Decide whether this is a new layer or a top-up
A target depth describes the finished total layer. Existing usable mulch already occupies part of it. In the mulch calculator, entering 75 mm target and 25 mm existing leaves a 50 mm top-up. Entering 75 mm as an additional layer would answer a different question.
Take several measurements if the existing layer varies. Divide the area into zones where one depth assumption is useful, rather than treating a single convenient spot as representative of the entire bed.
Draw rings and exclusions deliberately
For a ring, the inner diameter should describe the complete area you intend to leave unmulched. It is not automatically the diameter of the trunk. The calculator uses the annulus between the two diameters; it does not select a suitable clear space for a plant.
A rectangular bed can contain positioned exclusions. The tool checks supported exclusions within that bed, but different zone rows have no shared site coordinates. Avoid adding the same patch twice.
Compare quantities before comparing prices
For 12 m² at 75 mm new depth, geometric volume is 0.9 m³. A chosen 5% allowance makes 0.945 m³. With 50 L bags, nineteen supply 0.95 m³; with bulk increments of 0.25 m³, the order is 1 m³. These are different purchased amounts for the same target.
That distinction affects a fair price comparison. A bag price applies to full bags and a volume price applies to the rounded bulk purchase. Compare both in the same currency; the selector does not convert exchange rates.
Include the charge that changes the decision
A cheaper bulk material rate can be outweighed by delivery. Enter a known free charge as zero and leave an unknown charge blank. The latter must not be read as a free delivery promise.
For soil beneath a mulch layer, use the topsoil calculator. Save separate estimates when products or delivery arrangements differ.