by a quarter of the total in shallow anchorages. And the tide is still coming in — a 7:1 scope set at low water on a four-metre range is a very different number by midnight.
Getting it to hold
- Measure depth at HIGH water, not at the depth sounder reading when you arrive. Anchoring on a rising tide with scope calculated for the depth you see is how boats end up at 3:1 by midnight.
- Add the height of your bow roller above the water. On a small boat in three metres of water that can be a quarter of the total, and leaving it out quietly shortens your scope by the same fraction.
- All-chain rode can use slightly less scope than rope, because the weight of the chain keeps the pull horizontal. Rope rodes need more, and a length of chain at the anchor end regardless.
- Swinging room is a circle of the full rode length plus the boat, and every boat around you has its own. Boats on chain and boats on rope swing differently in the same wind, which is how neighbours end up alongside each other at slack water.
- Set the anchor properly — back down on it under engine and watch a transit ashore rather than the chartplotter. An anchor that has not been set is decoration.
How to use
- Measure water depth at your anchoring spot.
- Add the height of your bow roller above the water.
- Add the expected tidal rise.
- Pay out the calculated scope and set the anchor properly.
Frequently asked questions
What is scope and how is it measured?
The ratio of rode paid out to the vertical distance from the bow roller to the seabed. The critical point is that the vertical distance is not the depth sounder reading — it is depth plus bow height plus any tidal rise still to come. Leaving those out is the commonest anchoring error.
What ratio should I use?
Seven to one is the traditional figure for good holding in settled conditions with all-rope rode, and five to one is often treated as a working minimum. All-chain rode can hold at lower ratios because its weight keeps the pull horizontal, but in strong wind more scope is always better.
Why does scope matter so much?
Because an anchor holds by pulling horizontally along the seabed. A steep angle of pull lifts the shank and breaks it out, however heavy it is. Scope is what keeps the pull flat, which is why paying out more rode is the first response to worsening conditions.
What does tide do to my scope?
Reduces it, silently, while you sleep. Anchoring at low water in a six-foot tidal range and paying out for the depth you see means your effective scope shrinks substantially by high water. Calculating for the highest expected water is the only safe approach.
How do I know the anchor is set?
Back down on it under power and watch a transit — two fixed objects lined up ashore. If they stay aligned, the anchor is holding; if they separate, it is dragging. Setting properly and confirming it is worth more than any amount of extra chain.
What about swinging room?
The boat will swing through a circle of radius equal to the rode paid out plus the boat's length, and boats on chain swing differently from boats on rope. Checking that circle is clear of other boats, shallows and hazards at all states of tide is part of choosing the spot.
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