Estimate mooring block weight from your boat’s length and exposure, or find a concrete, granite or steel block’s dry and submerged weight from its size.

Rule-of-thumb dry weight of a concrete deadweight block by boat length and exposure.

Choose the roughest conditions the mooring must survive, not a calm-day average.

Seabed bottom type
Blocks on rock hold by friction alone, so they are sized heavier; soft mud adds some suction.

Mooring Block Weight (Heuristic) Formula

The boat mode gives a fast rule-of-thumb estimate of the dry weight of a concrete deadweight mooring block from boat length, exposure and bottom type. It is useful for early planning and sanity checks, but it is not a substitute for a full mooring design or your harbor's mooring rules.

MBW = B(L) × E × S
Variable Meaning
MBWTypical dry block weight in pounds; the result shows a range of ±15% around it
B(L)Base weight for boat length L in feet: 20 ft 600 lb, 25 ft 750 lb, 35 ft 1,200 lb, 45 ft 2,000 lb, 55 ft 2,800 lb, 65 ft 3,500 lb, 80 ft 5,000 lb, 100 ft 7,000 lb, interpolated between points
EExposure factor: protected 0.75, moderate 1, exposed 1.35, hurricane or storm mooring 1.9
SBottom factor: sand or average 1, soft mud 0.9, rock or hard bottom 1.2

The second mode finds the weight of a block you have or plan to pour:

Wdry = V × ρblock
Wsub = Wdry × (1 - (ρwater) / (ρblock))

Concrete is about 150 lb/ft³, granite 165 lb/ft³, steel 490 lb/ft³, seawater 64 lb/ft³ and freshwater 62.4 lb/ft³.

How to Calculate the Estimate

  1. Measure or enter the boat length in feet or meters.
  2. Select the exposure that represents the roughest conditions the mooring must survive, not a calm-day average.
  3. Optionally choose the bottom type; sand or average is the default.
  4. Multiply the base weight for the boat length by the exposure and bottom factors to find the typical dry block weight.

Examples

Example 1: 32 ft boat, moderate exposure, sand bottom

MBW = (750 + (32 - 25) / (10) × 450) × 1 × 1 = 1,065 lb
Range = 0.85 × 1,065 to 1.15 × 1,065 ≈ 905 to 1,225 lb

Example 2: 40 ft boat, exposed water, rock bottom

MBW = 1,600 × 1.35 × 1.2 = 2,592 lb
Range ≈ 2,203 to 2,981 lb

Example 3: weight of a 24 in concrete cube

Volume = 2 ft × 2 ft × 2 ft = 8 ft³, so the dry weight is 8 × 150 = 1,200 lb. In seawater its effective weight is 1,200 × (1 − 64/150) = 688 lb.

How to Interpret the Result

The number returned by this calculator should be treated as an estimate for screening purposes. The formula is intentionally simple, which makes it quick to use but also means it does not directly account for:

  • Boat displacement
  • Freeboard and windage area
  • Current and tidal flow
  • Wave and wake exposure
  • Bottom type and holding characteristics
  • Chain length, scope, pennant setup, and hardware ratings
  • Safety factors or local code requirements

Because of that, two boats with the same length may need different real-world mooring systems if their hull shape, weight, or exposure conditions are different.

Dry Weight vs. Underwater Effect

Mooring blocks are often described by their weight in air. Once submerged, the block has a lower effective underwater weight because buoyancy offsets part of its load. When comparing products or existing installations, make sure you know whether the listed number refers to:

  • Dry weight in air, or
  • Effective submerged weight in the water

This distinction matters because two blocks with the same advertised material and shape can perform differently if the specified weight basis is not the same.

Using the Calculator More Effectively

  • Choose the exposure that reflects the roughest conditions you want the mooring to withstand.
  • Keep units consistent, especially when switching between feet and meters or inches and centimeters.
  • Use the estimate as a starting point, then compare it against local practice or a detailed marine recommendation.
  • Check the full mooring system, not only the block weight. Chain, shackles, swivels, pennants, and attachment points must also be sized appropriately.

Common Input Mistakes

  • Entering overall boat length in one unit while assuming another.
  • Choosing a calm-day exposure instead of the roughest conditions the mooring must survive.
  • Assuming block weight alone determines mooring safety.
  • Confusing pounds with kilograms when reviewing the result.

Quick Reference

What You Know What the Calculator Can Solve
Boat length and exposure (optionally bottom type) Recommended dry mooring block weight range
Block shape, dimensions, material and water type Dry and effective submerged block weight

For best use, treat this calculator as a quick estimating tool that helps frame the problem before making any final mooring decision.