1. Measure the existing layer
Check several representative spots instead of measuring only the deepest edge. Move aside loose leaves, insert a ruler to the soil surface, and note the typical remaining depth. If different beds have noticeably different depths, calculate them as separate projects.
2. Choose a target depth for the material and site
There is no single correct depth for every mulch, soil, and plant. Iowa State University Extension notes that the appropriate layer depends on mulch type, soil texture, and plant material, and that overly deep mulch can create problems. Keep mulch away from direct contact with trunks and plant stems.
Use local horticultural guidance for your site before choosing the target. The calculator's suggested range is a starting point, not a prescription.
3. Subtract current depth from target depth
Top-up example
A 200 ft² bed has about 1 in of mulch remaining and your chosen target is 3 in. The added depth is 2 in.
200 ft² × 2 in ÷ 12 = 33.33 ft³, before any allowance. That is about 1.23 yd³, or 17 whole 2 ft³ bags.
If the current layer already meets or exceeds the target, the added depth is zero. Inspect the material and site rather than adding mulch automatically.
4. Separate beds when their needs differ
The current calculator applies one top-up depth to all areas in a calculation. If one bed needs 1 in and another needs 2 in, run them separately so each shopping estimate uses the right added depth.
Open the mulch calculator, choose “Topping up existing mulch,” and enter the target and current depth. Review the calculation method to check each conversion.
Source
Iowa State University Extension: How deep should the mulch layer be around trees and shrubs?