About the Anchor Scope Calculator
This anchor scope calculator tells you how much anchor line (rode) to pay out for a given scope ratio. Scope is the ratio of rode length to the vertical distance from the bow roller down to the seabed — not just the depth of the water — so the calculator adds the height of your bow above the water and the tide still to rise before multiplying by the ratio you choose.
It is useful for sailors and powerboaters anchoring overnight, skippers checking whether their rode is long enough for a particular anchorage, and crews estimating how much room they need to swing clear of other boats. Enter your boat length to see the swing radius: the horizontal distance the boat can travel around the anchor when the wind shifts.
Common rules of thumb are about 7:1 for rope-and-chain rode in normal conditions, 5:1 for an all-chain rode, around 3:1 or 4:1 for a short lunch stop in calm weather, and 10:1 when a storm is forecast. These are starting points — holding ground, anchor type and weather all matter, so always check the anchor is set.
With the default inputs, the rode to let out is 189. Change any value above to recalculate instantly.
How to use the anchor scope calculator
- 1Choose feet or metres and read the current depth from your sounder.
- 2Add how much the tide will rise before you leave, using a tide table.
- 3Measure the height of your bow roller above the waterline.
- 4Pick a scope ratio for your rode type and the forecast.
- 5Let out the rode shown, set the anchor, and check the swing radius is clear of other boats.
Formula and method
Scope is measured from the bow roller, where the rode leaves the boat, to the seabed. The vertical height is therefore the current depth D plus any tide T still to rise plus the bow height B. Multiplying that height by the scope ratio S gives the length of rode to pay out.
The swing radius treats the rode as a straight line from the anchor to the bow: by Pythagoras its horizontal reach is the square root of rode² minus height², and adding the boat length gives the circle the stern can sweep. In reality chain sags in a curve (a catenary), so the true reach is a little shorter and this estimate errs on the safe side. The rode angle at the seabed for a taut line is arcsin(1 ÷ S) — the lower the angle, the better most anchors hold.
- D
- Water depth now
- T
- Tide still to rise
- B
- Bow roller height above the water
- S
- Scope ratio (e.g. 7 for 7:1)
- LOA
- Boat length overall
Worked examples
35 ft sailboat, 20 ft depth, 7:1 scope
The vertical distance is 20 ft depth + 3 ft tide + 4 ft bow = 27 ft. At 7:1 you need 27 × 7 = 189 ft of rode. Its horizontal reach is √(189² − 27²) ≈ 187 ft, so with a 35 ft boat the swing radius is about 222 ft.
All-chain rode at 5:1 in metres
Depth 8 m + tide 1.5 m + bow 1.2 m = 10.7 m vertical. At 5:1 that is 53.5 m of chain. The reach is √(53.5² − 10.7²) ≈ 52.4 m, plus the 12 m boat gives a 64.4 m swing radius.
Storm scope of 10:1 in non-tidal water
With 15 ft of water and a 5 ft bow the vertical distance is 20 ft, so 10:1 scope needs 200 ft of rode. The horizontal reach is √(200² − 20²) ≈ 199 ft, and the 40 ft boat swings within about 239 ft.
Frequently asked questions
What is anchor scope?+
Scope is the ratio between the length of rode you let out and the vertical distance from your bow roller to the seabed. A 7:1 scope in 10 ft from bow to bottom means 70 ft of rode. More scope pulls the anchor more horizontally, which helps it dig in and hold.
What scope ratio should I use?+
A common guideline is 7:1 for a rope-and-chain rode in normal conditions and 5:1 for an all-chain rode, because heavy chain lies on the seabed and keeps the pull low. Use 10:1 or more for storms, and 3:1 to 4:1 only for brief stops in calm water while someone stays aboard.
Should I include bow height and tide in the scope?+
Yes. Scope is measured from the bow roller, not the water surface, and you should plan for the deepest water you will see while anchored. Ignoring a 3 ft bow and a 5 ft tidal rise in 10 ft of water would leave you with roughly half the scope you intended.
How do I know how much rode I have let out?+
Mark the chain or rope at regular intervals with paint, cable ties or commercial markers, or use a windlass chain counter. Knowing the length lets you match the scope this calculator gives you.
How much room do I need to swing at anchor?+
Roughly the horizontal reach of your rode plus your boat length, in every direction around the anchor. Boats on all-chain and boats on rope swing differently, so leave extra space from neighbours who may be using a different rode.