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.
Why Scope Matters Mechanically
At 3:1 scope, the pull on the anchor is roughly 18° above horizontal.
That upward angle is what lifts the anchor out of the seabed instead of driving it in.
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.
The angle the calculator prints is the straight-line one, arctan(vertical ÷ rode), and it deliberately ignores the catenary. That makes it pessimistic on all-chain rode and about right on rope. Chain sags under its own weight, so the last few meters at the anchor lie flatter than the geometry suggests, and in light air the pull can be genuinely horizontal at scope where the straight-line figure says 14°. That sag is exactly why the all-chain column above asks for less scope than the rope column. In a real blow the chain straightens out and the straight-line angle is the honest one, which is the case worth planning for.
How Much Chain, and How Heavy
| 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) |
Weight matters twice over. It is what creates the catenary that keeps the pull horizontal, and it is also weight carried in the worst possible place, right up in the bow, where it makes the boat pitch. On a light modern hull a full locker of 10 mm chain is a noticeable change in how she rides. That trade is the reason a mixed rode exists at all: a boat length or two of chain to hold the angle down and to take the chafe on the bottom, then nylon for the rest, which also stretches and absorbs the snatch loads that chain transmits straight to the bow roller.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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