Anchor scope calculator
Calculate how much rode to lay based on depth, rode type and expected conditions.
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Worked example
Anchoring in 4 m of water with all-chain rode, moderate overnight conditions, bow roller 0.6 m above waterline:
- Total depth: 4.0 + 0.6 = 4.6 m
- Recommended scope for all-chain, moderate: 5:1
- Rode needed: 4.6 × 5 = 23 m
- In rough conditions at the same anchorage: 4.6 × 7 = 32.2 m
Always let out more scope if conditions deteriorate overnight.
What is scope and why does it matter?
Scope is the ratio of rode length to the total depth (water depth + bow roller height). A 5:1 scope means you lay 5 m of rode for every 1 m of total depth. More scope means the anchor pulls horizontally rather than being lifted vertically — which is how anchors hold.
All-chain rode is heavier and requires less scope than rope because the chain catenary (the curve it makes when laid out) absorbs shock loading. Rope rode is lighter but needs more scope to achieve the same low angle of pull.
Rode needed = (Water depth + Bow roller height) × Scope ratio
Scope recommendations by rode type
| Rode type | Calm | Moderate | Rough / storm |
|---|---|---|---|
| All chain | 4:1 | 5:1 | 7:1 |
| Chain + rope | 5:1 | 6:1 | 8:1 |
| All rope | 6:1 | 7:1 | 10:1 |
These are minimum recommendations. Never skimp on scope in unfamiliar anchorages or when the forecast is uncertain.
Frequently asked questions
How much anchor chain does a 30ft boat need?
A 30ft (9m) cruising yacht typically carries 40–60 m of 8mm chain for normal cruising. For a 4 m anchorage at 5:1 scope you need ~23 m — but you want reserve for deeper anchorages, more scope in rough conditions, and chain scope overlap. 50 m of 8mm chain weighs about 45 kg — significant for bow weight and trim.
What is a 5:1 scope?
A 5:1 scope means you have laid 5 metres of rode for every 1 metre of total depth (water depth plus bow roller height above water). So in 4 m of water with a 0.5 m bow roller, total depth is 4.5 m and 5:1 scope = 22.5 m of rode.
Should I account for tidal rise when anchoring?
Yes — always calculate scope using the maximum depth you’ll experience during your stay, not just the current depth. If tidal range is 2 m and you arrive at low water, add the full tidal range to your depth calculation. A 4 m depth at low water becomes 6 m at high water — recalculate scope accordingly.
What’s the difference between chain and rope for anchoring?
Chain is heavier and creates a natural catenary curve that absorbs snatch loads and encourages horizontal pull. It’s more durable and less prone to chafe. Rope is lighter (better for bow-heavy boats) and stretches to absorb shock, but needs more scope for the same holding power. Most cruising boats use a combination: several metres of chain at the anchor, then rope for the bulk of the rode.