Markup percentage answers: “How much did I stack on top of my cost when I set the selling price?” The denominator is almost always cost, not revenue. That single choice distinguishes markup from profit margin, where the denominator is revenue.
If you import goods for $40 and sell them for $65, your gross profit dollars are $25. Markup on cost is 25 ÷ 40 = 62.5%, while gross margin on revenue is 25 ÷ 65 ≈ 38.5%. Same story, different camera angle—mixing them inflates pitches to investors.
Seasonal categories (snow gear, holiday decor) often need higher markup earlier in the season to absorb end-of-season clearance risk—percents help you simulate those arcs quickly.
Freelancers who quote fixed project fees should still compute implied markup on internal hours—otherwise you cannot tell which clients subsidise others.