Bend Allowance Calculator
Calculate bend allowance and the flat blank for one sheet-metal bend before the press brake.
Calculator
About this calculator
This bend allowance calculator is one sheet-metal bend. It does not estimate press-brake tons. Browse the sheet metal calculator hub.
Calculation Results
0.1357 in
Bend allowance · outside setback 0.1200 in · bend deduction 0.1043 in
Formula: BA = (π/180) × angle × (R + K × T)
Flat blank 3.1357 in = inside flanges + bend allowance.
How to use this result
Use the bend allowance when the flange lengths stop at the inside of the bend, and the bend deduction when they are outside dimensions. This is one bend. It ignores grain direction and springback, and it does not tell you press-brake tonnage. Confirm the K-factor with a coupon before cutting a production blank.
Continue Your Calculation
The flat blank does not set a spindle speed. Open speeds and feeds only when the next job is milling or drilling.
Bend allowance examples
Single 90° bend. Click a preset above to load the same inputs.
| Case | Inputs | BA | BD | Flat |
|---|---|---|---|---|
| 16 ga inside | 0.060 in, R 0.060 in, 90°, K 0.44, legs 2 in and 1 in | 0.1357 in | 0.1043 in | 3.1357 in |
| 16 ga outside | Same bend, outside legs 2 in and 1 in | 0.1357 in | 0.1043 in | 2.8957 in |
| 2 mm inside | 2 mm, R 2 mm, 90°, K 0.40, legs 50 mm and 30 mm | 4.398 mm | 3.602 mm | 84.398 mm |
Bend allowance formula
Bend allowance: BA = (π/180) × θ × (R + K × T)
Outside setback: OSSB = tan(θ/2) × (R + T)
Bend deduction: BD = 2 × OSSB − BA
Inside flat: Flat = L1 + L2 + BA. Outside flat: Flat = L1 + L2 − BD.
Example: T = 0.060 in, R = 0.060 in, θ = 90°, K = 0.44. BA = (π/180) × 90 × (0.060 + 0.44 × 0.060) = 0.1357 in. OSSB = 0.1200 in. BD = 0.1043 in. Inside legs 2 in and 1 in give a flat blank of 3.1357 in.
Need spindle RPM after the blank is cut? Use the speeds and feeds calculator. Hub: sheet metal calculator.
Bend allowance chart (90° bends)
Air-bend allowance with the inside radius at 1× thickness and K = 0.44. BA = (π/180) × 90 × (R + K × T). Confirm with a coupon for your die set.
| Material | Thickness T (in) | Inside radius R (in) | BA at 90° (in) | BA (mm) |
|---|---|---|---|---|
| 18 ga mild steel | 0.048 | 0.048 | 0.1086 | 2.758 |
| 16 ga mild steel | 0.060 | 0.060 | 0.1357 | 3.447 |
| 14 ga mild steel | 0.075 | 0.075 | 0.1696 | 4.308 |
| 11 ga mild steel | 0.120 | 0.120 | 0.2714 | 6.894 |
| 3 mm plate | 0.118 | 0.118 | 0.2669 | 6.779 |
| 6 mm plate | 0.236 | 0.236 | 0.5338 | 13.559 |
The K assumption dominates: at R = 1×T the allowance swings roughly ±10% between K = 0.33 and K = 0.50. Bend a coupon and measure the flange before trusting any chart row for production.
K-factor starting values
Starting ranges by forming method and material. A coupon measurement always wins over a table value.
| Material | Air bend | Bottoming | Coining |
|---|---|---|---|
| Mild steel | 0.33 – 0.44 | 0.44 – 0.50 | ≈ 0.50 |
| Stainless steel | 0.36 – 0.45 | 0.45 – 0.51 | ≈ 0.52 |
| Aluminium (soft) | 0.33 – 0.42 | 0.42 – 0.48 | ≈ 0.50 |
Air bend K rises toward 0.50 as the inside radius grows past 2× thickness; tighter radii sit near the low end. Bottoming and coining stretch the neutral axis outward, so K climbs. These are starting values only — the coupon on your material, die, and grain direction is the number you program.
Coupon procedure in one pass: cut two 2-inch flanges, bend at production pressure, measure both legs outside, solve K from the measured flat, and re-run the calculator with that K. Two coupons bracket the value; three give you a trend across grain directions.
Bend deduction: from setback to flat blank
Bend deduction is the amount the outside dimensions overstate the flat blank:
BD = 2 × OSSB − BA, where OSSB = tan(θ/2) × (R + T).
Why it exists: outside flange lengths include material that bends around the corner twice. The deduction removes that double-counted corner. Inside dimensions take the opposite path — you add bend allowance instead.
16 ga worked example: T = 0.060 in, R = 0.060 in, 90°, K = 0.44. OSSB = tan(45°) × 0.120 = 0.1200 in. BA = 0.1357 in. BD = 2 × 0.1200 − 0.1357 = 0.1043 in. Outside legs of 2.000 in and 1.000 in give flat = 3.000 − 0.1043 = 2.8957 in — the same blank the inside method reaches by 3.000 + 0.1357 = 3.1357 in from inside flange lengths.
Programming the brake from outside dimensions? Subtract BD per bend. From inside dimensions? Add BA per bend. Both land on the same flat blank for the same bend.
Which shop step this is
| Question | Use |
|---|---|
| Flat blank for one bend | This page |
| Milling or drilling RPM and feed | Speeds and feeds calculator |
| Torque at that spindle RPM | Motor torque calculator |
| Horsepower in kilowatts | HP to kW |
Frequently Asked Questions
How do I calculate bend allowance?
Use BA = (π/180) × angle × (R + K × T). Example: 0.060 in thickness, 0.060 in inside radius, 90°, and K = 0.44 gives 0.1357 in.
What is the difference between bend allowance and bend deduction?
Add bend allowance to inside flange lengths. Subtract bend deduction from outside flange lengths. For the 16-gauge example, BD = 2 × 0.1200 − 0.1357 = 0.1043 in.
What K-factor should I use?
Start near 0.33 to 0.50 for an air bend. This page does not look up a material chart. Bend a coupon and replace 0.44 if your die and grain direction differ.
Does this calculator size a press brake?
No. Tonnage depends on thickness, die opening, bend length, and material strength. This page stops at the flat blank.
Is the bend angle the inside angle of the part?
Enter the angle the sheet turns. A square bracket is 90°. The same 90° is not entered as 180°.
How do I calculate bend deduction?
BD = 2 × OSSB − BA. For the 16-gauge example: OSSB = 0.1200 in, BA = 0.1357 in, so BD = 0.1043 in. Subtract BD from outside flange lengths per bend; add BA instead when working from inside dimensions.
What is outside setback (OSSB)?
OSSB is the horizontal distance from the bend tangent to the outside mould point: OSSB = tan(θ/2) × (R + T). At 90°, tan(45°) = 1, so OSSB = R + T exactly — 0.1200 in for the 16-gauge example. It is the setback the outside legs see.
How does K-factor relate to the inside radius?
K shifts the neutral axis inward as the radius tightens. A sharp R < 1×T pushes K toward 0.33; a radius past 2×T drifts toward 0.50. That is why air-bend K is not one number — measure it at the radius you will actually run, not at a different die.
Does springback change bend allowance?
No — BA is geometry of the bent part and does not change with springback. Springback changes the overbend angle you program so the part relaxes to target. Feed springback into the press angle, not into BA or BD.
Does grain direction change the K-factor?
Indirectly. Bending across the grain needs a smaller inside radius than bending with it, and radius change moves K. The grain itself does not rewrite the K table — the die and radius it forces do. Coupon both orientations if the part bends both ways.