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Sacrificial Anode Weight Calculator

Zinc, Aluminum, and Magnesium Anode Sizing for Hull, Shaft, Rudder, and Heat Exchanger Protection per ABYC E-2

Free sacrificial anode sizing calculator for marine mechanics, boatyards, and corrosion engineers who need to determine the correct weight of sacrificial anode material to protect underwater metals from galvanic corrosion. Enter the wetted surface area of the metals to be protected (hull, shafts, props, rudders, trim tabs, heat exchangers), the water type (salt, brackish, or fresh), the desired protection period, and the anode material (zinc, aluminum, or magnesium). The calculator returns the total anode weight required and the recommended number and placement of anodes.

Galvanic corrosion is the number one cause of underwater metal failure on boats. When dissimilar metals are immersed in an electrolyte (seawater), the more active metal corrodes preferentially. Sacrificial anodes are intentionally more active than the metals they protect — they corrode instead, sacrificing themselves to preserve your propeller shafts, bronze through-hulls, and aluminum outdrives. The sizing calculation is based on current density requirements from NACE SP0169 and ABYC E-2: each square foot of exposed metal surface requires a certain amount of protective current (typically 5-15 mA/ft2 in seawater), and each pound of anode material provides a known amp-hour capacity over its service life.

The calculator supports zinc (the traditional choice for saltwater), aluminum (works in both salt and brackish water, 50% more capacity per pound than zinc), and magnesium (required for freshwater because the higher driving voltage overcomes the lower conductivity). Choosing the wrong anode material for your water type is a common and expensive mistake — zinc anodes passivate in fresh water and provide no protection.

Pro Tip: If your zinc anodes are lasting more than 2 years without significant wastage, they may have passivated (formed a non-conductive oxide layer) and are no longer providing cathodic protection even though they look intact. Switch to aluminum anodes, which resist passivation and provide about 50% more amp-hour capacity per pound than zinc. If you operate primarily in fresh water, magnesium is the only anode material that provides adequate driving voltage.

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Sacrificial Anode Weight Calculator
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How It Works

  1. Enter Protected Metal Surfaces

    Input the wetted surface area of each metal component: hull plating (steel or aluminum boats), propeller shafts, propellers, rudders, trim tabs, through-hulls, and heat exchanger waterboxes. The calculator provides typical surface area estimates for common component sizes.

  2. Select Water Type

    Choose saltwater, brackish, or freshwater. The water type determines the required current density for cathodic protection and which anode materials are effective. Saltwater requires 5-15 mA/ft2, brackish 3-8 mA/ft2, and freshwater 1-3 mA/ft2.

  3. Select Anode Material and Service Life

    Choose zinc, aluminum, or magnesium based on your water type. Enter the desired protection period (typically 12-24 months between haul-outs). The calculator uses the electrochemical capacity of each material to determine the required weight.

  4. Review Anode Weight and Placement

    Check the total anode weight required, the recommended number of individual anodes, and suggested placement locations (hull, shaft, rudder, engine heat exchanger). Verify that commercial anode sizes are available to meet the calculated weight.

Built For

  • Boatyards specifying anode replacement during seasonal haul-outs and bottom paint jobs
  • Marine mechanics sizing shaft anodes and hull anodes for steel and aluminum workboats
  • Yacht owners transitioning from zinc to aluminum anodes and needing to recalculate the required weight
  • Corrosion engineers designing cathodic protection systems for marina pilings and floating dock hardware

Features & Capabilities

Three Anode Materials

Calculates requirements for zinc (354 Ah/lb, 0.5V driving voltage), aluminum (1150 Ah/lb, 0.55V driving voltage), and magnesium (500 Ah/lb, 0.7V driving voltage). Shows the weight advantage of aluminum over zinc and explains when magnesium is required.

Current Density by Water Type

Applies the correct cathodic protection current density based on water type per NACE SP0169 guidelines: seawater (5-15 mA/ft2), brackish water (3-8 mA/ft2), and fresh water (1-3 mA/ft2).

Component Surface Area Estimator

Includes typical wetted surface area values for common marine components: propeller shafts by diameter and length, propellers by diameter, rudders, trim tabs, and heat exchanger waterboxes, so you can estimate without measuring every part underwater.

Service Life Calculation

Calculates the anode weight needed for a user-specified protection period (6 to 36 months). Longer intervals between haul-outs require proportionally more anode material. Includes an 85% utilization factor since anodes lose electrical contact before they are fully consumed.

Frequently Asked Questions

In saltwater, both zinc and aluminum work well. Aluminum provides about 3 times the amp-hour capacity per pound compared to zinc, so you need fewer or smaller anodes. In brackish water (estuaries, river mouths), aluminum is preferred because zinc tends to passivate at lower salinity. In fresh water, only magnesium provides enough driving voltage (0.7V vs 0.5V for zinc) to push protective current through the low-conductivity water. Using zinc in fresh water is one of the most common anode mistakes.
Replace sacrificial anodes when they have lost approximately 50% of their original mass. At this point, the remaining material may not maintain adequate current output for full protection. Most boatyards check anodes during haul-out and replace any that are more than half consumed. If your anodes are completely gone before your next haul-out, you need more anode weight or a shorter maintenance interval.
Yes. Over-protection (too much cathodic current) can cause paint blistering and disbondment on coated steel and aluminum surfaces, hydrogen embrittlement on high-strength steel components, and accelerated deterioration of wooden hull planking in way of the anodes. The goal is to maintain a hull potential of -0.80 to -1.10V vs. Ag/AgCl reference electrode. A corrosion survey with a reference electrode can verify you are in the correct range.
If anodes show little or no wastage after a full season, they have likely passivated — formed a non-conductive oxide or carbonate layer that prevents current flow. This is common with zinc anodes in brackish or fresh water, and with any anode type if the electrical bonding connection is broken or corroded. Check the bonding wire connections first. If bonding is intact and you are in lower-salinity water, switch from zinc to aluminum or magnesium anodes.

Learn More

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Marine Cathodic Protection & Anode Sizing

How to select and size sacrificial anodes (zinc, aluminum, magnesium) for boats based on water type, hull material, and wetted surface area.

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