How much anchor chain for which depth?
The rule of thumb, a calculator and a table: how much chain to lay for your depth, wind and tidal range – and why the bow roller counts too.
The short answer
With an all-chain rode, lay five times the height from your bow roller to the seabed for a night at anchor, measured at high water. In calm weather, on good holding ground and for a short stop, three times is enough; with strong wind in the forecast, or with a chain-and-rope rode, make it seven times.
The formula:
Chain length = (water depth + tidal rise + bow roller height) × scope
An example: 8 m of water, 1.5 m still to rise before high water, the bow roller 1.2 m above the surface, a night in moderate wind. That gives (8 + 1.5 + 1.2) × 5 = 53.5 m. Lay 55 m – rounded up to the next marking, never down.
The calculator
Enter the depth, the tidal rise and the height of your bow roller, and pick the conditions. The calculator shows how much chain to lay and the widest swing radius your boat will need with it.
Chain calculator
Chain to lay: m
Chain length for common depths
Chain in metres with the bow roller 1 m above the water and no tidal rise:
| Water depth | 3 : 1 | 5 : 1 | 7 : 1 |
|---|---|---|---|
| 3 m | 12 m | 20 m | 28 m |
| 5 m | 18 m | 30 m | 42 m |
| 8 m | 27 m | 45 m | 63 m |
| 10 m | 33 m | 55 m | 77 m |
| 12 m | 39 m | 65 m | 91 m |
| 15 m | 48 m | 80 m | 112 m |
| 20 m | 63 m | 105 m | 147 m |
The bottom rows show why the limit is often not the rule of thumb but the chain aboard. Many cruising yachts between 10 and 14 m carry 50 to 80 m in the locker. How much yours has is one of the first questions to ask when taking over a charter boat – and the answer decides which anchorages are open to you at all.
What does scope mean?
Scope is the ratio between the chain you have laid and the vertical height from the bow roller to the seabed. 5 : 1 means five metres of chain for every metre of height.
The ratio decides the angle at which the pull reaches the anchor. An anchor holds best when it is pulled horizontally – it then digs itself deeper. Pulled at an upward angle, the same force levers it out of the ground.
Even in the worst case, when the wind pulls the chain into a straight line, scope makes the difference:
| Scope | Angle of pull with the chain stretched straight |
|---|---|
| 3 : 1 | about 19° |
| 5 : 1 | about 12° |
| 7 : 1 | about 8° |
| 10 : 1 | about 6° |
As long as the chain sags, its last stretch lies flat on the seabed and the pull reaches the anchor horizontally. That curve – the catenary – is why an all-chain rode gets by with less length than rope. It is also a shock absorber: a gust has to lift the heavy chain before it can snatch at the anchor.
Why the bow roller counts
The most common mistake is to multiply the water depth alone. But the chain does not start at the surface; it starts at the bow roller.
In 4 m of water with the bow roller 1.5 m above it, "five times the depth" gives 20 m of chain. The real height, though, is 5.5 m, so the real scope is only 3.6 : 1 – a true 5 : 1 would need 7.5 m more. The shallower the anchorage and the higher the bow, the larger the error, and it is largest in exactly the shallow, sheltered bays where a boat feels safest.
Tidal range: work it out for high water
Where there is tide, what counts is the depth at high water, not the depth when you drop the anchor. Anchor at low water in 4 m, lay 25 m (5 : 1 with a 1 m bow roller), and with 3 m of tide to come, six hours later you are in 7 m of water. The 5 : 1 has become barely more than 3 : 1, without anyone aboard having done anything.
In the Mediterranean the tidal range is usually small enough to leave out. In the North Sea, the English Channel and along the French Atlantic coast it runs to several metres. Tide tables or the pilot book give the figure for your stay: use how much the water will still rise before the next high water.
Which scope for which conditions?
| Situation | Scope with an all-chain rode |
|---|---|
| Swimming stop in calm weather, good holding, crew aboard | 3 : 1 |
| Overnight in moderate conditions | 5 : 1 |
| Strong wind or gusts forecast, poor holding | 7 : 1 and more |
| Chain with rope | at least 7 : 1 |
These are rules of thumb from practice, not a standard. The type and weight of the anchor, the seabed – sand holds differently from weed or mud – and the wind all move them in either direction. Choose the scope for the forecast overnight, not for the weather while you are anchoring. And when in doubt: more chain rarely does harm, as long as there is room for it.
All chain or chain and rope?
Chain is heavy: 10 mm chain weighs around 2.2 kg per metre, so 50 metres is about 110 kg. That weight makes it sag and keeps the pull flat at the anchor. Rope has no such weight. Under load it runs almost straight from bow to anchor – which is why a rode of chain leader and rope needs considerably more length, and why the leader has to be long enough that it is chain, not rope, lying at the anchor.
An all-chain rode needs a snubber: a chain hook on a stretchy line, made fast to a cleat. It takes the load off the windlass, whose gypsy is not built to hold the boat through gusts, and it absorbs the shocks that a chain pulled taut passes straight to the anchor.
When there is not enough room: the swing circle
More chain means a larger circle. When the wind shifts, the boat swings around its anchor, and the radius can be as large as the chain stretched out horizontally plus the length of the boat. With 50 m of chain in 10 m of water and a 12 m boat, that is about 61 m – a circle more than 120 m across.
In a crowded bay, simply laying less chain is not the answer. Better:
- Pick another spot, at the edge of the bay or a little deeper, where the circle stays clear.
- Watch the neighbours. A motorboat on rope swings differently and further than a yacht on chain. Whoever was there first has priority.
- Go down to 3 : 1 only in calm weather with crew aboard – never below, and with an anchor watch.
- Use a kellet: a weight lowered part of the way down the chain towards the anchor. It holds the chain down, flattens the angle of pull and makes the circle smaller.
How much is actually out?
The calculation is the easy part. The harder part is knowing, while anchoring, that 55 m really are out – and not 40. The usual ways:
- Paint marks or chain markers every 5 or 10 m. Cheap, but paint wears off, markers go missing, and both are hard to see at night or against the light. Which method lasts how long is compared in Marking anchor chain: six methods compared.
- Counting the seconds the windlass runs. Imprecise, because a windlass runs at different speeds depending on the load and the battery.
- A chain counter from the windlass maker. Accurate, but a fixed installation: a sensor at the gypsy, a cable through the boat, a display at the helm. What installing one involves, and how to do without, is covered in A chain counter without installation.
This is the question AnkerSepp is built for: a device that clips onto the windlass you already have and shows the chain paid out in metres – with no installation and no wiring. It is still a prototype; the footage on the home page shows it counting on a real windlass.
The most common mistakes
- Multiplying the water depth alone and forgetting the height of the bow roller.
- Working from the depth while anchoring instead of the depth at high water.
- Dumping the chain in a heap. The anchor needs time to orient itself and the chain should lie stretched out on the seabed: pay it out steadily while the boat drifts or motors slowly astern.
- Not setting the anchor. Once the chain is out, build up pull with gentle reverse and check against two landmarks that the boat is really holding.
- Choosing scope for the weather while anchoring. What matters is the forecast for the hours you will be asleep.
More mistakes – from the wrong seabed to a missing anchor watch – are collected in The ten most common anchoring mistakes.
Rules of thumb do not replace judgement on the spot. They are the starting point from which the seabed, the wind and the room around you decide how much it has to be that evening.