Anodize Growth Calculator
Anodizing is not a coat of paint on top of the part. Some of it grows outward, and that part changes your dimensions. Enter the finished size you need and get the size to machine before the bath.
Last checked September 27, 2026. Runs in your browser, nothing is sent. Typical figures, the anodizer's process sheet governs.
How the number is built
- Outward growthCoating thickness × growth share. With a 12 micron Type II coating and a 50 percent share, each surface moves 6 microns outward.
- BoreTwo surfaces face each other, so the diameter closes by 2 × growth. Machine the bore larger by that amount.
- Shaft or pinTwo surfaces face away, so the diameter grows by 2 × growth. Machine it smaller by that amount.
- Flat faceOne surface, moves by the growth once. Matters on a stack height or a shim.
- Tolerance checkThe spread between the thinnest and thickest coating the type allows, times the same geometry. If that alone eats most of the band, mask the feature or put the thickness on the drawing.
Thickness ranges and the growth rule are in the finishes table. Why the fatigue penalty makes Type III a poor choice for a stem is in the anodizing guide.
What each coating does to a dimension
At a 50 percent growth share, the two ends of each typical thickness range. Microns, so 25 microns is 0.025 mm.
| Coating | Thickness | Bore diameter | Shaft diameter | Flat face |
|---|---|---|---|---|
| Type II, decorative | 5 µm | -5 µm | +5 µm | +2.5 µm |
| Type II, decorative | 25 µm | -25 µm | +25 µm | +12.5 µm |
| Type III, hard anodize | 25 µm | -25 µm | +25 µm | +12.5 µm |
| Type III, hard anodize | 50 µm | -50 µm | +50 µm | +25 µm |
Sources: MIL-A-8625F, anodic coatings for aluminum and aluminum alloys, Type II and Type III, Wikipedia, Anodizing. Growth share is a working rule and varies with alloy and process.
What to write on the drawing
Three lines in the notes block stop the argument between the shop and the anodizer before it starts.
- Before or after"Dimensions apply after finishing" or "before finishing". Pick one and say it. Most disputes over a tight bore start here.
- Type and thickness"Type II, 10 to 15 µm, black" is a specification. "Anodized black" is a wish.
- Mask listName the features to be masked or plugged. Bearing seats, threads and any bore under ±0.02 mm are the usual list.
- Go deeperWhere tolerance belongs on a bike part drawing and the RFQ checklist.
Questions
Does anodizing make a bore smaller or a shaft bigger?
Both. The oxide layer grows partly into the metal and partly outward. The outward part is what changes a dimension: a bore closes up by twice the outward growth and a shaft grows by twice the outward growth, because each has two surfaces on a diameter. A flat face moves by the outward growth once.
How thick is Type II anodize, and Type III?
Type II decorative anodize is typically 5 to 25 microns. Type III hard anodize is typically 25 to 50 microns. Within those ranges the thickness depends on the alloy, the bath and how long the part is in it, so specify a thickness on the drawing if a dimension depends on it.
Why 50 percent, and is it a specification?
It is a working rule, not a specification. Roughly half the coating thickness builds outward and half converts metal below the original surface. Hard anodize is close to that figure. Type II varies more with alloy and process. Ask the anodizer for their figure and measure a first article before you trust it on a tight fit.
Should bearing seats and threads be masked?
On hard anodize, yes, unless the drawing carries an allowance. Type III on a bearing seat can take a press fit to a crushed bearing. Threads are masked or chased after coating. On Type II the change is smaller, and whether it matters depends on the tolerance band of the feature.
Does the growth matter on a stem clamp bore?
Usually not for Type II. A 28.6 mm steerer bore held to ±0.05 mm has a 100 micron band and a 12 micron Type II coating moves it by about 12 microns. Type III at 50 microns moves the same bore by about 50 microns, half the band, which is one reason hard anodize belongs on wear faces rather than on clamps.
Need the part machined and anodized?
Send the drawing and we match it with up to 3 shops that machine bike components and have finishing sorted, within 24 business hours.