Press Brake Tonnage: How to Calculate It

In short: For air bending, tonnage rises with thickness squared and bend length, and falls with die opening.

A 3 m bend in 6 mm steel of 414 MPa over a 48 mm V needs about 1,490 kN, or 152 tonne-force. Multiply by material, method and tooling factors, then check the tool and machine limits. Treat any formula as an estimate and confirm on your own tooling.

The formula, with units

A widely used shop formula comes from Steve Benson, a press brake specialist writing in The Fabricator (source 1). It is for air bending a baseline steel of 60,000 psi tensile strength, with all dimensions in inches and the result in US tons:

Forming tonnage = [575 × t² ÷ V ÷ 12] × L × M × F × T

t = material thickness (in)
V = die opening (in)
L = length of bend (in)
M = material factor = tensile strength in psi ÷ 60,000
F = method factor: 1 air bending, 5+ bottoming, 10+ coining
T = tooling factor: 1 conventional, 4 hemming, 5 offset or hat tools

The factors are as published in source 1, which also notes that offset tools in thick material can reach a factor of 10.

The same formula in metric

Converting units gives the version below. This is our conversion of the formula above (1 in = 25.4 mm, 1 US ton = 8.896 kN, 60,000 psi = 414 MPa), not a separate source:

F (kN) ≈ 660 × t² × L ÷ V × M × F × T

t = material thickness (mm)
V = die opening (mm)
L = length of bend (m)
M = tensile strength in MPa ÷ 414

Worked example

Air bend 6 mm steel, tensile strength 414 MPa, over a length of 3 m, in a die opening of 48 mm (8 × thickness), with conventional tooling. So M, F and T are all 1.

Metric:   660.7 × 6² × 3 ÷ 48 = 1,487 kN  (about 152 tonne-force)
Imperial: t = 0.236 in, V = 1.890 in, L = 118.1 in
          [575 × 0.0558 ÷ 1.890 ÷ 12] × 118.1 = 167 US tons
Check:    167 × 8.896 = 1,487 kN

Now change one thing at a time:

Change	Calculation	Result
Baseline	6 mm, 414 MPa, V48, 3 m	1,487 kN
304 stainless at 84,000 psi (M = 1.4, source 1)	1,487 × 1.4	2,082 kN
Plate 10 % thicker (6.6 mm)	1,487 × 1.1²	1,799 kN
Wider die (V60)	1,487 × 48 ÷ 60	1,190 kN
Bottom bending (F = 5)	1,487 × 5	7,435 kN

Our arithmetic from the formula above, for illustration.

Two lessons stand out. Thickness is squared, so plate at the top of its tolerance raises force by far more than you might expect. And a wider die lowers force, but in air bending the die opening also sets the inside bend radius, so changing it changes your bend deduction (source 1).

Force per metre at V = 8 × thickness

For a quick estimate with M = 1 in air bending:

Thickness (mm)	V (mm)	Force per metre (kN/m)
2	16	165
3	24	248
4	32	330
6	48	496
8	64	661
10	80	826
12	96	991

Calculated by Halcyon Laser with the metric formula above. Thicker plate often uses wider dies: follow your tooling supplier.

Why formulas disagree

You will find other formulas and they will not match exactly. Source 2 quotes one found in steel and press maker literature:

P (metric tons) = (b × t² × Rm) ÷ [(W − Rd − Rp) × 9,800]

b = bend length (mm), t = thickness (mm), Rm = tensile strength (MPa)
W = die width (mm), Rd = die entry radius, Rp = punch radius

With our example and radii of 4 mm each (our assumption), this gives about 114 metric tons, some 25 % below the 152 calculated above. Benson notes that this formula does not seem to work well for middle and softer materials, and that chart values depend on the conditions behind them (source 2). So do not quote a single formula as the answer: use the higher estimate for planning, then confirm with your machine and tooling maker's charts.

Then check the limits

Tooling: precision-ground tools carry a rated load in the catalogue or on the tool. Do not exceed it (source 1).
Short bends: a short, heavy bend concentrates load on a small part of the bed and tool, so a job within total tonnage can still exceed a limit. Ask the machine maker for the concentrated-load rating.
Deflection: source 1 gives an average design limit for bed and ram deflection of 0.0015 in per foot between the side frames. Long bends near capacity need crowning to stay straight.
Consult the makers: the source itself advises consulting your press brake and tooling manufacturer to stay within limits.

Sizing a press brake

Calculate your heaviest realistic job: the thickest plate at the top of its tolerance, the strongest material and the longest bend. Add a margin for your future work. We suggest about 20 %, which is our rule of thumb and not a standard. If you bend stainless or high-strength steel regularly, size for that rather than for mild steel, and consider a longer bed only if your parts need it.

Our HB CNC press brakes cover electro-hydraulic, electric, hybrid, heavy-duty and tandem versions, and bending automation is available for repeat parts. Send your thickest part through the enquiry form and we will check the tonnage with you.

Frequently asked questions

How do you calculate press brake tonnage?

For air bending in metric: about 660 × thickness² × length ÷ die opening, in kN, with mm and metres, for 414 MPa steel. Multiply by material, method and tooling factors, then confirm against your maker's charts.

How much tonnage for 6 mm steel over 3 m?

With a 48 mm die opening and 414 MPa steel, about 1,487 kN, or 152 tonne-force. Stainless at 84,000 psi needs about 1.4 times that.

What die opening should I use?

V = 8 × thickness is a common starting point. A wider die lowers force but increases the inside radius in air bending. Follow your tooling supplier, especially on thick plate.

How much more force does bottom bending need?

The Fabricator's published formula uses a factor of 5 or more for bottom bending and 10 or more for coining, compared with air bending.

Why do tonnage calculators give different answers?

They use different constants and assumptions about radii and friction. In our example two published formulas differ by about 25 %. Plan with the higher value and confirm with test bends.

Next step

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Sources

Steve Benson, “Four steps to calculate press brake tonnage limits”, The Fabricator, 18 May 2023 — www.thefabricator.com (accessed 2026-11-01)
Steve Benson, “Taking a deep dive into press brake tonnage, Part I: Improving formulas”, The Fabricator, 12 November 2019 — www.thefabricator.com (accessed 2026-11-01)

Press brake tonnage: how to calculate it, with formula and worked example