Boat Anchor Size Calculator
Find the right anchor weight, rode length, chain leader, and line diameter for your boat. Sizing follows modern manufacturer charts (Rocna, Mantus, Fortress class) and adjusts for anchor type, holding conditions, and your boat's weight and windage.
How anchor sizing works
Anchor charts key off boat length because length tracks the wind and current loads a hull generates, but two boats of the same length can pull very differently. A flybridge cruiser or a catamaran presents far more windage than an open center console, which is why the calculator asks for a weight and windage tier on top of length.
- Anchor type matters. Modern scoop anchors (Rocna, Mantus class) should be bought at the maker's chart size, never smaller. Steel fluke anchors can run roughly half the weight in the sand and mud they excel in, but are poor in grass and rock.
- Scope is the other half of holding. The calculator uses 7:1 for overnight anchoring and 10:1 for storm conditions, measured on water depth plus bow height.
- Chain leader. A boat length of chain between anchor and nylon line keeps the pull on the shank horizontal and resists chafe.
Anchor sizing FAQ
What size anchor do I need for a 24 foot boat?
For a typical 24 ft boat of moderate displacement, plan on a 16 lb plow or modern scoop anchor for overnight holding, or around 8 lb in a steel fluke for sand and mud. Size up for heavy weather or a high-windage boat.
What size anchor do I need for a 30 foot boat?
About 22 lb in a plow or modern scoop for overnight use, which matches the manufacturer chart size for a 30 ft boat of moderate displacement. A heavy or high-windage 30 footer should step up to roughly 29 lb.
How much anchor rode do I need?
Multiply your deepest expected anchorage (water depth plus bow height) by your scope ratio: 7:1 for overnight, 10:1 for storm conditions. Anchoring in 20 ft of water with a 4 ft bow at 7:1 takes about 175 ft of rode.
How much chain should I put between the anchor and the line?
A common working rule is one foot of chain per foot of boat. The chain's weight keeps the pull on the anchor shank horizontal so it stays set, and it takes the bottom chafe the nylon line should never see.
Should I size a modern scoop anchor down because it holds better?
No. Rocna, Mantus, and similar makers publish charts that already assume their geometry, and their guidance is to size up or stay at chart size, never down. Extra holding power is reserve for the night the forecast is wrong.