Understanding the Bend Allowance and consequently the Bend Deduction of a part is a crucial first step to understanding how sheet metal parts are fabricated.  When the sheet metal is put through the process of bending the metal around the bend is deformed and stretched.  As this happens you gain a small amount of total length in your part.  Likewise when you are trying to develop a flat pattern you will have to make a deduction from your desired part size to get the correct flat size.  The Bend Allowance is defined as the material you will add to the actual leg lengths of the part in order to develop a flat pattern.  The leg lengths are the part of the flange which is outside of the bend radius.  In our example below a part with flange lengths of 2” and 3” with an inside radius of .250” at 90° will have leg lengths of 1.625” and 2.625” respectively.  When we calculate the Bend Allowance we find that it equals .457”.  In order to develop the flat pattern we add .457” to 1.625” and 2.625” to arrive at 4.707”.  As you can see the Bend Allowance and Bend Deduction are closely related below.

Bend Allowance Calculator

Below is our simple Bend Allowance Calculator, it works by inputing the Material Thickness, Bend Angle, Inside Radius and K-Factor. It simply processes these variables through the equations we will discuss below.  Note that the Bend Angle is the excluded angle, not the included angle.

Bend Allowance Formula

The Bend Allowance Formula takes into account the geometries of bending and the properties of your metal to determine the Bend Allowance.  You will need to know your Material Thickness (MT), the Bend Angle (B<), the Inside Radius (IR), and the K-Factor (K).  The Material Thickness will be measured in decimal form, not by the gauge number.  For more information on gauges and their decimal equivalents and tolerances view our Gauge Chart page.  The Bend Angle will be something that you determine based on what the complimentary angle of your part is going to be.  It is important to convert from the included angle to the complimentary angle before performing any calculations.  The Inside Radius will be the finished radius of the included angle.  For information on how the Inside Radius is determined see our post on the Air Bend Force Chart.  Finally the K-Factor is a property of the material which you are bending.  This property determines how the material is stretched when bending.  See our post on the K-Factor for better understanding as well as charts and formulas.

The Formula itself is rather simple:

 Bend Allowance = {\dfrac{\pi}{180} \cdot B< \cdot (IR + K \cdot MT)}

Bend Allowance Chart

Because Bend Allowance is often confused with Bend Deduction it is important to understand which value you intend on working with.  Once you understand what these values represent you can use them to accurately and quickly develop flat patterns for your sheet metal parts.  Creating a chart with your standard values is a key component of speeding up this process.  Many software packages like Solid Works, Inventor and Solid Edge will let you incorporate a Bend Allowance Chart into its calculations when developing flat patterns.

Unfortunately sheet metal bending is not always going to be the same in every shop.  The largest variations come from the materials themselves.  Protective coatings, variations in the alloy and thickness as well as many other small factors all add up to give you Bend Allowances unique to your operation.  This chart will get you close enough for most applications and may not require fine tuning on your part, however if you are truly dedicated to precise bending, download the excel sheet and begin plugging in your own values.

The below Microsoft Excel chart is for even numbered gauges 8 through 24 and has a default K-Factor of .33 for each.  The inside radii are based off of a standard Air Bend Force Chart.  You can easily modify the Material Thickness, Inside Radius and K-Factor for each thickness at the top of each column.

Sheet Metal Bend Allowance Chart (PDF)

Sheet Metal Bend Allowance Chart (Excel)

BEND ALLOWANCE
8 Gauge10 Gauge12 Gauge14 Gauge16 Gauge18 Gauge20 Gauge22 Gauge24 Gauge
K factor0.33K factor0.33K factor0.33K factor0.33K factor0.33K factor0.33K factor0.33K factor0.33K factor0.33
MTIRMTIRMTIRMTIRMTIRMTIRMTIRMTIRMTIR
0.1630.2100.1350.1640.1050.1180.0750.1050.0600.0920.0480.0660.0360.0200.0300.0200.0240.020
AngleAllowanceAllowanceAllowanceAllowanceAllowanceAllowanceAllowanceAllowanceAllowance
100.04610.03640.02660.02260.01950.01430.00560.00520.0049
150.06910.05460.03990.03390.02930.02140.00830.00780.0073
200.09210.07270.05320.04530.03900.02850.01110.01040.0097
250.11520.09090.06650.05660.04880.03570.01390.01300.0122
300.13820.10910.07990.06790.05850.04280.01670.01560.0146
350.16120.12730.09320.07920.06830.05000.01950.01820.0170
400.18420.14550.10650.09050.07800.05710.02220.02090.0195
450.20730.16370.11980.10180.08780.06420.02500.02350.0219
500.23030.18180.13310.11310.09750.07140.02780.02610.0243
550.25330.20000.14640.12450.10730.07850.03060.02870.0268
600.27640.21820.15970.13580.11700.08560.03330.03130.0292
650.29940.23640.17300.14710.12680.09280.03610.03390.0316
700.32240.25460.18630.15840.13650.09990.03890.03650.0341
750.34550.27280.19960.16970.14630.10700.04170.03910.0365
800.36850.29100.21290.18100.15600.11420.04450.04170.0389
850.39150.30910.22630.19230.16580.12130.04720.04430.0414
900.41460.32730.23960.20360.17550.12840.05000.04690.0438
950.43760.34550.25290.21500.18530.13560.05280.04950.0462
1000.46060.36370.26620.22630.19500.14270.05560.05210.0487
1050.48360.38190.27950.23760.20480.14990.05840.05470.0511
1100.50670.40010.29280.24890.21450.15700.06110.05730.0535
1150.52970.41820.30610.26020.22430.16410.06390.05990.0560
1200.55270.43640.31940.27150.23400.17130.06670.06260.0584
1250.57580.45460.33270.28280.24380.17840.06950.06520.0608
1300.59880.47280.34600.29420.25350.18550.07230.06780.0633
1350.62180.49100.35940.30550.26330.19270.07500.07040.0657
1400.64490.50920.37270.31680.27300.19980.07780.07300.0681
1450.66790.52740.38600.32810.28280.20690.08060.07560.0706
1500.69090.54550.39930.33940.29250.21410.08340.07820.0730
1550.71400.56370.41260.35070.30230.22120.08620.08080.0754
1600.73700.58190.42590.36200.31200.22830.08890.08340.0779
1650.76000.60010.43920.37340.32180.23550.09170.08600.0803
1700.78300.61830.45250.38470.33150.24260.09450.08860.0827