Anchor Scope Calculator
How much rode to pay out: scope ratio × (depth + bow height), with tide rise included — the arithmetic behind every quiet night at anchor.
The two classic shortchanges: forgetting the bow roller's height (it's part of the triangle) and anchoring at low water without adding the rise. Both errors shrink real scope exactly when the wind arrives at 2 am.
Formula
⚠️ For planning and education only — verify with official charts, tide tables and your vessel's documentation. Not for navigation.
How much rode to pay out: scope ratio × (depth + bow height), with tide rise included — the arithmetic behind every quiet night at anchor.
About Anchor Scope Calculator
Anchors don't hold by weight — they hold by digging, and they only dig when pulled horizontally. Scope is the geometry that buys that horizontal pull: rode length as a multiple of the height from seabed to bow roller, tide included. This calculator does the full arithmetic the 2 a.m. wind will audit — effective depth with bow height and rise, the rode to veer, and the resulting pull angle at the shank.
How to use Anchor Scope Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula rode = scope × (depth + bow height + tide rise); pull angle = asin(1/scope) substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Anchor Scope Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula rode = scope × (depth + bow height + tide rise); pull angle = asin(1/scope) with sources cited on the page
- ✓The two classic shortchanges: forgetting the bow roller's height (it's part of the triangle) and anchoring at low water without adding the rise. Both errors shrink real scope exactly when the wind arrives at 2 am.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why does scope matter more than anchor size?+
Because the anchor's hold collapses with pull angle: most modern anchors develop full resistance only when the shank is pulled within a few degrees of horizontal, and lose half or more of it by 15° of upward pull. At 3:1 scope the geometry alone gives ~19°; at 7:1, ~8°. An oversized anchor at short scope drags before a modest one on proper scope digs.
What scope ratios fit which situations?+
Working consensus: 5:1 for a settled-weather lunch stop with all-chain; 7:1 for overnight on chain or mixed rode; 10:1 when it's going to blow (and room permits). Crowded harbors force compromises — which is what oversized anchors, kellets and anchor alarms exist to backstop. The ratio is rode ÷ (depth + bow + rise), never rode ÷ charted depth.
Does all-chain rode change the scope requirement?+
It relaxes it in moderate conditions: chain's weight sags into a catenary that keeps the pull at the anchor horizontal even at 4–5:1 — until the wind pulls the catenary straight (around 30+ knots for typical cruisers), after which chain behaves like very expensive rope and wants the same 7:1 geometry. Storm scope recommendations converge for chain and nylon for exactly this reason; our catenary tool quantifies the crossover.
How do I track tide in the depth term?+
Anchor at any state, but compute for high water: tonight's effective depth is depth-now plus the remaining rise (tide tables or this site's tide tools), plus the bow roller. Anchoring in 8 ft at low water with a 9-ft range nearly doubles the depth term overnight — the boat that 'dragged for no reason' at 3 a.m. usually just had its scope halved by the moon.
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