Bend Allowance Calculator

Created by Dominik Czernia, PhD candidate
Last updated: Sep 08, 2022

When it comes to bending sheet metal, a few factors come into play to get the desired result, and our bend allowance calculator is the right tool to help you. Bend allowance is the distance between the points where the metal is bent. This measurement determines how much material needs to be removed in order to create a smooth, finished product.

This sheet metal bend radius calculator can help you quickly and easily determine your project's bend allowance. Simply enter the dimensions of your project into the calculator, and it will calculate the bend allowance for you. Below, we also discuss what bend allowance is and show an example of bend allowance chart. With this information, you can ensure that your products are made with precision and accuracy!

What is bend allowance?

Bend allowance is the amount of metal that is required to fill in the space between the two points of maximum curvature on a bent part. This is necessary because, when sheet metal is bent, the inside wall of the bend stretches while the outside wall compresses. This results in a thinner cross-section on the inside of the bend, which needs to be taken into account when calculating the amount of material needed.

💡 What is bend deduction?

Bend deduction is the amount of material that is gained when a part is bent. It takes into account both the thickness of the material and the radius of the bend.

Our other tools might interest you if you're working on a bigger project. Check the belt length calculator or speeds and feeds calculator to find the optimal belt for two pulley systems and determine rotation speed and feed rate for drilling, milling, boring, etc.

How do you find bend allowance?

There are a few different ways to find bend allowance:

  1. One way is to use a bend allowance calculator. Simply enter the dimensions of your project into the calculator, and it will calculate the bend allowance for you.

  2. Another way is to use a bend allowance chart. This chart lists the most common dimensions and their corresponding bend allowances and should be provided by a supplier.

  3. Finally, you can use a formula, which we present in the next section.

Bend allowance and bend deduction formulae

To estimate the sheet metal bend allowance, use the following equation:

BA=θ(r+KT)BA = \theta \cdot (r + K\cdot T)

where:

  • BABA - Bend allowance;
  • θ\theta - Bend angle (in radians);
  • rr - Inside radius;
  • K=t/TK = t/T – K-factor, where tt is the distance from the inner surface to the neutral axis; and
  • TT - Material thickness.

❗ Be aware that the above formula is an estimation, and you need to know the empirical K-factor beforehand from the chart provided by the supplier or an experiment.

Our tool can also work as the bend deduction calculator. We use the following bend deduction formula:

BD=2(r+T)tanθ2BABD = 2(r + T) \cdot \tan\frac{\theta}{2} - BA

where BDBD is the bend deduction, and the rest parameters are the same as before.

Bend allowance chart

We generated the following bend allowance chart using the sheet metal bend allowance calculator for steel, assuming the K-factor of 0.45, bend angle of 90 degrees, and inside radius of 0.05 inches (1.3 mm). The actual K-factor may depend on the type of bending operation, the thickness of the sheet, and the material. Please, treat the chart as a rule of thumb.

TT [inch (mm)]

BABA [inch (mm)]

BDBD [inch (mm)]

0.02 (0.5)

0.045 (1.14)

0.034 (0.86)

0.024 (0.6)

0.051 (1.3)

0.036 (0.9)

0.028 (0.7)

0.057 (1.45)

0.037 (0.95)

0.031 (0.8)

0.063 (1.61)

0.039 (0.99)

0.035 (0.9)

0.07 (1.77)

0.041(1.03)

0.039 (1)

0.076 (1.92)

0.042 (1.08)

0.047 (1.2)

0.088 (2.24)

0.046 (1.16)

0.055 (1.4)

0.1 (2.55)

0.049 (1.25)

0.063 (1.6)

0.11 (2.87)

0.052 (1.33)

0.071(1.8)

0.13 (3.18)

0.056 (1.42)

0.078 (2)

0.14 (3.5)

0.059 (1.5)

As you can see, we also used the bend deduction formula to find the corresponding values of BDBD. If you want to know precisely what the bend allowance is, use our bend deduction calculator with the appropriate K-factor.

How to use the bend allowance calculator

To use the bend allowance calculator:

  1. Enter the values for thickness, radius, K-factor, and angle.
  2. Read the bend allowance and deduction from the last two fields.
  3. Switch the units if you want to use millimeters instead of inches or vice-versa.
  4. To use the tool as a bend radius calculator, input all known parameters and leave the inside radius field empty. The sheet metal bend radius calculator should find it for you.

Are you using our tool to build a vehicle? Then keep in mind how to calculate power-to-weight ratio metric, which we described in another article.

Dominik Czernia, PhD candidate
Bend angle (θ)
deg
Inside radius (r)
in
Material thickness (T)
in
K-factor
Bend allowance (BA)
in
Bend deduction (BD)
in
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