Garden Mulch Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The amount of mulch needed is calculated with surface × thickness. For 20 m² at 7 cm thick, 1,400 liters of mulch are needed, or 28 bags of 50 liters.
Explanation
Mulching means covering the soil around plants with a layer of organic matter (straw, bark, wood chips, dried grass clippings) or mineral material, to limit weed growth, retain soil moisture, and regulate its temperature. The thickness of mulch to apply varies by use: a vegetable patch generally does well with 5 to 7 cm, enough to smother weed germination without excess, while a flower bed or shrub border often benefits from a more generous layer, around 7 to 10 cm, for better moisture retention on longer-lived plantings. This thickness should be doubled in regions with harsh winters, where mulch also plays a role in protecting against frost. The volume needed is simply calculated by multiplying the surface to cover by the desired thickness, converted to liters to match the usual packaging of mulch bags sold at garden centers; this calculator also directly gives the number of bags to buy based on the chosen bag size. Remember to leave a gap of about 5 cm around plant stems when spreading mulch, without directly covering the crown, to avoid any risk of rot from prolonged contact with the mulch's moisture — a principle to combine with the right fertilizer dosage, and in a raised bed or large pot, with a good depth of potting soil underneath before the mulch layer goes on top — the plant spacing in a vegetable garden matters too, since mulch has to fit around each plant without smothering it.
Example: 20 m² of vegetable patch at 7 cm thick
Inputs
Surface to mulch: 20 m². Thickness: 7 cm. Volume per bag: 50 L.
Calculation
Volume = 20 × (7 ÷ 100) × 1000 = 20 × 0.07 × 1000 = 1,400 L. Number of bags = ⌈1,400 ÷ 50⌉ = 28.
Result
1,400 liters of mulch are needed, or 28 bags of 50 liters.
Frequently asked questions
What mulch thickness should I choose for my vegetable patch?
A thickness of 5 to 7 cm is generally enough for a vegetable patch, sufficient to smother weed germination and retain soil moisture without excess. A layer that's too thin quickly loses its effectiveness against weeds, while an excessive layer can trap stagnant moisture that's unfavorable to some crops.
Should mulching be different in winter?
Yes, in regions with harsh winters, it's common to double the usual mulch thickness to protect perennial roots from frost, a practice especially useful for recent plantings or cold-sensitive species.
Should mulch directly touch plant stems?
No, it's recommended to leave a gap of about 5 cm around the crown (base) of each plant, without directly covering the stems. Prolonged contact between damp mulch and stems encourages rot or fungal disease.