The boat anchor size calculator recommends the correct anchor weight and scope for your vessel based on boat length, beam, conditions, and bottom type. Anchoring with too small an anchor is a leading cause of boats going adrift.
Size Your Anchor
How to Choose the Right Boat Anchor
Anchor size depends on your boat size, expected conditions, and bottom type. A commonly used rule is 1 lb of anchor per foot of boat length for calm conditions, increasing to 1.5-2 lbs/ft for rough or open-water anchorage. Heavier boats and high-freeboard vessels (more windage) need larger anchors.
Anchor Types by Bottom
Fluke/Danforth anchors excel in sand and soft mud — they self-set quickly and hold well but can break out if the boat swings 180 degrees. Plow anchors (CQR, Delta) reset well when the boat swings and handle mixed bottoms. Bruce/claw anchors grip grass and rocky bottoms that Danforths miss. For mixed-use recreational boating, a plow or claw is the most versatile choice.
Chain vs. Rope Rode
Using 5-10 feet of chain as a snubber between the anchor and rope rode improves holding significantly. The chain's weight keeps the anchor shank angle low (ideally horizontal), which is the optimal angle for setting and holding. Rope absorbs shock loads from wave action. All-chain rode (common on cruising sailboats) provides maximum holding but is very heavy.
FAQ
Is this boat anchor size calculator free?
Yes, completely free with no signup required.
How is anchor size determined?
Anchor weight is primarily determined by boat length, then adjusted for beam (wider boats need more anchor), conditions (wind, current, bottom type), and duration of anchorage. A general rule: 1 lb of anchor per foot of boat length for calm conditions, doubling to 2 lbs/ft for exposed or rough anchorage.
What anchor type is best for my boat?
Fluke/Danforth anchors work best in sand and mud — the most common recreational anchorage. Plow/CQR anchors handle varied bottoms and reset well if the boat swings. Bruce/claw anchors hold well in grass and rocky bottoms. Mushroom anchors work only for small, calm-water boats in soft bottoms.
How much anchor rode (chain and rope) do I need?
The anchor scope — the ratio of rode length to water depth — should be at least 5:1 for good holding, and 7:1 or more in rough conditions. In 20 feet of water, you need at least 100 feet of rode at minimum scope. Add the freeboard height to the water depth when calculating.
Should I use chain or rope anchor rode?
Best practice is to use 5-10 feet of chain between the anchor and rope rode. The chain's weight helps keep the anchor angle flat for better holding, and the chain resists chafe on rough bottom. The rope portion (usually three-strand nylon) absorbs shock loads from waves and wakes.