Choosing the wrong tonnage for a CNC press brake is one of the most common — and costly — mistakes buyers make. Too little tonnage and the machine cannot complete the bend. Too much and you’re paying for capacity you’ll never use. This guide walks you through exactly how to calculate the press brake tonnage you need, based on your material, thickness, and bending length.
What Is Press Brake Tonnage?
Press brake tonnage refers to the maximum bending force the machine can apply, measured in metric tons (T) or kilonewtons (kN). A 100-ton press brake can exert up to 1,000 kN of downward force on the workpiece.
The required tonnage depends on four factors: material type, material thickness, bending length, and V-die opening width. Get any of these wrong and your calculation will be off.
The Tonnage Calculation Formula
The standard formula for calculating required bending tonnage is:
Tonnage (T) = (650 × t² × L) / V
Where:
- t = material thickness in millimeters
- L = bending length in meters
- V = V-die opening width in millimeters
- 650 = constant for mild steel (tensile strength ~450 MPa)
Worked Example
You need to bend 4mm mild steel plate over a 2-meter length, using a V-die with 32mm opening:
Tonnage = (650 × 4² × 2) / 32 = (650 × 16 × 2) / 32 = 20,800 / 32 = 650 kN ≈ 65 tons
In this case, an 80-ton press brake would give you sufficient capacity with a safety margin.
Material Correction Factors
The formula above uses 650 as the constant for mild steel. For other materials, apply these correction multipliers:
| Material | Multiplier | Example (vs mild steel) |
|---|---|---|
| Mild Steel (Q235) | 1.0× | Baseline |
| Stainless Steel (304) | 1.5× | 65T becomes 97T |
| Aluminum (5052) | 0.5× | 65T becomes 33T |
| High-Strength Steel | 2.0× | 65T becomes 130T |
| Copper | 0.7× | 65T becomes 46T |
V-Die Opening Width: The Rule of Thumb
The V-die opening (V) is typically 6–10 times the material thickness for optimal bending results. Use this quick guide:
- 1–2mm material: V = 10–16mm
- 2–4mm material: V = 20–32mm
- 4–6mm material: V = 32–50mm
- 6–10mm material: V = 50–80mm
- 10mm+ material: V = 80mm+
A narrower V-die produces a tighter inside radius and requires more tonnage. A wider V-die requires less tonnage but produces a larger inside radius. Choose based on your part drawing requirements.
Practical Tonnage Selection Guide
Use this table as a quick reference for common mild steel bending scenarios (2m bending length, V = 8× thickness):
| Material Thickness | Required Tonnage (2m length) | Recommended Machine |
|---|---|---|
| 1mm | ~10T | 40T press brake |
| 2mm | ~25T | 40T press brake |
| 3mm | ~40T | 63T press brake |
| 4mm | ~65T | 80T press brake |
| 6mm | ~90T | 100T press brake |
| 8mm | ~130T | 160T press brake |
| 10mm | ~170T | 200T press brake |
| 12mm | ~220T | 250T press brake |
5 Common Tonnage Mistakes to Avoid
- Ignoring material tensile strength — Always check the actual tensile strength of your material, not just the grade. Two batches of “Q235” can have different properties.
- Forgetting safety margin — Always add 20–30% to your calculated tonnage. Running a press brake at 100% capacity reduces tool life and machine longevity.
- Not accounting for partial bending length — If you only bend in the center of a long machine, tonnage distribution changes. Consult the manufacturer for off-center loading limits.
- Choosing tonnage for today only — If you plan to bend thicker material in the future, buy for your 3-year plan, not just your current job.
- Ignoring bending length — A 100T/3200mm press brake and a 100T/4000mm press brake have different force-per-meter ratings. Match the machine length to your longest part.
Summary: Key Takeaways
- Use the formula: T = (650 × t² × L) / V for mild steel
- Apply material correction factors for stainless, aluminum, or high-strength steel
- Choose V-die opening at 6–10× material thickness
- Add 20–30% safety margin to your calculated result
- Select machine tonnage for your future needs, not just today’s work
Need Help Choosing the Right Press Brake?
Our engineering team at Hudong Heavy Industry can help you select the correct tonnage and configuration for your specific application — at no cost.
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Frequently Asked Questions About Press Brake Tonnage
How much safety margin should I add to my press brake tonnage calculation?
As a practical rule, add 20% to 30% safety margin to your calculated bending tonnage. This helps protect tooling life, reduces machine stress, and gives you room for slightly harder material batches.
Does stainless steel require more tonnage than mild steel?
Yes. Stainless steel typically requires around 1.5 times the tonnage of mild steel for the same thickness, bend length, and V-die opening. Always verify the actual material grade and tensile strength before buying a machine.
What is the best V-die opening for air bending?
For most air bending applications, the V-die opening is usually 6 to 10 times the material thickness. Narrower dies increase tonnage demand, while wider dies reduce tonnage but increase inside bend radius.
Should I buy a larger press brake than my current job requires?
In many cases, yes. Buyers often choose a press brake based on their expected 2- to 3-year production plan rather than their current single project, especially when future materials may be thicker or parts may be longer.
Related Resources
Need a machine recommendation? Explore our sheet metal machinery product range, review our after-sales support services, or send your bending requirements to Hudong Heavy Industry.
Recommended Manufacturer Pages
If you are comparing suppliers as well as tonnage, review our CNC press brake manufacturer in China page and browse all Hudong buying guides before sending your RFQ.
