Anchor Scope Calculator ⚓
Calculate the correct anchor rode length based on water depth, freeboard, tidal range, and sea conditions. Stay safe at anchor.
How to Use the Anchor Scope Calculator
Enter water depth, freeboard, tidal range, and sea conditions to calculate recommended anchor rode length. Scope ratios: calm 5:1, moderate 7:1, rough 8:1, storm 10:1.
Ensuring adequate scope is the key to safe anchoring. Always account for tidal changes.
Chain rode is effective with shorter scope than nylon rode. Adjust calculations for mixed rode.
Boat & Marine Calculator - Introduction
Find the rode length you should pay out for the depth, freeboard, tidal rise, and weather you actually have. The calculator uses standard scope ratios — 5:1 for calm anchorages, 7:1 for moderate wind, 8:1 for rough conditions, and 10:1 for storm holding — so you get a defensible number for your night plan rather than a guess.
How It Works
Rode length = (water depth + tidal range + freeboard) × scope ratio. Tidal range matters because the anchor needs enough scope at high water, not just at the moment you set; freeboard is the height of your bow roller above the water. Example: 15 ft depth, 4 ft tide, 4 ft freeboard, moderate 7:1 ratio → (15 + 4 + 4) × 7 = 161 ft of rode.
Usage Scenarios
- Overnight in a quiet cove: 12 ft of water, 2 ft of tidal rise, 3 ft freeboard, forecast calm. (12 + 2 + 3) × 5 = 85 ft — set a rounded 90 ft on a snubber and you have a comfortable margin.
- Front coming through after dark: Same anchorage, but the forecast shifts to 25-knot gusts. Recalculating at 8:1 raises the rode requirement to (12 + 2 + 3) × 8 = 136 ft. Re-rig before the wind builds, not after.
- Calculating swing radius for a crowded anchorage: A 100 ft rode means your stern can swing in a 100 ft radius around the anchor. Multiply by two for the diameter of clear water you need, and check that no other anchored boat's circle overlaps yours.
Frequently Asked Questions
What is anchor scope and why does it matter?
Scope is the ratio of anchor rode length to the depth from bow to seabed (depth + freeboard + tidal range). A 7:1 scope means 7 feet of rode for every 1 foot of depth. More scope = better holding power. Minimum 5:1 in calm conditions, 7:1 in moderate, 10:1 in storms.
How much anchor chain should I use?
Chain provides weight that keeps the anchor pull horizontal (critical for holding). Use at least 1 foot of chain per foot of boat length. For a 30-foot boat, use 30 feet of chain minimum. Chain also resists chafe on rocky bottoms better than nylon rode.
What is the best anchor type for different bottom conditions?
Plow/CQR anchors: excellent in sand and mud. Danforth/fluke anchors: best in sand, poor in rock. Bruce/claw anchors: good all-around, especially in weed. Rocna/Mantus: modern high-holding anchors that work well in most conditions. Always carry a backup anchor.
How do I know if my anchor is dragging?
Set an anchor alarm on your GPS/chartplotter. Take bearings on fixed objects ashore and check them periodically. Watch for the boat swinging differently than other anchored boats. Feel the anchor chain — a dragging anchor transmits vibration. If dragging, motor forward and reset.
How much swinging room do I need at anchor?
Your swinging circle radius equals your rode length. With 100 feet of rode, you need 200 feet of clear water diameter. Account for other anchored boats and their swinging circles. Wind shifts can cause boats to swing 180°, so ensure adequate clearance in all directions.