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Anchor Size Calculator

Calculate anchor size and rode length from boat length, weight, and conditions. Select ground tackle for safe anchoring in varied wind and bottom.

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Inputs that matter
Boat Length (LOA), Displacement, Primary Bottom Type
Output to expect
Anchor Size Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Anchor Size by Boat Length

Manufacturers size anchors by boat length and displacement ranges. A 30-foot cruiser typically needs a 15–22 lb (7–10 kg) modern anchor or equivalent holding power.

Anchor class = manufacturer chart (length + displacement)

Scope Calculation

Scope is rode length divided by water depth (including bow height). Minimum 5:1 for chain, 7:1 for all-rope in calm; 10:1 for overnight or storm conditions.

Rode length = (depth + bow height) × scope ratio

Wind Load Estimation

Wind force on boat increases with wind speed squared. Larger displacement boats need heavier anchors and longer scope for the same wind speed.

Force ∝ wind speed² × exposed lateral area

Updated: July 2026

Example Scenarios

10 ft depth + 4 ft bow height = 14 ft. At 7:1 scope: 98 ft rode minimum. Use 100 ft chain-rope combo; 15 kg Bruce or 14 kg Mantus for 30–35 ft range.

8 ft depth, 5:1 scope on chain: 40 ft rode sufficient for 15-knot max wind. Lighter lunch anchor acceptable if crew remains aboard.

Increase scope to 10:1: 140 ft rode at 14 ft depth. Verify anchor one size up from standard recommendation; deploy second anchor if exposed anchorage.

Common Mistakes to Avoid

Using insufficient scope in gusty anchorages

Gusts to 30 knots pull horizontally on the rode. Short scope lifts the anchor shank and breaks holding. Deploy at least 7:1 for overnight; 10:1 in exposed locations.

Oversizing rope but undersizing anchor

Strong rode with weak anchor still drags. The anchor must hold the boat; rode only transmits load. Upgrade anchor before upgrading rope diameter.

FAQ

Calm day: 5:1 with chain. Normal overnight: 7:1. Heavy weather: 10:1 or more. All-rope rode needs more scope than chain because chain's weight improves catenary holding.

Modern designs (Rocna, Mantus, Spade) hold better per pound than older CQR/plow styles. Size by manufacturer recommendation for your LOA and displacement, not legacy weight charts alone.

Minimum one boat length of chain for rope-chain rodes. All-chain rodes on larger boats improve holding and reduce scope needed — typical 200–300 ft for cruising yachts.

Both matter. A heavy-displacement 30-footer needs a larger anchor than a lightweight 30-footer. Use the larger recommendation when length and weight charts disagree.

Add water depth at high tide, bow roller height above water, and expected storm surge. Scope is measured from bow roller to anchor shank, not water surface to anchor.

About Anchor Size Calculator

Proper ground tackle keeps your boat secure in changing conditions. This calculator recommends anchor weight class and minimum rode length from boat length, displacement, and expected wind conditions.