The ounce scale is one division repeated
An ounce of leather is a sixty-fourth of an inch of thickness, so the scale is arithmetic and not a code. Eight ounce leather is 8/64, which is one eighth of an inch, which is 3.18 mm. Five ounce is 5/64, 0.078 in, 1.98 mm. Going the other way, a gauge reading of 2 mm is 0.0787 in, and multiplying by 64 gives 5.04 — five ounce leather, near enough that nobody would argue. The conversion never needs a table.
What does need care is that a piece of leather is not one thickness. A single side can run two ounces thicker over the backbone than at the belly, which is why the field asks for a gauge reading on the part you will cut rather than for the number printed on the shelf label.
A lap is the sum of everything in it
Glue eight ounce onto five ounce with a four thousandth glue line and the lap stands at 0.207 inches — thirteen ounce, two-thirds thicker than the piece it is joining. Skive it down to match the eight ounce piece and you have to take 0.082 inches out, and if you split that evenly each side loses 0.041 inches, which is over half the thickness of the five ounce piece. That is why an even split is often the wrong choice: it asks more of the thinner piece than the thinner piece has to give.
Push the whole removal onto the eight ounce side instead and it loses 0.082 of its 0.125 and keeps 0.043 under the lap. Neither answer is right in general. The page prints what each split costs each piece so the choice is made with numbers rather than by feel.
The ramp is shallower than it feels
Taking 0.041 inches out over a one inch lap is a ramp of 2.35 degrees, or one in twenty-four. That is a slope you cannot see. It is why skiving is done with the blade held almost flat and checked against a raking light, and why a skive that feels aggressive under the knife usually is not. Halve the lap to half an inch and the same removal becomes 4.7 degrees, which starts to read as a step through the grain side once the joint is glued.
Two full-length tapers do not give a constant joint
There is a tidy-sounding rule that if both pieces taper to nothing across the lap the joint stays even. It is only true when both pieces start at the same thickness. Taper eight ounce to zero and five ounce to zero across the same lap and the sum runs from 0.125 at one end to 0.078 at the other, sliding the whole way — a wedge rather than a step, which is usually an improvement, but not the flat joint the rule promises. The page works from a target thickness for that reason instead of assuming the tapers cancel.
Questions people ask
How many millimetres is 8 oz leather?
An ounce is 1/64 inch, so eight ounce is 8/64 = 0.125 in, and 0.125 times 25.4 is 3.175 mm. The same arithmetic backwards: a gauge reading of 3 mm is 0.1181 in, times 64 is 7.56 ounces, so somebody would call it 7 to 8 ounce. Note that leather is sold in ranges — a hide labelled 8 to 9 ounce will measure differently at the shoulder than at the belly.
How much should I skive off a lap joint?
The page does not answer that and neither should any page. It works the other way round: you decide what the joint should measure, and it tells you what has to come off each side to get there and what each side has left afterwards. Matching the thicker of the two pieces is a common starting point because the joint then disappears into the line of the thicker piece.
Why does the split between the two pieces matter?
Because the removal is a fixed amount but the two pieces have different amounts to give. Joining 8 oz to 5 oz and splitting evenly takes 0.041 in off each, which is more than half the thickness of the 5 oz piece and barely a third of the 8 oz. Shifting the split moves the loss onto whichever piece can spare it, and the page prints the percentage each side loses so the trade is visible.
What is the skive angle for a one inch lap?
It depends entirely on how much you are removing. Taking 0.041 in out over a one inch lap is 2.35 degrees, one in twenty-four. Taking the same amount over half an inch is 4.7 degrees. The angle is the arctangent of the depth over the lap length, and the reason it is worth printing is that both of those feel much steeper under a knife than the numbers suggest.
Does a skived joint hold as well as an unskived one?
That is a strength question and this page does not answer strength questions. What it will tell you is the geometry: how thick the joint ends up, how much leather is left on each side of it, and how much material came out. Whether that is enough for what the piece has to do depends on the leather, the adhesive, whether it is stitched, and the load, and none of those are in the form.