Bend Deduction & Allowance Chart
Make your own bend deduction, bend allowance or outside setback chart for one thickness and K-factor: any inside radii, any angles, in millimetres or inches. Read it on screen, copy it, or download it as CSV.
Dimensions
Results
Flat length = outside flanges added − one deduction per bend.
Preview
Chart
How this tool is verified
- Every cell is checked against the single-bend tool on this site for the same thickness, K, radius and angle, and against hand-worked values.
- The downloaded CSV is read back by a second implementation in Python, which recomputes every cell from the definitions, to the CSV’s own rounding, on every release.
- With K = 0.447 the allowance agrees with the FAA handbook’s fixed formula to within 0.15 % over a grid of radii, thicknesses and angles.
The method, the reports and the responsibility boundary are on the Verification page.
Built by Ruthen Systems
Free, with no account, and the dimensions you type never leave your browser. If a result looks wrong, or the option your shop needs is missing, say so — reports of a wrong result are dealt with first.
Report a problem or request a toolAssumptions and limits
Read these before you cut anything. They describe exactly what this tool does and does not account for.
- One thickness and one K-factor per chart. K is yours: work it out from a test bend with the K-factor tool, or start from a published value and confirm it.
- Angles are angles bent through: 90° is a square corner, not the included angle between the flanges. 1° to 179°.
- Bend allowance = angle (radians) × (inside radius + K × thickness). Outside setback = tan(angle ÷ 2) × (inside radius + thickness). Bend deduction = 2 × outside setback − bend allowance.
- Flat length from outside dimensions = sum of the outside flanges − one bend deduction per bend.
- Nothing about springback, tooling or minimum radius is applied. A chart is only as good as the K in it.
Output is a planning and fabrication aid. You are responsible for verifying dimensions, tolerances and material behaviour before production — full disclaimer.
Worked example
2 mm sheet, K 0.44, 2 mm inside radius, bent 90°.
- Bend allowance4.524 mm
- Outside setback4.000 mm
- Bend deduction3.476 mm
By hand. Allowance = π/2 × (2 + 0.44 × 2) = π/2 × 2.88 = 4.524. Setback = tan 45° × (2 + 2) = 4. Deduction = 2 × 4 − 4.524 = 3.476. A part with outside flanges of 50 and 30 is cut 50 + 30 − 3.476 = 76.524 long.
Method
The relations used
Bend allowance is the arc length of the neutral axis, angle × (R + K·T) — in degrees, π × (R + K·T) × A ÷ 180, the form in the SOLIDWORKS help on the K-factor. Outside setback is tan(A ÷ 2) × (R + T), and bend deduction is twice the outside setback less the bend allowance (Steve Benson, “Sheet metal bending calculation basics”, The Fabricator; SOLIDWORKS help on allowance and deduction).
The FAA's airframe handbook prints a fixed-K version for inch work: bend allowance = (0.01743 R + 0.0078 T) × N, with N the degrees of bend (FAA-H-8083-31B, Ch. 4, p. 4-64, public domain). Its 0.01743 is π/180 rounded, and 0.0078 ÷ (π/180) is K ≈ 0.447, so a chart here with K = 0.447 agrees with it to within 0.15%.
How this tool is checked
Hand-worked values; every cell held to this site's single-bend tool for the same radius and angle; and a second implementation in Python that reads the downloaded CSV back and recomputes every cell from the definitions, to the CSV's own rounding, on every release.
Questions
Bend deduction or bend allowance — which do I need?
Deduction if your drawing dimensions the flanges to the outside sharp corners (the usual case): add the outside flanges and take one deduction off per bend. Allowance if you work from the straight legs between bends: add the legs and one allowance per bend. Outside setback is the distance from the tangent point to the outside corner, the link between the two.
Can I type fractions?
Yes: radii like 1/32, 1/16, 3/32 are read as fractions of an inch in inch mode. Separate values with commas or spaces. The row labels keep what you typed.
What is in the CSV?
One row per inside radius and, for every angle, all three values — allowance, outside setback and deduction — in the chart unit, with a first comment line naming the thickness and K. It opens in any spreadsheet.
Where does the K-factor come from?
From a test bend, ideally: the K-factor tool on this site works it out from a strip you bent and measured, and its result links straight here with the thickness, K and radius filled in. Generic tables put K between about 0.33 and 0.5.
Missing an option?
A different datum, an offset, a shop allowance, another shape altogether — say what this tool does not cover yet. Requests decide what gets built next, and a report of a wrong result is dealt with before anything new.
Rather have the whole part drawn — a shop drawing or a cut file from your sketch? That is paid drafting, quoted per job: drafting for small shops.
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