Bend allowance calculator
Bend allowance is the arc length the neutral axis travels through a bend — the material the bend consumes. Add it to the flange lengths measured to the bend line and you have the flat blank length.
The formula
- A — bend angle in degrees, measured from flat (a right-angle fold is 90°)
- R — inside bend radius
- T — material thickness
- K — the K-factor, where the neutral axis sits as a fraction of thickness (typically 0.33–0.5)
The term R + K × T is the radius of the neutral axis. Multiplying it by the bend angle in radians gives the arc length that axis follows — and because the neutral axis is by definition the layer that neither stretches nor compresses, that arc length is the true amount of flat material the bend uses.
Worked example
A 90° bend in 2 mm mild steel, air-bent to a 2 mm inside radius, with a K-factor of 0.4:
If the two flanges measure 30 mm and 50 mm from the bend line to their outer edges, the flat blank is 30 + 50 + 4.398 = 84.398 mm long.
Using bend allowance on a real part
Bend allowance assumes your flanges are dimensioned to the bend line. Most shop drawings are not — they give outside dimensions, because that is what you can put a caliper on. For those, use bend deduction instead, which subtracts from the sum of the outside flanges.
The K-factor is the only value here that is not a hard number. It depends on the material, the forming method and the tooling. If accuracy matters, bend a test coupon, measure it, and back out your own K-factor — then every calculation after that matches your press brake rather than a textbook.
Parts with more than one bend compound these numbers quickly, and each bend interacts with hole positions and flange reach. The Coobic editor runs this same formula across a whole part, unfolds and refolds it live, and exports the DXF flat pattern ready for cutting.
Frequently asked questions
What is bend allowance?
Bend allowance is the arc length of the neutral axis through the bend region. It is the amount of material the bend itself consumes, so adding it to the two flange lengths measured to the bend line gives the total flat length.
What is the bend allowance formula?
BA = (π / 180) × angle × (R + K × T), where angle is the bend angle in degrees measured from flat, R is the inside bend radius, T is the material thickness and K is the K-factor.
Is the bend angle measured from flat or from the closed position?
From flat. A right-angle fold is 90°, not 270°. This is the complementary angle, which is what press brake operators and most CAD packages use.
What is the difference between bend allowance and bend deduction?
Bend allowance is added to flange lengths measured to the bend line. Bend deduction is subtracted from flange lengths measured to the outside mould line. Both produce the same flat length — they just suit differently dimensioned drawings.
Design the whole part instead
Draw the flat pattern, add as many bends as you need, and export a DXF or a folded STEP file — free, in your browser.
Open the editor