Anchor Scope Calculator

Calculate the ideal chain length and scope ratio for safe anchoring. Enter your conditions below and get instant results.

Your Conditions

5 m
1.5 m
0 m
10 m

Recommended Chain Length

46 m

at 7:1 scope ratio

Total Depth

6.5 m

water + freeboard + tide

Scope Ratio

7:1

moderate conditions

Swing Radius

56 m

chain + boat length

Swing Circle

112 m

total diameter needed

Scope Visualization

Surface

⛵
⚓

7:1 scope • 46m chain

6.5m deep

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What Is Anchor Scope?

Anchor scope is the ratio of the total length of anchor rode (chain and/or rope) deployed to the depth of water at the bow. It is one of the most important factors in determining whether your anchor will hold securely.

For example, if you are anchored in 5 meters of water and you let out 35 meters of chain, your scope ratio is 7:1. A higher scope ratio means a more horizontal pull on the anchor, which provides better holding power.

Recommended Scope Ratios

  • 3:1 scope — Minimum for lunch stops in calm, protected conditions. Not recommended for overnight stays.
  • 5:1 scope — Standard for fair weather overnight anchoring with all-chain rode. Suitable for most calm conditions.
  • 7:1 scope — Recommended for moderate weather and the general best practice. Provides good holding in most conditions.
  • 10:1 scope — Heavy weather anchoring. Use this ratio when expecting strong winds, storms, or if anchoring on poor holding ground.

Chain vs. Mixed Rode

All-chain rode generally requires less scope than chain-and-rope combinations because the weight of the chain creates a natural catenary curve, keeping the pull on the anchor more horizontal. For rope-and-chain combinations, add 1-2 to the recommended scope ratio.

How to Calculate Anchor Chain Length

The formula for calculating chain length is:

Chain Length = (Water Depth + Freeboard Height) × Scope Ratio

Remember to account for your bow height above the waterline (freeboard) and to add extra length for tidal changes if applicable. Always round up to be safe.

Swinging Room

When anchored, your boat will swing in a circle around the anchor point. The radius of this circle equals the total length of rode deployed plus your boat's length. Make sure you have enough room to swing freely without hitting other boats, the shore, or underwater obstacles.

Swinging Circle Radius = Chain Length + Boat Length

Tips for Safe Anchoring

  • Always check the weather forecast before anchoring overnight
  • Inspect the seabed type — sand and mud offer better holding than rock or weed
  • Set the anchor properly by reversing gently and applying load
  • Use an anchor alarm app like Safety Anchor Alarm to monitor your position while you sleep
  • Check for adequate swinging room relative to other boats
  • Account for tidal changes when calculating depth

Frequently Asked Questions

What is the ideal anchor scope ratio?
The ideal scope ratio depends on conditions. Use 5:1 for calm weather with all-chain rode, 7:1 for moderate conditions (the general standard), and 10:1 for heavy weather. For chain-and-rope combinations, add 1-2 extra to these ratios.
How do I calculate how much anchor chain I need?
Multiply the total depth (water depth + bow height above water + expected tidal rise) by your chosen scope ratio. For example: 6m water depth + 1.5m freeboard = 7.5m total, at 7:1 scope = 52.5m of chain. Always round up.
Does the type of rode affect scope requirements?
Yes. All-chain rode requires less scope because the chain's weight creates a catenary curve that keeps the pull on the anchor more horizontal. Chain-and-rope combinations need 1-2 extra scope ratio because rope is lighter and creates a more direct pull angle.
What is swinging room and why does it matter?
Swinging room is the circle your boat traces as it swings around the anchor with changes in wind and current. The radius equals your total chain length plus boat length. You need enough room to swing freely without hitting other boats, shore, or obstacles.
Should I account for tidal changes when calculating scope?
Absolutely. Rising tides increase the effective water depth, which reduces your scope ratio. Always calculate based on the maximum expected depth (high tide), not the current depth. This is especially important in areas with large tidal ranges.