Sheet Metal Blank Size Calculator
Estimate flat blank length and width from finished part dimensions, bend allowance, bend deduction, trim allowance, corner relief, thickness, K-factor, and multiple bend counts.
Blank size results
Calculation breakdown
| Material | Typical thickness | Starting K-factor | Inside radius cue | Blank layout note |
|---|---|---|---|---|
| Cold rolled steel | 16 ga 0.060-0.063 in | 0.40-0.44 | 1.0T typical | Stable bends, verify coating-free thickness. |
| 5052-H32 aluminum | 0.050-0.125 in | 0.30-0.36 | 1.0T to 1.5T | Good brake forming; coupon values improve accuracy. |
| 6061-T6 aluminum | 0.063-0.125 in | 0.32-0.38 | 2.0T or larger | Avoid tight bends; grain direction matters. |
| 304 stainless | 20 ga to 14 ga | 0.38-0.45 | 1.5T to 2.0T | Higher springback; use shop BD when available. |
| Copper or brass | 0.032-0.063 in | 0.35-0.42 | 1.0T to 1.5T | Soft alloys may mark easily; trim after forming. |
| Layout method | Use when | Formula idea | Best input source | Common caution |
|---|---|---|---|---|
| K-factor estimate | Early design or one-off bracket | BA = angle x (R + K x T) | Material data and tooling radius | K changes with material, radius, and die opening. |
| Shop bend allowance | Inside/tangent dimensions are used | Flat = legs + total BA | Brake test coupon or bend table | Confirm angle matches the coupon angle. |
| Shop bend deduction | Outside virtual sharp dimensions are used | Flat = outside legs - total BD | Approved shop chart or CAD bend table | BD is not the same as BA. |
| Trim after bending | Edges need square cleanup | Add trim after bend math | Shear, laser, or forming practice | Do not hide bend error by over-trimming. |
| Blank layout preset | Finished part style | Bend count | Relief detail | Typical shop check |
|---|---|---|---|---|
| U channel | Base with two side flanges | 2 same direction | Usually none unless ends close | Verify inside width after springback. |
| Four-sided tray | Bottom with two long and two short flanges | 4 total | Square or tear-drop corner relief | Check corner gaps and flange collision. |
| Z bracket | Offset web with opposing flanges | 2 total | Often no corner relief | Confirm offset height and parallel faces. |
| Cover pan | Shallow lid with returns | 4 total | Relief slightly larger than thickness | Measure outside fit before batch cutting. |
| Door skin return | Large panel with edge returns | 2 to 4 total | Corner notches at intersections | Leave controlled trim for final squaring. |
| Corner relief style | Starting size | Best for | Effect on blank | Inspection cue |
|---|---|---|---|---|
| Square notch | 1T to 2T per side | Simple trays and pans | Removes square corner area | No tearing at flange intersection. |
| Round relief | Diameter 1.5T to 2.5T | Fatigue-sensitive corners | Less sharp inside corner | Smooth radius, no punch burr. |
| Obround relief | Width 1T, length 2T+ | Long returns and boxes | Allows flange movement | Relief extends beyond bend tangent. |
| No relief | 0 | Single-bend channels | No area removed | Only if flanges do not collide. |
It uses K-factor, radius, thickness, bend count, and finished dimensions to calculate the finished width, length, trim, relief area, bend allowance, and bend deduction for corresponding blank size. It will get you remarkably close to the right blank size (length x width), though real metal and springback will always have final say when you bend your piece on the brake. Mess up here and you not only waste material but you also cost yourself time too, or worst case scenario…scrap a sheet.
Each time you unfold a tray or bracket in your mind there’s a little slice of metal being added/removed. The length are never quite equal to the total of both legs because material gets compressed and stretched in curve area. This calculator figures out math so you don’t have to try and guess.
How to Calculate Sheet Metal Bends Correctly
Gauge is one of those things where most folks think thickness makes no difference. On paper a couple thousandths doesn’t seem like anything, but that’s what moves the neutral axis and alters metal’s desire to spring back. Use real measured gauge. Using actual measured gauge keeps your numbers honest.
Inside radius is also subject to same rule. That radius are determined by tooling, material, and die opening. If you make an incorrect assumption there, the error build up through each flange height on the completed part.
There are two sides to this coin: Bend deduction and bend allowance. Shops usually draws and dimension with one or the other, and they prefer to do it that way. If you have an inside tangent line callout on your print you add allowance; if it is a outside virtual sharp dimension then you subtract deduction. Where the devil hides is when you mix them.
Which calculation method you use either has a proven K-factor behind it, or it’s something the shop likes because it came from test coupon for them. It’s up to you which method, but it will tell you whether or not your flat pattern match what happens on floor.
Notches prevent big headaches It is no big headache. If you have two flange that intersect at a 90 degree angle, where does all that material go? If your notch is the correct size, the flange will be able to lift up cleanly without ripping or bunching out bend lines. The notch typically begins at a distance about twice the thickness of material. It may not matter for something as simple as a channel. But if you attempt same ruse on a four sided tray, you’ll find yourself watching corners buckle and crack.
Plan for grain direction. When bending metals such as aluminum or some of the stainless steels, be aware that the material can become brittle near bend radius if it is bent with the grain. Rotating the blank so that bends is across the grain typically results in less springback and safer forming. Sometimes this will fight the way the sheet nests on the raw stock, so consider this balance before sending to shear or laser.
Trim allowance is easy to overlook until the edges come back looking ragged. After forming, that additional piece on the outside of each edge provides the shear or laser with a clean path to follow. It allows you to square everything off so you don’t have to chase bend inaccuracies into final dimension. Trim is added at the end, after the bend math has been figured, as it isn’t a band-aid for miscalculated brake settings.
Even seasoned fabricators will cut and form a test blank before cutting a stack of them. For a tight tolerance part, small variations in thickness, spring back, or die deflection can throw off a dimension just enough to be significant. You get amazingly close with the calculator but it’s always best to let real metal on a real brake have last word. That initial coupon should of been cheap insurance.
A good blank calculation is about using precise math to understand exactly how the metal react and behaves in motion. Respect those little details that make all the differance, like grain and relief. Get your inputs correct so the flat pattern you cut one day turns into a part that fits seamlessy with another, with no drama whatsoever. That’s the difference between knowing and guessing, just like the size of your first scrap sheet that you’ll never need to order.
