You will need to know your material thickness mt the bend angle b the inside radius ir and the k factor k.
Bend allowance formula in sheet metal.
The following illustration shows the equation calculation for determining the bend allowance when forming sheet metal.
Sheet metal calculator bend allowance equations and calculator.
Where ba is bending allowance.
If you have a sheet metal bending product as shown in above picture with two legs of length x and y and you unbend it you will see that total length of the unbend sheet will not x y rather the length will be x y ba.
The total length of the flatten sheet is equal to the sum of l1 that s the first length and l2 plus the bend allowance.
The bend allowance formula takes into account the geometries of bending and the properties of your metal to determine the bend allowance.
Lf l1 l2 ba.
This page also includes a link to a on line sheet metal bend allowance calculator.
Most sheet metal materials will conform to the calculations.