⚓ Anchor Size Calculator
Enter your boat's length, the conditions you anchor in, and the depth to get a recommended anchor weight range, chain diameter, and the rode to pay out at a 7:1 scope.
🌊 Size Your Ground Tackle
⚓ Recommended ground tackle
What is an Anchor Size Calculator?
Ground tackle is a system — anchor, chain, and rode sized together. This calculator distils the sizing charts anchor makers publish into one quick lookup: your boat length picks the weight band and chain diameter, your expected conditions scale the holding power up, and the depth (plus your bow height, since scope is measured from the roller) sets how much rode the 7:1 rule wants over the side.
The result is a sensible starting point for outfitting a new boat or sanity-checking the rusty lump that came with a used one. It is indicative guidance, not professional marine advice — bottom type, windage, and anchor design all shift the answer, so confirm against your manufacturer's chart.
❓ Frequently Asked Questions
How is anchor size chosen?
Manufacturer sizing charts band anchors by boat length — a 20-footer typically carries a 4–8 lb fluke anchor, a 30-footer 13–16 lb, and so on. This calculator applies a typical fluke (Danforth-style) chart and then scales the band by conditions: ×1.5 for a moderate blow, ×2 for storm holding.
What is scope, and why 7:1?
Scope is the ratio of rode paid out to the vertical distance from your bow roller to the bottom — depth plus bow height. At 7:1, the classic all-round figure, the pull on the anchor stays near-horizontal so the flukes dig in rather than lever out; in a crowded calm anchorage 4–5:1 may do, in a storm let out more.
Why put chain between the anchor and the rope?
A boat length or so of chain does two jobs: its weight holds the pull on the shank horizontal so the anchor sets properly, and it shrugs off the chafe of rock, coral, and shell that would saw through nylon. The calculator suggests a proof-coil diameter matched to your boat length.
Is this professional anchoring advice?
No — it's a deterministic rules-of-thumb table, a starting point. Bottom type matters enormously (fluke anchors excel in sand and mud, less so in grass or rock), as do your boat's windage and the anchor design you actually carry. Check your anchor maker's chart and local knowledge, and always set the hook properly.