What one size up actually changes
A size step moves several measurements at once, not just the one you were unhappy with. That is why sizing up to fix a single dimension so often produces a shirt that is wrong in a new way.
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Nothing is printed on either shirt here — two tees, one size apart, with tapes laid across chest, length and shoulder to measure exactly what the step between them adds.
People reach for the next size up to fix one thing. A size step changes more than one thing.
What actually changes when you go up a size?
Every measurement on the chart, at the same time — chest, length, shoulder and sleeve all move together, each by its own amount.
That's the part worth internalising, because it's what makes sizing up a blunt instrument. You don't get to move one dimension. You move all of them, in a ratio the pattern decided, and the ratio is rarely the one you had in mind.
Is sizing up a good way to get a longer shirt?
Usually not, and it's the most common way to end up with the wrong garment.
On a t-shirt grade, width moves considerably faster than length. So a size up in pursuit of a little more length hands you a noticeably wider shirt as the price of it — and width is the dimension people actually notice on a finished garment.
If length is the thing that matters, the size chart's length column is the number to read directly, rather than hoping a size step delivers enough of it.
Then how should I decide?
Read the chart for the two dimensions you care about, rather than reasoning from the letter.
Every size chart lists its own measurements, and the step between adjacent sizes is visible right there. Comparing the two rows you're weighing tells you exactly what you're trading — which is a different question from choosing between two sizes you're genuinely between.
Why do chest numbers on different charts disagree so much?
Because some charts give the full circumference and others give the flat width, and they differ by a factor of two.
A garment laid flat and measured across is half of what goes around the body. A chart quoting the larger figure and one quoting the smaller can describe the same shirt. Before comparing any chart to a shirt you already own, check which of the two it's giving you — this single ambiguity causes more sizing mistakes than any other.
How do I get one made?
Describe the design at JustOG — a garment, a size, and a second size to compare it against. Pick a direction, drag the crop frame, see it composited on the real garment, and it's made to order and shipped.
Designs other people have published are in the shop.
One size up isn't one change. It's every change, in a ratio you didn't pick.