Anchor Chain Length Calculator
Calculate anchor rode and chain length from water depth using the 7:1 scope rule.
Covers calm, moderate, and storm conditions with chain minimums by boat size.
“Scope” is the ratio of total anchor rode (chain + rope) deployed to the vertical distance from the bow roller to the seabed. Getting scope right is the single most important factor in secure anchoring.
The Scope Formula
Total Rode Needed = Scope Ratio x (Water Depth + Freeboard + Tide Rise)
Where:
- Scope Ratio = the multiplier (typically 5:1 to 10:1)
- Water Depth = current charted depth at the anchorage
- Freeboard = height of the bow roller above the waterline
- Tide Rise = expected maximum tide change during your stay
Recommended Scope Ratios
| Conditions | All-Chain Rode | Chain + Rope Rode |
|---|---|---|
| Calm, day anchor (lunch stop) | 4:1 | 5:1 |
| Normal overnight | 5:1 | 7:1 |
| Moderate wind (15–25 kt) | 6:1 | 8:1 |
| Strong wind (25–35 kt) | 7:1 | 9:1 |
| Storm conditions (35+ kt) | 8:1–10:1 | 10:1+ |
All-chain rode requires less scope because the chain’s weight creates a catenary (sag) that keeps the pull on the anchor horizontal. Rope rode is lighter and tends to lift the anchor, requiring more length.
Worked Example — Overnight Anchorage
Water depth: 4 meters (13 feet). Freeboard (bow roller height): 1.5 meters (5 feet). Expected tide rise: 1 meter (3.3 feet). Total vertical distance: 4 + 1.5 + 1 = 6.5 meters. Conditions: Normal overnight, all-chain rode. Scope: 5:1. Total rode = 5 x 6.5 = 32.5 meters of chain.
If using chain + rope (7:1 scope): 7 x 6.5 = 45.5 meters total. Typical setup: at least 1 boat-length of chain (e.g., 10 m) + 35.5 m of nylon rope.
Swing Radius
When anchored, your boat will swing in a circle with radius equal to the total rode deployed plus your boat length. Swing radius = Rode + Boat length. In our example: 32.5 + 10 = 42.5 meters from the anchor. Ensure there are no obstructions, other boats, or shallow patches within this radius.
Chain Weight Reference
| Chain Size | Weight per Meter | Typical Boat Size |
|---|---|---|
| 6 mm (1/4") | 0.8 kg/m (0.5 lb/ft) | Under 8 m (26 ft) |
| 8 mm (5/16") | 1.4 kg/m (0.9 lb/ft) | 8–12 m (26–40 ft) |
| 10 mm (3/8") | 2.2 kg/m (1.5 lb/ft) | 12–15 m (40–50 ft) |
| 12 mm (1/2") | 3.1 kg/m (2.1 lb/ft) | 15–20 m (50–65 ft) |
Why Scope Matters Mechanically
At 3:1 scope, the pull on the anchor is roughly 18° above horizontal — this tends to pull the anchor out of the seabed. At 5:1 scope, the pull angle drops to about 11°. At 7:1 scope, the pull is nearly horizontal (about 8°), which buries the anchor deeper. The goal is always to keep the pull as horizontal as possible to maximize holding power.