Cubic yards of mulch or soil.
Cubic yards = (sq ft × depth in inches) ÷ 324.
3 inches is typical for weed suppression and moisture retention; 4+ for high-traffic beds.
Too much mulch piled against a plant stem or tree trunk can actually trap moisture against the bark and promote rot, so depth recommendations aren't just about coverage — they're also about not overdoing it.
200 sq ft × 3 in = 600 ÷ 324 = 1.85 cu yd.
Bulk (yards) is usually 30–50% cheaper per volume, delivered. Bags are easier for small jobs.
Top up 1–2 in each spring as mulch decomposes.
It comes from converting inches of depth over a square-foot area into cubic yards — 1 cubic yard covers 324 square feet at exactly 1 inch of depth, so the formula scales from that baseline.