How to Use This Calculator
- 1
Enter Boat Length & Displacement
Input your vessel's overall length in feet and fully loaded displacement in pounds (e.g., 22 ft and 4,500 lbs).
- 2
Select Anchor Type & Wind Conditions
Choose your anchor style (e.g., Plow / CQR) and expected wind/sea severity (e.g., Moderate 10–20 kts).
- 3
Configure Advanced Depth Options (Optional)
Expand Advanced Options to enter actual water depth at high tide and bow freeboard height to refine rode scope calculations.
- 4
Review Ground Tackle Results
Examine your primary anchor weight, kedge anchor weight, recommended rode length, holding power, anchor-to-length ratio, scope safety rating, and the chain/rope breakdown panel.
Example Calculation
A boater anchoring a 22 ft vessel weighing 4,500 lbs with a Plow/CQR anchor in moderate wind (10–20 kts) in 10 ft of water with 4 ft bow freeboard.
Boat Length (ft)
22
Boat Displacement (lbs)
4,500
Anchor Type
plow
Wind / Sea Condition
moderate
Water Depth (ft)
10
Bow Freeboard (ft)
4
Results
Primary Anchor Weight
22 lbs
Kedge / Secondary Anchor
12 lbs
Recommended Rode Length
84 ft
Estimated Holding Power
4,400 lbs
Anchor-to-Length Ratio
1.00 lbs/ft
Scope Safety Rating
6:1
Tips
Prioritize Holding Efficiency
Anchor weight is only one component of ground tackle safety. High-efficiency modern spade or claw anchors deliver greater holding power per pound than traditional designs, especially in soft sand or clay.
Size for Storm Preparedness
Always maintain a safety margin. In winds over 35 knots, wind force on the hull increases exponentially with wind velocity. Keep an auxiliary or storm anchor sized at least 50% heavier for rough weather.
Deploy an Adequate Chain Leader
A chain leader equal to or exceeding your boat's length (minimum 15 ft) keeps the shank down for a horizontal pull, prevents rope chafe on rocks, and absorbs dynamic wave energy through catenary weight.
Sizing Your Anchor: Matching Weight to Boat Dimensions
The Anchor Weight to Boat Size Calculator helps boaters calculate ground tackle requirements, incorporating vessel length, displacement, anchor style, expected weather conditions, and water depth.
Selecting an undersized anchor is one of the leading causes of vessel dragging in anchorages.
For a 22-foot boat weighing 4,500 lbs, a 22 lb primary plow anchor provides approximately 4,400 lbs of holding force in moderate wind conditions (10–20 kts).
Ground Tackle Sizing Logic and Formulas
Anchor sizing moves beyond crude rules of thumb by applying multipliers for anchor design and environmental drag forces.
1. Primary Anchor Weight
Primary Anchor Weight = Boat Length (ft) × Base Weight per Foot × Wind Multiplier
- Base Weight per Foot: Plow/CQR =
1.0 lb/ft, Fluke/Danforth =0.85 lb/ft, Delta =0.95 lb/ft, Bruce/Claw =1.05 lb/ft, Mushroom =1.3 lb/ft. - Wind Multiplier: Calm (<10 kts) =
0.8, Moderate (10–20 kts) =1.0, Rough (20–35 kts) =1.3, Storm (>35 kts) =1.7.
2. Kedge Anchor Weight
Kedge Weight = Primary Anchor Weight × 0.55
3. Total Rode Length & Scope
Total Rode Length = Scope Ratio × (Water Depth + Bow Freeboard)
- Scope Ratio: Calm =
5:1, Moderate =6:1, Rough =7:1, Storm =8:1. - Chain Leader Minimum: Equal to boat length (minimum 15 ft).
Step-by-Step Worked Example
Consider sizing ground tackle for a 22 ft vessel weighing 4,500 lbs with a Plow/CQR anchor in moderate wind (10–20 kts) in 10 ft of water with 4 ft bow freeboard:
Base Factor & Wind Multiplier:
- Base factor for Plow anchor =
1.0 lb/ft - Wind multiplier for Moderate winds =
1.0
- Base factor for Plow anchor =
Calculate Primary Anchor Weight:
22 ft × 1.0 lb/ft × 1.0 = 22 lbs
Calculate Kedge Anchor Weight:
22 lbs × 0.55 = 12.1 lbs→ rounded to12 lbs
Calculate Total Rode Length:
- Total vertical height =
10 ft depth + 4 ft freeboard = 14 ft - Moderate scope ratio =
6:1 - Total rode length =
6 × 14 ft = 84 ft
- Total vertical height =
Determine Rode Line Breakdown:
- Chain leader length =
max(15 ft, 22 ft) = 22 ft - Nylon rode length =
84 ft - 22 ft = 62 ft
- Chain leader length =
Calculate Holding Capacity & Weight Ratio:
- Estimated holding power =
22 lbs × 200 = 4,400 lbs - Weight-to-length ratio =
22 lbs / 22 ft = 1.00 lbs/ft
- Estimated holding power =
Frequently Asked Questions
How does boat length determine primary anchor weight?
Boat length serves as the standard baseline for anchor sizing because windage area increases proportionally with length. Marine standards recommend approximately 1 lb of anchor per foot of vessel length for standard plow anchors in moderate conditions, adjusted for specific anchor styles and weather factors.
What is the difference between primary and kedge anchors?
A primary anchor is your main working anchor used for daily mooring and overnight stays. A kedge (or secondary) anchor is smaller—typically 50% to 60% of the primary's weight—used for stern mooring to prevent swinging, breaking ground, or backing up the primary anchor during strong currents.
How is recommended rode length calculated?
Rode length is calculated by multiplying the required scope ratio by total vertical height (water depth + bow freeboard). For example, a 6:1 scope ratio in 10 ft water depth with 4 ft freeboard requires 6 × (10 + 4) = 84 ft of total rode line.
Why is a chain leader necessary with a nylon anchor line?
A chain leader adds low-point weight right at the anchor shank. This weight lowers the angle of pull to keep the anchor dug into the seabed, absorbs shock loads from swell, and protects the rope from seabed abrasion.
What is a good scope ratio for overnight anchoring?
A minimum scope ratio of 5:1 is suitable for calm daytime anchoring, while 7:1 is recommended for overnight stays and moderate winds. Severe storm conditions warrant an 8:1 to 10:1 scope ratio to prevent anchor breakout.