Anchor Chain Length Calculator
Why Anchor Rode Isn't Just "Water Depth"
An anchor holds by digging in and pulling roughly horizontally along the seabed — if the rode runs too steeply up to the bow, it lifts the anchor rather than setting it. That's why the length of chain paid out, called scope, has to be a multiple of the total depth from bow roller to seabed, not the depth alone.
The Formula
Chain Length = Scope Ratio × (Water Depth + Bow Height)
Worked Example
| Scope ratio | Calculation | Chain length |
|---|---|---|
| 5:1 (minimum) | 5 × (15 + 3) | 90 ft |
| 7:1 (typical overnight) | 7 × (15 + 3) | 126 ft |
| 10:1 (storm conditions) | 10 × (15 + 3) | 180 ft |
These scope ratios are widely used rules of thumb among cruisers, not fixed regulations — actual holding also depends on bottom type (sand and mud hold better than grass or rock) and how much all-chain versus rope-and-chain rode is being used.
Where This Matters
- Anchoring for the night — a 7:1 scope is the commonly cited starting point for comfortable overnight holding in typical conditions.
- Tight anchorages — knowing the chain length needed at a given scope helps judge whether there's enough swinging room before setting the hook.
- Foul weather — increasing scope toward 10:1 or more is standard practice when wind and wave conditions pick up, since more scope lowers the angle of pull on the anchor.
How to Use This Calculator
- Enter the Water Depth (ft) at the anchorage.
- Optionally enter Bow Height Above Water (ft) for a more precise total drop.
- Optionally adjust the Scope Ratio (defaults to 7).
- Select Calculate to see the recommended chain length.
Related Calculations
Check tide swings that affect depth overnight with the Tide Time Calculator, or plan the approach distance with the Marine Distance Calculator.