How do you calculate bend allowance for sheet metal?

Multiply the bend angle in radians by the inside bend radius, material thickness, and K-factor together. With a 1.5708 rad (90 degree) bend, 0.125 in radius, 0.0747 in thickness, and 0.42 K-factor, the allowance is about 0.0062 in for this term.

What is the K-factor in bend allowance?

The K-factor is the ratio of the distance from the inside surface to the neutral axis, divided by material thickness. It typically runs 0.3 to 0.5; 0.42 is a common starting value for many mild steels and aluminums.

Why use radians for the bend angle?

Bend allowance is an arc length, and arc length equals angle in radians times radius. Using radians lets the formula multiply cleanly — a 90 degree bend is 1.5708 radians (pi/2).

Bend allowance vs bend deduction — what's the difference?

Bend allowance is the arc length added along the neutral axis; bend deduction is the amount subtracted from the sum of the outside flange dimensions to get the flat length. They're related but used differently in flat-pattern math.

What happens if my K-factor is wrong?

An off K-factor shifts the neutral axis, so the computed allowance is short or long and your flat blank is sized wrong. For precise parts, confirm the K-factor with a physical test bend on your actual material and tooling.

Does material thickness affect bend allowance?

Yes — thickness is a direct multiplier in the formula. Thicker material consumes more length in the bend, so the same 90 degree bend on heavier stock yields a larger allowance.

Welding & Fabrication calculator

Sheet Metal Bend Allowance Calculator

Bend allowance is the length of material consumed in a sheet metal bend, measured along the neutral axis — the plane inside the material that neither stretches nor compresses as it forms around the radius. Sheet metal designers and press brake programmers use it to calculate accurate flat patterns so a formed part hits its final dimensions instead of coming off the brake long or short. It ties together four things: the bend angle, the inside bend radius, material thickness, and the K-factor, which locates that neutral axis as a fraction of thickness. Get bend allowance wrong and every downstream flange length is off, which is why it's the foundation of any correct flat blank.

What this calculator does

Estimate sheet metal bend allowance from bend angle (radians), inside bend radius, material thickness, and K-factor.
Use it when developing a flat blank length for a sheet metal part so the bend zone material is accounted for before nesting and shearing.
It computes bend allowance as the bend angle in radians times the product of inside bend radius, material thickness, and K-factor.

Formula used

Combined bend zone term = inside bend radius × material thickness × K-factor
Sheet metal bend allowance = bend angle in radians × combined bend zone term

Inputs explained

Bend angle in radians:
Inside bend radius:
Material thickness:
K-factor:

How to use the result

Use it when developing a flat pattern for a press-brake part, so the blank length accounts for material consumed in each bend.
The K-factor is empirical and varies with material, tooling, and radius-to-thickness ratio; a book K-factor may not match your actual brake, so verify against a test bend for tight-tolerance work.

Current U.S. benchmarks

The producer price index for steel mill products stands at 348.53 (BLS, May 2026), up 6.7% from a year earlier. Quotes priced off last quarter's material cost miss this move.
Industrial electricity averages 8.66 cents per kWh across the U.S. (EIA, Apr 2026), up 5.5% from a year earlier. Energy-intensive steps carry this directly into unit cost.
U.S. iron and steel imports ran $2.1B in May 2026 (Census International Trade). The U.S. ran a trade deficit of $0.4B in the category that month. Import volumes are the pressure gauge behind tariff and reshoring decisions.
The U.S. has 53,790 fabricated metal products establishments employing about 1,441,471 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate bend allowance for sheet metal? Multiply the bend angle in radians by the inside bend radius, material thickness, and K-factor together. With a 1.5708 rad (90 degree) bend, 0.125 in radius, 0.0747 in thickness, and 0.42 K-factor, the allowance is about 0.0062 in for this term.
What is the K-factor in bend allowance? The K-factor is the ratio of the distance from the inside surface to the neutral axis, divided by material thickness. It typically runs 0.3 to 0.5; 0.42 is a common starting value for many mild steels and aluminums.
Why use radians for the bend angle? Bend allowance is an arc length, and arc length equals angle in radians times radius. Using radians lets the formula multiply cleanly — a 90 degree bend is 1.5708 radians (pi/2).
Bend allowance vs bend deduction — what's the difference? Bend allowance is the arc length added along the neutral axis; bend deduction is the amount subtracted from the sum of the outside flange dimensions to get the flat length. They're related but used differently in flat-pattern math.
What happens if my K-factor is wrong? An off K-factor shifts the neutral axis, so the computed allowance is short or long and your flat blank is sized wrong. For precise parts, confirm the K-factor with a physical test bend on your actual material and tooling.

Last reviewed 2026-05-12.