The 720 Rule for anodizing is a practical method for estimating how long an aluminum part should remain in an anodizing bath to produce a desired oxide coating thickness. It connects three important process variables: current density, anodizing time, and coating thickness.
Although the calculation looks simple, many anodizing problems occur because the rule is applied incorrectly. Surface area may be counted twice, total amperage may be confused with current density, voltage may be treated as the controlling variable, or the calculated coating thickness may be assumed to be exact regardless of alloy and bath conditions.
For CNC-machined aluminum parts, another issue is equally important: the oxide thickness calculated by the 720 Rule is not necessarily the same as the dimensional increase of the finished part. Part of the aluminum surface is converted into oxide and part of the coating grows outward. Ignoring this effect can cause bearing bores, sliding fits, threads, pins, and precision assemblies to become too tight after anodizing.
This guide explains how the 720 Rule works, how to calculate anodizing time and required amperage correctly, why actual coating thickness may differ from the theoretical result, and how engineers should account for anodizing when designing CNC aluminum parts.
What Is the 720 Rule for Anodizing?
The 720 Rule states that approximately 720 amp-minutes per square foot are required to produce 1 mil of anodic oxide on aluminum under suitable constant-current anodizing conditions.
One mil equals 0.001 inch, or 25.4 microns.
The basic relationship is:
720 = Current Density × Time ÷ Coating Thickness
waar:
- Current density is measured in amps per square foot, or ASF.
- Time is measured in minutes.
- Coating thickness is measured in mils.
The equation can then be rearranged depending on which value needs to be calculated.
What Is the Formula for the 720 Rule?
To calculate anodizing time:
Time = 720 × Desired Thickness ÷ Current Density
To calculate expected coating thickness:
Thickness = Current Density × Time ÷ 720
To calculate required current density:
Current Density = 720 × Thickness ÷ Time
These equations use current density rather than total amperage. This distinction is one of the most important parts of using the rule correctly.
720 Rule Calculation Example
Suppose an aluminum CNC component requires a 0.5-mil anodized coating and the anodizing process is operated at 12 ASF.
The required time is:
720 × 0.5 ÷ 12 = 30 minutes
Therefore, under ideal constant-current conditions, approximately 30 minutes would be required.
If the required coating were increased to 1 mil:
720 × 1 ÷ 12 = 60 minutes
Doubling the target coating thickness doubles the theoretical processing time when current density remains unchanged.
Where Does Surface Area Enter the 720 Rule?
This is one of the most common sources of confusion.
Surface area is used to calculate the total amperage required to achieve a selected current density. Once current density has been calculated, surface area should not be multiplied into the time formula again.
The relationship is:
Total Current = Surface Area × Current Density
If a part has 0.5 square foot of anodized surface and the process requires 12 ASF:
0.5 ft² × 12 A/ft² = 6 amps
The power supply therefore needs to provide approximately 6 amps to the part to achieve 12 ASF.
The anodizing time for a 0.5-mil coating remains:
720 × 0.5 ÷ 12 = 30 minutes
You do not multiply those 30 minutes by 0.5 again. The part size has already been included when the required amperage was calculated.
How Do You Calculate Surface Area for Anodizing?
The surface area calculation should include the aluminum surfaces exposed to the anodizing electrolyte and intended to receive the coating.
For a simple rectangular CNC part, calculate the areas of all exposed faces. For cylindrical components, include the outside cylindrical area, end faces, bores, and other surfaces that are exposed to the bath.
For surface area measured in square inches:
Surface Area in ft² = Surface Area in in² ÷ 144
Then:
Required Amps = Surface Area in ft² × ASF
Complex CNC components containing pockets, large bores, fins, internal surfaces, and multiple holes can have substantially more true surface area than their external dimensions suggest. CAD software is therefore useful for estimating total exposed area.
However, simply including an internal surface in a CAD area calculation does not guarantee that the feature will develop exactly the same coating thickness as an open external surface. Current distribution and electrolyte access also matter.
Why Does the 720 Rule Require Constant Current?
An anodized oxide layer is electrically resistive. As the layer becomes thicker, more voltage is generally required to maintain the same current through the process.
For this reason, a constant-current power supply adjusts voltage as necessary while maintaining the selected current.
The 720 Rule assumes that the required current density remains approximately constant during the anodizing cycle. If the current falls significantly as the oxide grows, the actual number of amp-minutes delivered to the aluminum becomes lower than the simple calculation assumes.
Therefore, operating a process at a fixed voltage and assuming that the same current flows throughout the cycle can produce inaccurate thickness predictions.
Why Is Voltage Not Included in the 720 Rule?
Another common question is why the equation contains amperage but not voltage.
Oxide growth is related to the electrical charge passing through the anodizing system. The 720 Rule therefore focuses on current density and time.
Voltage is still important because sufficient voltage must be available to drive the desired current through the increasing electrical resistance of the oxide film. However, voltage is normally treated as a response variable rather than the direct thickness-setting variable in this calculation.
A power supply that reaches its maximum available voltage before the target current can be maintained may cause the current to fall. Once that occurs, the actual anodizing process no longer follows the simple constant-current assumption.
Can the 720 Rule Be Used for Type II Anodizing?
Yes. The rule is commonly applied to sulfuric acid Type II anodizing when the process operates under controlled current-density conditions.
Type II anodizing is frequently used for corrosion protection, decorative finishes, dyed components, electronic housings, brackets, knobs, covers, and general CNC-machined aluminum parts.
However, selecting a current density should not be based on processing time alone. A higher current density can shorten the theoretical anodizing time, but bath temperature, pore structure, alloy response, heat generation, dye requirements, and process capability must also be considered.
For this reason, it is incorrect to assume that doubling current density always produces an identical coating in exactly half the time.
Can the 720 Rule Be Used for Type III Hard Anodizing?
The same current-density and amp-minute concept can also be useful when planning Type III hard anodizing, but process conditions become more demanding.
Hardcoat anodizing normally targets a thicker and more wear-resistant oxide layer than conventional Type II anodizing. Higher current densities and lower bath temperatures are commonly used in commercial hardcoat processes.
For example, if a process operates at 24 ASF and the target theoretical coating thickness is 2 mils:
720 × 2 ÷ 24 = 60 minutes
This provides a useful process estimate, but it does not mean every aluminum alloy will produce exactly 2 mils after 60 minutes. Hardcoat formation is strongly influenced by alloy chemistry, temperature control, agitation, electrical contact, and oxide dissolution.
Why Can Actual Thickness Differ From the 720 Rule?
The 720 Rule describes a useful relationship between current and theoretical oxide growth, but a real anodizing bath is not perfectly efficient.
While oxide is being formed electrochemically, the acidic electrolyte also dissolves some of the oxide. The final coating is therefore the result of oxide formation minus oxide dissolution.
Several conditions can change that balance.
Aluminum Alloy
Pure aluminum does not behave exactly like 6061, 6082, 2024, 7075, or cast aluminum. Copper, silicon, magnesium, zinc, and other alloying elements affect oxide formation and coating appearance.
High-alloy compositions can therefore produce a different actual thickness from the value predicted by a simple theoretical calculation.
Bath Temperature
Higher bath temperatures generally increase chemical dissolution of the oxide. If dissolution becomes excessive, the coating may not continue growing at the theoretically predicted rate.
Temperature control becomes particularly important for thick hard-anodized coatings because the electrical process itself generates heat.
Acid Concentration
Electrolyte concentration affects conductivity, pore formation, and oxide dissolution. A poorly controlled bath can therefore behave differently from the process conditions used to establish expected production results.
Agitation and Cooling
Good solution movement helps remove heat from the aluminum surface and maintain a more uniform electrolyte environment. Insufficient agitation can contribute to localized heating and uneven coating development.
Electrical Contact
Poor rack contact creates additional resistance. The rectifier may indicate electrical output, but the component may not receive the intended current distribution.
This can cause thin coatings, localized defects, burning near contact areas, or inconsistent results between parts in the same load.
Does 720 Rule Accuracy Depend on Aluminum Alloy?
Yes. This is an important limitation for CNC components manufactured from different aluminum grades.
6061 and 6082 are widely anodized because they generally respond well to conventional sulfuric anodizing. However, high-copper 2000-series alloys and high-strength 7000-series alloys can require tighter process control.
Cast aluminum presents another challenge. Silicon content and casting porosity can affect both coating appearance and uniformity.
The 720 Rule can therefore be used as a process starting point, but coating thickness should still be verified on the actual alloy when dimensional or functional requirements are critical.
Does the 720 Rule Control Anodizing Color?
No. The 720 Rule predicts oxide thickness rather than final color.
Thickness can influence the pore structure available for dye absorption, but color consistency also depends on the alloy, surface finish, pretreatment, dye concentration, dye temperature, immersion time, sealing process, and previous processing history.
Two parts processed for the same anodizing time may therefore show slightly different shades if they are manufactured from different alloy lots or have different machined surface conditions.
This is why cosmetic CNC parts often require more than simply specifying “black anodize.” The drawing may also need to identify the anodizing type, class, required coating thickness, surface preparation, and cosmetic expectations.
Does Anodizing Thickness Equal Dimensional Growth?
No. This distinction is particularly important for CNC designers.
Anodizing is a conversion process rather than a conventional deposited coating. Some aluminum below the original surface is converted into aluminum oxide while some of the resulting oxide grows outward.
For Type III hard anodizing, a practical engineering estimate is that approximately half of the total coating thickness appears as outward growth.
A 2-mil hardcoat would therefore produce approximately 1 mil of outward growth on each coated surface.
For Type II sulfuric anodizing, the outward-growth proportion can be lower under typical process conditions, so the exact dimensional allowance should be confirmed with the anodizer when tolerances are tight.
How Does Anodizing Affect CNC Bores and Shafts?
Consider a shaft that receives Type III anodizing with a 2-mil total coating thickness.
If approximately 1 mil grows outward on each surface, an outside diameter can increase by approximately 2 mils in total because the diameter includes two opposite surfaces.
A bore behaves in the opposite direction. Oxide growth from both internal walls reduces the usable diameter.
This can create serious problems for:
- bearing bores;
- sliding shafts;
- dowel-pin holes;
- precision bushings;
- valve components;
- press fits;
- locating features;
- close-tolerance assemblies.
For critical features, the machining drawing should clearly distinguish the required pre-anodize dimension from the final coated dimension.
Should Threads Be Anodized?
Threads require special consideration because coating growth changes both the effective major and minor geometry of the thread.
A thick hardcoat can make an originally acceptable thread difficult to assemble. Internal threads are especially sensitive because coating growth reduces available clearance.
Depending on the functional requirements, possible strategies include machining additional allowance, using appropriate thread tolerances, masking the thread before anodizing, or finishing specific surfaces afterward.
The correct strategy should be determined before CNC machining begins rather than after the finished part fails assembly.
When Should Areas Be Masked Before Anodizing?
Masking is commonly considered when anodizing would interfere with electrical contact, sealing faces, bearing fits, precision bores, grounding surfaces, threads, or other functional interfaces.
Not every dimension needs masking. Excessive masking increases finishing complexity and cost.
A better approach is to identify the functional features where coating buildup creates a real risk and allow the remaining surfaces to receive the specified anodized finish.
How Should CNC Parts Be Designed for Anodizing?
The 720 Rule is primarily a finishing calculation, but its implications should be considered during CNC design and machining.
First, specify the aluminum alloy. Anodizing response varies significantly between alloys, so simply writing “aluminum” is not sufficient for a precision component.
Second, specify the anodizing type and required thickness rather than relying only on color.
Third, identify whether tolerances apply before or after anodizing. Tight bores, pins, sealing surfaces, and fits require particular attention.
Fourth, consider surface finish. Anodizing follows the underlying machined surface rather than hiding tool marks or scratches.
Finally, discuss masking and contact locations for cosmetic components. Rack contact is required to conduct electricity into the part, so completely eliminating all possible contact marks may not be realistic.
How Should the 720 Rule Be Used in Production?
For production anodizing, the rule is best treated as a controlled starting point rather than a substitute for process qualification.
A typical workflow is:
- Identify the aluminum alloy.
- Determine the required coating type and thickness.
- Calculate the exposed surface area.
- Select an appropriate current density for the qualified anodizing process.
- Calculate the total required amperage.
- Calculate theoretical anodizing time using the 720 Rule.
- Maintain controlled bath temperature and agitation.
- Monitor current and voltage throughout the cycle.
- Measure actual coating thickness.
- Adjust the qualified process when repeat production data shows a consistent difference from theoretical values.
This approach is particularly important for production CNC parts where the final anodizing thickness influences dimensional acceptance.
How Is Anodizing Thickness Verified?
Even when the 720 Rule is used correctly, the finished coating should be measured when coating thickness is specified as a drawing requirement.
Commercial anodizers can use coating-thickness instruments or other qualified inspection methods depending on the specification and part geometry.
Measurement location also matters. Sharp corners, deep recesses, blind holes, and areas near electrical contacts may not develop exactly the same coating thickness as accessible external surfaces.
The drawing should therefore avoid assuming perfectly uniform oxide growth on every microscopic surface of a complex part.
Common 720 Rule Mistakes
Using Total Amps Instead of Current Density
A 10-amp setting means something very different for a 0.25-ft² part than for a 2-ft² load. Current density must be calculated from actual anodized area.
Multiplying Surface Area Into the Formula Twice
Once ASF has been calculated, surface area has already been accounted for. Multiplying the calculated anodizing time by area again produces an incorrect result.
Assuming Constant Voltage Is Constant Current
As oxide thickness increases, electrical resistance changes. A fixed-voltage supply may not maintain the required current density throughout the process.
Ignoring Alloy Chemistry
A recipe that works well for 6061 cannot automatically be expected to produce identical thickness and appearance on every aluminum grade.
Treating Theoretical Thickness as Guaranteed Thickness
The equation predicts ideal oxide formation. Production results must still account for temperature, dissolution, bath condition, contact quality, and alloy behavior.
Ignoring Finished Dimensions
A technically correct anodized coating can still create a rejected CNC part if coating growth closes a precision bore or changes a critical fit.
720 Rule for CNC Machined Aluminum Parts
For a CNC manufacturer, anodizing should be integrated into the complete manufacturing process rather than treated as an isolated cosmetic operation.
Op Tuofa CNC Duitsland, aluminum components requiring anodizing should be reviewed according to alloy, machined geometry, coating type, specified thickness, tolerance, masking requirements, surface finish, and final assembly condition.
For example, a decorative 6061 enclosure may tolerate ordinary Type II dimensional growth without special machining allowance. A hard-anodized piston, sliding shaft, bearing housing, or precision valve component may require considerably more careful compensation.
A useful DFM review therefore asks not only “What anodizing color is required?” but also “Which surfaces are functional, what coating thickness is required, and which dimensions must remain within tolerance after finishing?”
FAQs About the 720 Rule for Anodizing
What Does 720 Mean in Anodizing?
It represents approximately 720 amp-minutes per square foot required to form 1 mil of anodic oxide under the assumptions behind the rule.
How Long Does It Take to Anodize 1 Mil at 12 ASF?
Using 720 × 1 ÷ 12, the theoretical anodizing time is 60 minutes.
Does Part Size Change Anodizing Time?
Not directly when current density remains constant. A larger part requires more total current to maintain the same ASF, but the theoretical time at that current density remains the same.
Can I Use the 720 Rule With a Constant-Voltage Power Supply?
The simple rule assumes controlled current density. If current changes significantly during a constant-voltage process, the calculated time will no longer predict thickness reliably.
Does the 720 Rule Work for 7075 Aluminum?
It can provide a useful process estimate, but 7075 and other highly alloyed aluminum grades may not achieve exactly the theoretical film thickness. Actual process data should be used for critical work.
Does the 720 Rule Apply to Titanium Anodizing?
No. Titanium anodizing follows a different relationship in which applied voltage is closely related to oxide thickness and resulting interference color. The aluminum 720 Rule should not be transferred directly to titanium.
Does a 1-Mil Anodized Coating Increase a Part Dimension by 1 Mil?
Not necessarily. Anodizing both consumes aluminum below the original surface and grows outward. The dimensional change depends on anodizing type and process conditions.
Conclusion
The 720 Rule for anodizing is a valuable method for connecting current density, anodizing time, and aluminum oxide thickness. Its most common form is:
Time = 720 × Thickness ÷ Current Density
However, correct use requires more than entering three numbers into a calculator. Surface area must first be used to determine total amperage, constant current must be maintained, and alloy chemistry, temperature, electrolyte condition, electrical contact, and oxide dissolution must be controlled.
Most importantly for CNC manufacturing, theoretical coating thickness should not be confused with final dimensional growth. Bores, shafts, threads, bearing seats, sliding fits, and assembly interfaces may require machining allowance or masking before anodizing.
Tuofa CNC Duitsland provides CNC machining for custom aluminum components and can evaluate anodizing requirements together with material selection, dimensional tolerances, surface finish, masking, and final assembly requirements so that finishing is considered before the component reaches the anodizing stage.