Fillet weld: throat a and leg z
The drawing says a5, the gauge in your hand measures z. This converts between the two, and shows what the difference costs in filler metal.
The conversion
For a fillet with equal legs, a = z / √2 ≈ 0.7·z and
z = a · √2 ≈ 1.41·a. So a5 and z7 are the same weld, and the two
notations get confused constantly — usually in the expensive direction, because
somebody reads a5 as a leg of 5 mm and welds a joint that is a third too
small.
The cost runs with the square. Going from a4 to a5 is a 56 % increase in cross-section, filler and arc time. That is why over-welding is worth catching: it is not free, and it is not stronger than the drawing asked for.
Sensible limits
Below about 3 mm a fillet does not fuse reliably and cracks as it cools. Above about 0.7 of the thinner part it is pointless — the parent metal will fail first. Between those bounds the size comes from the design, and the calculator's load figure is a sanity check on the order of magnitude, not a substitute for the structural calculation.
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