Mulch volume, bulk units, and whole-bag costs
Layer volume comes from the area after excluded ground is removed and the new depth. An ordering allowance increases the purchase volume without changing that depth. Whole bags add their own surplus. Bulk cost uses the selected per-cubic-meter or per-cubic-yard quote for the order volume.
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The formula
Depth must first be converted to metres. Multiplying square metres by metres gives cubic metres, and each cubic metre contains 1,000 litres. Bulk cost uses the unrounded volume, while bagged cost uses complete bags. Cubic feet and cubic yards are equivalent volumes derived from the exact international foot and yard definitions. Circle area uses numerical pi. Inch depth converts with 2.54 cm per inch; cubic-foot bags contain 28.316846592 L per ft³. Bulk quotes in yd³ use 0.764554857984 m³ per yd³. Bag and bulk costs both include the optional order allowance, while layer depth does not.
Worked example
For 20 m² receiving 5 cm of new mulch, volume is 1 m³ or about 1.308 yd³ (35.315 ft³). Twenty 50 L bags cover 1,000 L without surplus. At 6 per bag and 95 per cubic metre, bagged material costs 120 and bulk material costs 95 before delivery.
How to use this calculator
- Choose known area, a rectangle or a circle. Enter known area in m² or ft², or dimensions in meters. Optionally exclude ground in m².
- Enter the new layer depth in cm or inches, bag volume in liters or cubic feet and the corresponding bag price.
- Select the bulk quote unit and price. Optionally add an ordering allowance, then compare layer volume, order volume, whole bags and material costs.
Measure the layer you intend to add
For an irregular border, divide the shape into smaller rectangles and add their areas. Exclude paved sections and areas deliberately left bare around plants. If an existing layer is being topped up, enter only the missing depth; treating the full desired depth as new material can overstate the order.
Compare delivered quotes
The cheaper material subtotal may not be the cheaper purchase. A bulk supplier can charge delivery, set a minimum quantity, or sell in fixed increments. Add those charges when comparing quotes. Bag sizes describe volume, so bag weight is not a substitute unless the supplier provides a density. Loose material can settle after spreading.
Compare the same order in bag and bulk units
A 5 m by 4 m bed with 2 m² excluded has 18 m² receiving mulch. At 5 cm depth, the layer needs 0.9 m³. A 10% ordering allowance raises the order to 0.99 m³: twenty 50 L bags buy 1 m³ and leave 10 L above that order.
A cubic-yard price is multiplied by the order volume expressed in cubic yards; it is not treated as a cubic-meter price. Prices must use the same currency, and neither delivery charges nor a supplier’s minimum order is included. Enter new material depth rather than the full desired depth when topping up an existing layer.
Assumptions & limitations
What this calculation assumes
- One uniform additional layer covers the entire entered area.
- Bag volumes are usable volumes as sold.
What to keep in mind
- No compaction, delivery charge, or minimum bulk order is included.
Common questions
Should I enter the existing mulch depth?
No. Enter the thickness of the new material you plan to add after allowing for what remains.
Why does the bag count round up?
A partial final bag is still a whole purchase. The extra-volume result separates that purchase allowance from the amount needed on the ground.
Can I use cubic-foot bags and a cubic-yard bulk quote?
Yes. Select each label unit independently. Bag volume and bulk price convert using the international foot and yard factors before the purchase comparison.
Sources & further reading
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