Marine Corrosion and Sacrificial Anode Sizing Calculator
Protection and anode inputs
Anode sizing result
Enter the current requirement and anode data, then select Size anodes.
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Capacity-sizing formulas
Base current (A) = area (m²) × current density (mA/m²) ÷ 1,000
Design current = base current × (1 + margin)
Required Ah = design current × duty cycle × years × 8,766 h/year
Required net alloy mass = required Ah ÷ (capacity × utilization)
Capacity count = round up(required mass ÷ net mass per anode)
Using 365.25 days gives 8,766 hours per average year. Preset capacity is kept separate from utilization so the assumed usable mass remains visible.
Why there can be two anode counts
Capacity count provides enough usable alloy to supply the modeled amp-hours over the service interval. It does not prove that the anodes can deliver that current at any instant.
Output count is available only when you enter a verified continuous amperage per anode. It divides full design current by that rating and rounds up.
The reported governing count is the greater available count. Even when both checks pass, a design still needs resistance, driving-voltage, potential, distribution, placement, continuity, and interference verification.
How to use the marine anode calculator
- Obtain the required protection current from a qualified design, or enter electrically connected immersed surface area and an approved design current density. Do not copy the example placeholders as recommendations.
- Set a documented margin, immersed duty cycle, and replacement interval. Use 100% duty for continuously immersed structures.
- Select the aluminium or zinc seawater planning capacity only when it matches the design basis, or enter verified capacity for the actual alloy and environment.
- Enter an appropriate utilization factor and the manufacturer's net consumable alloy mass per anode—not gross shipping or assembly mass.
- If a competent calculation or manufacturer gives continuous output for the actual geometry and conditions, enter it for the second check. Review the governing count with a corrosion professional before purchase or installation.
Worked arithmetic example
Suppose an approved basis uses 20 m² at 10 mA/m², a 20% margin, 100% immersion, and a one-year life. Base current is 0.200 A and design current is 0.240 A. Required capacity is 0.240 × 8,766 = 2,103.84 Ah.
With an illustrative 2,000 Ah/kg capacity and 85% utilization, required net alloy mass is 2,103.84 ÷ (2,000 × 0.85) = 1.238 kg. If each anode contains 1.5 kg of net alloy, the capacity check rounds up to one anode. This is an arithmetic example only; it does not establish appropriate current density, alloy, utilization, output, location, or protection.
Frequently asked questions
How is required sacrificial-anode mass calculated?
Multiply average protective current by service-life hours to get required amp-hours. Divide by the selected electrochemical capacity and utilization factor. The calculator then divides by net alloy mass per anode and rounds upward.
Does enough anode weight guarantee protection?
No. Weight addresses capacity. The anodes must also produce and distribute enough current while maintaining the required structure-to-electrolyte potential. Geometry, water resistivity, driving voltage, coating, continuity, placement, shielding, and interference all matter.
What capacities do the presets use?
The seawater planning presets use 2,000 Ah/kg for aluminium-based anodes and 780 Ah/kg for zinc-based anodes, consistent with DNV-RP-B401 design values. Use the exact project or supplier value when it differs.
What does the utilization factor mean?
It is the fraction of net anode alloy assumed to remain usefully consumable. Attachment, shape, current distribution, and end-of-life geometry can leave some alloy unavailable. This is separate from electrochemical capacity in this calculator.
Should coated and bare areas use the same current density?
Not automatically. Coating type, breakdown over time, water conditions, flow, temperature, material, and design stage can require different current-density treatment. Calculate approved area groups separately and add their current demands before using the known-current method.
Can the tool choose zinc, aluminium, or magnesium for my boat?
No. Alloy selection depends on the protected metals, water chemistry and salinity, operating potential, coating, connected components, manufacturer requirements, and applicable standards. Use custom capacity only after suitability is established independently.
Are my vessel or structure details uploaded?
No. Calculations run locally in the browser, and this tool does not send, store, or attach input values to analytics events.
Engineering limits and safety disclaimer
- The tool is a steady-average capacity model. It does not calculate initial, mean, and final current-density cases; coating-breakdown progression; anode resistance; driving voltage; closed-circuit potential; or initial/final anode output.
- It does not verify electrical continuity, installation spacing, current distribution, shielding, isolation, bonding, reference-electrode criteria, overprotection, hydrogen effects, galvanic compatibility, stray-current corrosion, shore-power effects, or interference with nearby structures.
- Preset capacity values apply only as identified. Alloy composition, activation, temperature, salinity, sediment exposure, and supplier qualification can change performance.
- Protection of propellers, shafts, drives, trim tabs, aluminium hulls, steel hulls, underwater structures, tanks, and mixed-metal systems requires system-specific decisions. More anode material is not automatically safer.
Engineering and safety disclaimer: Do not purchase, install, remove, or resize anodes solely from this result. Use current standards, vessel and equipment manufacturer requirements, verified alloy certificates, a qualified cathodic-protection design, and commissioning potential measurements. Investigate unexpectedly rapid consumption, electrical leakage, damaged bonding, or persistent corrosion promptly with a competent marine electrician or corrosion professional.
Methodology and sources
Last reviewed: August 2, 2026. The capacity method and distinction between anode mass and current output were checked against the following engineering references:
- DNV-RP-B401: Cathodic protection design
- U.S. DoD TSEWG TP-17: Galvanic anode material selection and design considerations (PDF)
- WBDG Cathodic Protection Knowledge Area
- U.S. EPA: vessel cathodic-protection technical development document (PDF)
The DNV-based presets are 2,000 Ah/kg for aluminium-based anodes and 780 Ah/kg for zinc-based anodes in seawater. The EPA reference explicitly distinguishes current capacity per unit mass from maximum anode output, which depends on material, surface area, system resistance, and driving potential.