Pro tip: Lay a favorite tee flat. Measure against this guide.
A — Length
Place the tape at the highest point of shoulder (next to the collar). Measure straight down to the hem.
B — Width
Measure straight across the chest, seam to seam, about 1” below the armhole.
C — Sleeve
From the center back of the collar, run the tape along the top of the shoulder to the sleeve hem.
Notes
Minor variance is normal. Wash and wear can soften and relax measurements slightly.