Use the 720 rule to calculate anodizing amperage, run time or coating thickness from part area and current density, in US (ASF, mil) or metric units.
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Anodizing Amperage Formula
The calculator uses two formulas. Rectifier Setup gives the amps to dial in and the run time. The 720 Rule Solver relates thickness, time, and current density.
- Amps — rectifier output current
- ASF — current density in amps per square foot
- Area_sqft — total wetted surface area of the parts and rack contact
- Thickness_mil — target coating thickness in mils (1 mil = 25.4 µm)
- Time_min — anodize dwell time in minutes
The 720 rule is a nominal aluminum sulfuric-acid anodizing estimate at constant current density. Actual thickness depends on alloy, bath conditions and current efficiency; verify thickness using your qualified process. The time is full-current-equivalent time; a ramp profile is not modeled. To convert ASF to ASD (A/dm²), divide by 9.290304.
Reference Values
Illustrative current-density and thickness ranges, not operating instructions or specification limits. Use the qualified recipe for your alloy, bath and equipment:
| Process | Current Density | Typical Thickness |
|---|---|---|
| Type II Decorative | 10–15 ASF | 0.1–0.4 mil |
| Type II Architectural | 12–18 ASF | 0.4–0.8 mil |
| Type III Hardcoat | 24–30 ASF | 1.0–2.0 mil |
| Type III Heavy Build | 30–36 ASF | 2.0–4.0 mil |
Quick run-time lookup using the 720 rule:
| Thickness | @ 12 ASF | @ 18 ASF | @ 24 ASF |
|---|---|---|---|
| 0.5 mil | 30 min | 20 min | 15 min |
| 1.0 mil | 60 min | 40 min | 30 min |
| 2.0 mil | 120 min | 80 min | 60 min |
Worked Example
You have 8 square feet of 6061 parts and want a 2 mil hardcoat at 24 ASF.
- Amps = 24 × 8 = 192 A
- Time = 720 × 2 / 24 = 60 minutes
- Total charge = 192 × 1 = 192 A·hr
Why does my actual thickness come up short? Thickness can depart from the nominal estimate because of alloy, current efficiency, oxide dissolution and bath conditions. Confirm thickness and use the qualified process settings; increasing time or current is not a universal correction.
Should I ramp the current? Follow the validated ramp profile for your process. Oxide can form during the ramp, so do not assume that ramp time contributes no coating. This calculator models a constant current density only.
ASF or ASD? ASF is amps per square foot, common in North America. ASD is amps per square decimeter, common elsewhere. 1 ASD = 9.29 ASF.
