Boat Anchor Size Calculator
A 24 ft boat needs a 16 lb plow, claw, or scoop anchor or an 8 lb fluke, plus 175 ft of rode in 20 ft of water. Size yours for type, conditions, and windage.
- Free, no signup
- Plow, claw, scoop, fluke
- Rode, chain, and line size
- Adjusts for windage
Boat anchor size chart
Recommended anchor weight by boat length for a boat of moderate weight and windage, anchored overnight. Chain leader and nylon line diameter are sized for the longest boat in each row.
| Boat length | Fluke / Danforth | Plow, claw, or scoop | Storm sizing, plow or scoop | Chain leader | Nylon line |
|---|---|---|---|---|---|
| Up to 15 ft | 4 lb | 6 lb | 8 lb | 15 ft | 3/8 in |
| 16 to 20 ft | 6 lb | 11 lb | 14 lb | 20 ft | 3/8 in |
| 21 to 25 ft | 8 lb | 16 lb | 20 lb | 25 ft | 3/8 in |
| 26 to 30 ft | 11 lb | 22 lb | 28 lb | 30 ft | 7/16 in |
| 31 to 35 ft | 17 lb | 33 lb | 41 lb | 35 ft | 1/2 in |
| 36 to 40 ft | 22 lb | 44 lb | 55 lb | 40 ft | 5/8 in |
| 41 to 45 ft | 30 lb | 60 lb | 75 lb | 45 ft | 5/8 in |
| 46 to 50 ft | 40 lb | 80 lb | 100 lb | 50 ft | 3/4 in |
| 51 to 55 ft | 50 lb | 100 lb | 125 lb | 55 ft | 7/8 in |
| 56 to 60 ft | 60 lb | 120 lb | 150 lb | 60 ft | 7/8 in |
| 61 to 70 ft | 75 lb | 150 lb | 188 lb | 70 ft | 1 in |
Weights are the calculator's chart weight for the top of each length band. Steel fluke anchors run half the chart weight and belong in sand and mud. Storm sizing adds 25%. A heavy or high-windage boat (flybridge, liveaboard, catamaran) adds 30%, and a light open boat can drop 20%. Round up to the next size your anchor brand makes.
Anchor rode length by water depth
| Water depth | Lunch hook, 5:1 | Overnight, 7:1 | Storm, 10:1 |
|---|---|---|---|
| 10 ft | 75 ft | 100 ft | 150 ft |
| 15 ft | 100 ft | 150 ft | 200 ft |
| 20 ft | 125 ft | 175 ft | 250 ft |
| 25 ft | 150 ft | 225 ft | 300 ft |
| 30 ft | 175 ft | 250 ft | 350 ft |
| 40 ft | 225 ft | 325 ft | 450 ft |
| 50 ft | 275 ft | 400 ft | 550 ft |
Total rode (chain plus line) for a 4 ft bow height, rounded up to the next 25 ft. Add a tide allowance to the depth before you look it up.
How the anchor size calculator 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, so the calculator scales the chart weight for anchor type, conditions, and the boat's weight and windage.
- 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 5:1 for a lunch hook, 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 takes the bottom 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.
What size rope do I need for an anchor line?
The calculator sizes nylon anchor line at about 1/8 inch of diameter for every 9 ft of boat: 3/8 inch up to 27 ft, 7/16 inch to 31 ft, 1/2 inch to 36 ft, 5/8 inch to 45 ft, 3/4 inch to 54 ft, and 7/8 inch to 63 ft.
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.