How to use this calculator
- Enter thickness. Use the material thickness after selecting the correct unit system.
- Enter bend length. Use the total length of the bend line under the punch.
- Set the V die. Enter the lower die opening, commonly around eight times thickness for first-pass air bending.
- Set material strength. Enter tensile strength for the material, not yield strength.
- Read required force. Use the required force with margin, then verify press and tooling ratings.
How it works
Air-bending force scales with tensile strength, bend length and thickness squared,
and falls as the V-die opening gets wider:
F = 1.6 x UTS x L x t^2 / V
The constant normalizes the common shop formula
tons/ft = 575 x t_in^2 / V_in for 60 ksi mild steel.
The calculator reports the estimated force before margin and the required force after
your machine margin. It also shows V/t and the common
8t die-opening starting point so die selection is visible.
Worked example
Verified against the live calculator
A 3 mm mild-steel bend, 1000 mm long, in a
24 mm V die with 450 MPa tensile strength gives:
F = 1.6 x 450 x 1000 x 3^2 / 24 = 270,000 N, or about
27.5 metric tonnes-force.
With a 1.2 machine margin, the planning number becomes about
324 kN. If the die opening were narrowed, tonnage would rise in direct
proportion to 1/V.
Frequently asked questions
How do I calculate press brake tonnage?
For air bending, this calculator uses F = 1.6*UTS*L*t^2/V, where UTS is tensile strength, L is bend length, t is material thickness and V is the V-die opening. It is equivalent to the common mild-steel formula tons/ft = 575*t^2/V when t and V are in inches.
What V-die opening should I use?
A common first pass for air bending is V = 8t, with many jobs falling around 6t to 12t depending on material, bend radius and part geometry. Small V openings raise tonnage sharply; large V openings increase the formed radius.
Does this work for stainless or aluminum?
Yes. Enter the actual tensile strength for the material. Stainless often needs more force than mild steel, while softer aluminum usually needs less. Use the material cert or tooling chart when capacity is tight.
Is this for air bending, bottoming or coining?
It is an air-bending estimate. Bottoming and coining can require much higher force, so use the tooling manufacturer data for those operations.
Can I compare this to press capacity?
Yes, as a planning screen. Keep the required force below the press rating and check the machine load chart, off-center loading, tooling rating, punch tip radius, die rating and bend length limits.
Does this work in inches?
Yes. Toggle to imperial to enter thickness, length and die opening in inches, and tensile strength in ksi. The force output converts automatically.
Method & assumptions
- Air-bending estimate for a V die; not a bottoming or coining capacity check.
- Material is represented by ultimate tensile strength; actual batches vary.
- Tooling radius, die wear, bend angle, grain direction, lubrication and material springback are not modeled.
- Confirm press capacity, tooling tonnage rating, off-center loading and machine manufacturer load charts before running a job.