Letting out the right amount of rode is what makes an anchor hold. Too little scope and the pull lifts the anchor out; too much and you swing into your neighbours. This calculator works out the recommended scope and the exact rode length to deploy from the depth, tide, and conditions, and estimates the circle you will swing in.
How it works
Scope is measured from the bow roller down to the seabed at high water, so the calculation first builds the working depth:
working depth = charted depth (LW) + tidal rise + bow roller height
rode length = scope ratio * working depth
swing radius = sqrt(rode length squared - working depth squared)The recommended scope ratio comes from the rode type and the expected weather: all-chain rode holds at lower scope because its catenary keeps the pull horizontal, while nylon rope rode needs more scope to achieve the same low pull angle. You can override the ratio if your own anchor guidance differs.
Worked example
In 6 m of charted depth at low water, with a 2 m tidal rise and a 1.5 m bow roller height above the waterline:
- Working depth:
6 + 2 + 1.5 = 9.5 m - Scope ratio for all-chain in moderate conditions: 5:1
- Rode to deploy:
5 x 9.5 = 47.5 m of chain - Swing radius:
sqrt(47.5² - 9.5²) = sqrt(2256 - 90) = sqrt(2166) ≈ 46.5 m
If a gale is forecast, stepping up to 8:1 means 8 x 9.5 = 76 m of chain and a swing radius close to 75 m. The extra 28.5 m of chain dramatically increases how much room you need around the anchor point.
Scope ratios by rode type and conditions
| Conditions | All-chain | Mixed (chain+rope) | Mostly rope |
|---|---|---|---|
| Calm (lunch stop) | 4:1 | 5:1 | 7:1 |
| Moderate (overnight) | 5:1 | 6:1 | 7:1 |
| Strong breeze | 7:1 | 8:1 | 9:1 |
| Storm / blow | 8–10:1 | 10:1 | 10–12:1 |
These are general guides. The holding quality of the seabed, the anchor type, and any surge or swell also affect the required scope. In poor holding (weed, soft mud, rock), deploy more scope and consider a heavier anchor or a sentinel weight.
Why all-chain needs less scope
Chain sags between the bow roller and the seabed, forming a catenary curve. This catenary absorbs snatch loads when the boat surges and keeps the pull on the anchor shank closer to horizontal — the direction anchors are designed to resist. As the chain straightens under a hard gust, the rising catenary absorbs energy like a spring. Rope lacks this weight and sags much less, so a rope-rode boat must deploy more length to achieve the same near-horizontal pull on the shank.
Estimating swing clearance
The swing radius tells you how large a circle the boat will occupy as wind and tide cause it to swing around the anchor. When you choose a spot, ensure the swing circle clears:
- Other anchored boats and their own swing circles
- Mooring buoys and their scope
- Shoal water at the edges of the anchorage
- Fixed obstructions (docks, rocks, channel markers)
Remember that different boats swing at different rates depending on their hull shape and windage, so even a neighbouring boat on similar scope may swing out of phase with you.