Hem & Jog Allowance
Flat length for a hem (open or closed, a 180° bend) or a jog / offset (two opposite bends) from thickness, K-factor, radius and legs — with geometry checks.
Bend allowance, setback and deduction for the hem or for each jog bend, the sloping web length, the derived jog angle when you give the run, and the flat length — with a note when the hem return or the jog cannot form.
Example: An open hem, r 0.5 in 1 mm at K 0.44 on a 30 flange with an 8 return: BA 2.953, BD 0.047, flat 37.95. A 5 mm jog at 45°, R1, legs 30 / 30: web 7.071, flat 66.02.
Two shapes,
the same bend arithmetic.
A hem as a 180° bend, a jog as two opposite bends, and what the checks mean.
Hem
A hem is a 180° bend, so its allowance is BA = π(r + K·T) for an open hem with inside radius r, and π·K·T for a closed hem where r is zero. The tangent length of a 180° bend is r + T on each side, so BD = 2(r + T) − BA and the flat is flange + return − BD, with flange and return measured to the outside of the hem. A return shorter than 2(r + T) has no straight part left and cannot form.
Jog
A jog (offset, joggle) is two opposite bends of the same angle θ with a sloping web between them. The web between the mould lines is H / sin θ for an offset H; each bend has BA = θ(R + K·T) and the usual setback, and the flat is L1 + L2 + web − 2·BD when the legs are measured to the mould lines. Give the angle, or the horizontal run and the page takes θ = atan(H / run). When the web is shorter than the two setbacks the bends overlap — the offset needs a lower angle or a joggle tool.
What K is
The K-factor is yours; shops often use a smaller effective K for closed hems because the metal is crushed rather than bent, and the page says so rather than picking a number. No material or tooling data is embedded. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Hem as a 180° bend: BA = π(r + K·T), tangent length r + T; jog as two opposite bends of angle θ, web H / sin θ between mould lines, BA = θ(R + K·T) per bend — K-factor arithmetic
Last reviewed 20 September 2026. How results are checked: How we verify.