How the Bark Mulch Calculator Works

The calculation is length x width x depth. Measure your bed or border in metres, choose a depth in millimetres and calculator converts result into litres, because unlike aggregates bark is sold by volume in the UK rather than by weight.

The numbers behind it are simple, at 50mm depth you need 50 litres per square metre. A standard 1,000 litre bulk bag therefore covers about 20 square metres at 50mm or roughly 13 square metres at 75mm. For irregular beds split the shape into rectangles, calculate each and add them together.

How Deep Should Bark Mulch Be

Depth decides whether mulch actually works:

• 25mm: an annual top up to refresh existing mulch
• 50mm: basic moisture retention for densely planted established beds
• 75mm: the standard depth for proper weed suppression
• 75 to 100mm: paths and larger decorative areas

Under 50mm the layer lets light through and annual weeds germinate anyway. Over 100mm mulch can starve the soil of air and water, so deeper is not better. Play areas are the exception, where certified play bark needs a much deeper layer to meet impact absorption standards.

Bark Mulch vs Bark Chippings

They look similar on shelf but do different jobs. Bark mulch is finer shredded material around 5 to 20mm that breaks down over a year or two, feeding the soil as it goes. That makes it right choice for beds and borders. Bark chippings are chunkier 20 to 60mm pieces that last 2 to 4 years, which suits paths, play areas and decorative ground cover where longevity beats soil improvement.

One caution from experienced gardeners, fresh wood chip straight from a tree surgeon temporarily locks up nitrogen at the soil surface as it starts to decompose. Let it age for a few months before using it on planted beds or stick to composted bark which has already been through that stage.

Mistakes That Waste Your Mulch

Volcano mulching: Piling mulch against trunks and stems traps moisture against the bark and causes rot. Leave a 5cm gap around the base of every plant so mulch forms a ring rather than a cone.

Skipping the weeding: Mulch stops new weeds germinating but does nothing against established perennials. Dig out bindweed, couch grass and dandelions first or they will push straight through.

Mulching bone dry soil: A thick layer works both ways and can stop light rain reaching dry soil. Apply to moist soil, ideally in late spring after the ground has warmed or in autumn before the frosts.

Ignoring settlement: Bark settles by around 10 percent after first few waterings and finer mulch decomposes noticeably in year one. Order a little over and plan a 25mm top up each season to keep the layer effective.

Worked Example

Two borders each 4m x 1.2m plus a 2m x 2m bed around a tree, mulched at 75mm:

• Area: (2 x 4 x 1.2) + (2 x 2) = 13.6 square metres
• Volume: 13.6 x 0.075 = 1.02 cubic metres, which is 1,020 litres
• In 70 litre bags: 15 bags
• In bulk bags: just over one 1,000 litre bag, so order one and a couple of small bags

At that quantity the bulk bag wins easily on price. Bagged bark typically costs 30 to 50 percent more per litre than bulk.