Aluminium bend allowance & K-factor
Bending 2 mm aluminium to 90° over a 3 mm inside radius (K 0.35) gives a bend allowance of 5.81 mm and a bend deduction of 4.19 mm. Enter your own numbers below.
K-factor for aluminium: 0.33–0.4 (default 0.35) · air-bend starting value
Aluminium K-factor — typical starting values
| Air bending, general | 0.33–0.4 |
|---|---|
| Default used here | 0.35 |
| Tighter radius (R < T) | toward 0.33 |
| Bottoming / coining | toward 0.4 |
K-factor is a range, not a constant — it shifts with radius-to-thickness ratio, temper and bending method. Confirm against a test bend when accuracy matters.
K-factor is the one number people get wrong. It sets where the neutral axis sits, and it moves with the radius-to-thickness ratio. The values here are solid air-bend starting points for aluminium; a single test bend on your press and tooling beats any chart.
Bend allowance, deduction and flat length
When sheet is bent it stretches on the outside and compresses on the inside; somewhere between sits a neutral axis that neither stretches nor compresses. Its position, as a fraction of thickness, is the K-factor. Bend allowance is the arc length along that neutral axis — the material “used up” in the bend. Bend deduction is the shortcut most shops actually use: subtract it from the sum of the outside flange lengths to get the flat blank.
Picking K for aluminium
For aluminium, air bending, a K-factor around 0.35 is a good start (0.33–0.4 across conditions). Softer tempers and larger radii push K up toward the middle of the sheet; tight radii (inside radius below the material thickness) pull the neutral axis inward, lowering K toward 0.33. Bottoming and coining, which iron the bend, sit toward 0.4.
Minimum bend radius
Bend too tight and the outside fibres crack. A safe floor for aluminium is an inside radius roughly equal to the material thickness (1×T); ductile tempers go tighter, hard tempers need more. If a part cracks on the outside of the bend, open the radius before blaming the material.
From flat pattern to press brake
The flat length the calculator returns is what you cut before bending. Feed the outside leg lengths (mold-line to mold-line) and it subtracts the bend deduction for you; for multi-bend parts, subtract one bend deduction per bend. Get the flat right and the finished dimensions fall into place.
Need the stock thickness? Aluminium gauge chart
References
- Machinery’s Handbook — sheet-metal bending: bend allowance, bend deduction and setback.
- Press-brake / fabricator K-factor references — confirm K against a test bend for production work.
Last reviewed: 2026-07 · Calculation method & assumptions