Sleeve Taper Calculator
Upper arm and wrist measurements, your row count, and your gauge — it tells you exactly which rows to shape on, including whether your row count even has room for it.
Why this is harder than it looks
A sleeve taper isn't one calculation, it's three stacked on top of each other: converting two body measurements into stitch counts using your gauge, working out how many total shaping events that difference requires (at 1 stitch per edge per event, so 2 stitches per shaping row), and then fitting those events into your row count — which, if you're shaping on right-side rows only, gives you roughly half as many usable rows as you might assume. Missing any one of those three steps is exactly how a taper ends up uneven or simply doesn't fit the rows available, which is why this tends to trip people up doing it by hand.
When there isn't enough room
If the shaping needed doesn't fit into the available rows at the standard 1-stitch-per-edge rate (a real possibility with a short sleeve cap or a large arm-to-wrist difference), this calculator will flag it rather than silently producing an impossible plan. The usual fixes are working the shaping 2 stitches at each edge per event instead of 1 (fewer, more aggressive shaping rows), or adding a few rows to the taper if your length allows it — both are shown when relevant.
Getting the direction right
Bottom-up sweaters increase from wrist to upper arm as you work toward the body; top-down sweaters decrease from upper arm to wrist as you work toward the cuff. The stitch counts and row-by-row plan are mirror images of each other either way — what changes is simply whether row 1 in this plan is your cast-on/starting round at the wrist or at the underarm.