Calculate proper anchor and chain sizing for safe anchoring
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Download NowGround tackle is the only thing between your boat and a lee shore at 3am, and it is the system most skimped and most regretted. A correctly sized anchor and enough chain let the boat swing, dig and hold; an undersized rig drags politely until the moment it does not, and then there is no second chance.
Sizing runs in tiers. Anchor weight scales with boat length and is adjusted by type and displacement, chain size follows the boat, and holding power is really bought with scope, the ratio of chain paid out to water depth, because an anchor only digs when the pull on it lies along the seabed.
Enter boat length, displacement, water depth and conditions, and the calculator recommends anchor weight, chain size and the minimum chain and rope length for the scope your situation demands.
Anchor Weight = Boat Length × 1.5 × Type Factor; Chain Size = max(6, Length ÷ 5) + load allowances; Chain Length = Depth × Scope Ratio| Situation | Scope Ratio | Rode in 20 ft | Note |
|---|---|---|---|
| Calm lunch stop | 3:1 | 60 ft | Minimum acceptable |
| Overnight | 5:1 | 100 ft | Sound cruising default |
| Expecting wind | 7:1 | 140 ft | Serious holding |
| Storm | 10:1 | 200 ft | Maximum, where room allows |
Recommended scope ratio by situation.
Formula verified against standard naval-architecture references · Updated October 2026
Start from boat length and adjust for anchor type and displacement. A common rule is about a pound and a half of traditional anchor per foot of boat, reduced for high-holding modern designs and increased for heavier boats. Enter length and weight here and the calculator recommends a base weight and the type factor, but the true sizing insight is that holding comes mostly from scope, not just anchor mass.
Enough for the worst scope you will need in your deepest usual anchorage. Overnight cruising usually wants at least 5:1 and storm conditions 7:1 or more, so a boat that anchors in 20 feet wants 100 to 140 feet of rode at minimum, and more if it gets deep. The calculator multiplies depth by your scope ratio to give the rode length, so size to the conditions you actually face.
Scope is the ratio of rode paid out to the vertical depth. It matters because an anchor holds by digging along the seabed, and that only happens when the pull on it is near-horizontal, which long scope provides. A short scope pulls upward, the anchor breaks out and drags. Most anchoring failures are insufficient scope rather than an undersized anchor.
Chain near the anchor adds weight that keeps the pull low and resists chafe on rocky bottoms, while a rope warp is lighter, cheaper and stretches to absorb gusts. Many cruisers run an all-chain rode for heavy boats and long scopes, or a chain-and-warp splice for a balance. The trade is weight, cost and stowage against holding and shock absorption, and scope matters more than which you choose.
Modern high-holding designs such as Rocna, Mantus and Spade set quickly and hold strongly in most bottoms, often in a smaller, lighter unit than a traditional plough. Danforth and Fortress types are excellent in clean sand and stow flat but foul in weed and shell and drag in soft mud. There is no universally best anchor, so choose for your typical bottom and consider a second, different-style kicker.
Gauge grows with boat length and displacement, since heavier boats need chain whose breaking load comfortably exceeds the forces a good set can generate. The calculator estimates size from length and bumps it for heavier displacement. In practice, err toward the larger common size, because chain is the part you never want to be the weak link when a swell is pulling on the rode.
No, and treating it as gospel is dangerous. Published holding numbers assume an ideal set in good bottom with proper scope, and real conditions vary with weed, tide, gusts and seabed. Use the figures to size sensibly, then verify by setting the anchor and reversing hard on it, and always carry enough scope and a backup that a single imperfect set cannot put the boat at risk.