Mulch Calculator

Mulch is a layer of material applied to the surface of the soil. Many different types exist, both organic and synthetic, each with its own benefits and drawbacks. Mulch is used for aesthetic reasons as well as for what it does to the soil, including regulating soil temperature, helping the ground retain moisture and preventing erosion. The effects depend on the type of mulch, how much is used, and even when it is applied.

How much mulch do I need?

The amount needed depends on the type of mulch and what it is for. Different materials have different properties, so there is some variation in how much to use, and mulch spread purely for landscaping may be applied differently from mulch in a vegetable garden.

Generally, a layer of 2 to 4 inches, which is 5 to 10 cm, gives the soil real benefit without being too thick. Spread too thinly, the layer will not suppress weeds or protect the soil against heat and erosion. Spread too thickly, it can stop water reaching the soil at all and deprive plants of the moisture they need.

The arithmetic is simply area times depth. A bed of 200 square metres covered 2 cm deep needs 200 × 0.02 = 4 cubic metres, which is 4,000 litres, 5.23 cubic yards or 141.3 cubic feet. That single relationship is worth internalising, because it also explains why depth matters so much: doubling the depth doubles the order and the cost, while the area stays exactly the same.

Suppliers quote in different units depending on where you are and how you are buying. Loose bulk mulch is usually sold by the cubic yard in the United States and by the cubic metre or tonne elsewhere. Bagged mulch is sold by the bag, commonly 2 cubic feet in the US and 60 litres in Europe, which is why the price selector here covers both loose volumes and bag sizes. A cubic yard is about 13.5 of the 2 cubic foot bags, so the moment a job passes half a cubic yard, bulk delivery usually wins on price.

Order a little more than the calculation suggests. Beds are rarely perfect rectangles, mulch settles after the first heavy rain, and running out halfway across a bed leaves an obvious line.

The benefits of mulch

Mulch can do several things at once depending on when and how it is used. Often it is simply cosmetic, since it tidies the look of a garden, a walkway or a border. It can also protect and enrich the soil with nutrients, which matters for most gardening uses. If the point is to help seeding and growth, it is usually best applied at the beginning of the growing season, since it insulates the ground and helps the soil hold both heat and moisture.

Mulch regulates soil temperature and protects against weather

Many plants stay healthier when their roots sit at moderate temperatures. Mulch acts as an insulating layer, so it keeps soil from getting too hot or too cold and lets roots carry on delivering nutrients to the plant.

The same insulation reduces exposure to the sun. Too much sun dries the soil out, which reduces how much water can penetrate it and increases erosion from wind. Heavy rain will carry away uncovered topsoil, stripping out the organic matter and nutrients plants depend on. A mulch layer takes the impact of the raindrops instead.

Mulch can increase moisture retention

Because of that insulation, mulch also affects how much moisture the soil holds. It reduces evaporation from heat and wind, which improves retention and cuts both the frequency and the volume of watering needed.

It can also work the other way. Mulch can reduce moisture retention by absorbing water itself or by stopping light rain from reaching the soil at all. That happens when the layer is too thick or the material is too absorbent, which also reduces the oxygen reaching the soil below. Knowing which mulch suits the situation, and how much of it to use, is the whole point.

Mulch reduces weed growth and the chance of plant disease

Mulch significantly reduces the number of weeds in a garden. Where weed seeds are already in the soil, a layer of mulch stops many of them getting the sunlight they need to grow. It also prevents wind-blown seeds from reaching the soil and taking root in the first place.

Many plant diseases live in the soil. Some attack roots, while others need to reach the leaves. Mulch will not help against a disease that travels to the foliage, but where the pathogen has to contact the roots, or splashes up from the soil in the rain, mulch acts as a barrier. If plants are suffering or weeds are taking hold, adding or replenishing mulch may improve the situation.

Types of mulch

Mulches differ in availability, cost, appearance and how fast they break down. They also affect the soil differently, and some drive chemical reactions or shift the pH.

Using mulch is usually helpful but not always. Some mulches are more combustible than others and raise fire risk in the wrong conditions, and others may carry weed seeds, plant pathogens or even toxic substances depending on where and how they were sourced. It is worth knowing the trade-offs of each type.

Organic mulches

Organic mulches decompose over time. That is arguably their main advantage for gardening, since they feed nutrients into the soil as they break down and attract useful organisms such as worms. They also suppress weeds, insulate the ground, and improve how the planting looks.

The disadvantages are that they can attract termites and other pests, may contain weed seeds or contaminants, and because they decompose they need replacing more often than synthetic mulches.

Different organic mulches rot at different rates, absorb water differently, release different nutrients, shift pH in different directions and vary in how well they insulate and how easily they burn. A few common ones:

Leaves. Leaves from deciduous trees can be collected and shredded into a mulch that both protects the soil and feeds it. Depending where you live they are abundant and free, and they break down at a rate that leaves a soil-like consistency by autumn, when compost can be added straight over the top. The drawbacks are availability unless you collect and store them, the small pests they can harbour, and matting: if they are not shredded enough they form a mat that blocks air and water.

Wood mulch. This covers bark chips, wood chips, hardwood, pine and shavings. Like other organic mulches these decompose, feeding the soil and improving its structure while suppressing weeds. The variety is the point: different types suit different jobs depending on how quickly you want them to break down and how you want the bed to look. Wood mulch needs replenishing more often than synthetic alternatives, can carry contaminants particularly when dyed, and produces plastic waste when bought as bagged bark in bulk.

One caution specific to wood: fresh, un-composted wood chips dug into the soil can temporarily tie up nitrogen as microbes consume it while breaking the wood down. Left on the surface as mulch, which is how it is normally used, the effect is minor and confined to the top centimetre or two.

Grass. Grass clippings are usually to hand, easy to collect and dry, and relatively high in nitrates, which return to the soil as they decompose and increase the activity of worms and other beneficial microorganisms. They must be dried properly first: fresh clippings are too wet, and they mat down into a layer that grows mould, traps heat and damages plants. Avoid clippings from lawns recently treated with herbicide, which can carry over into a vegetable bed.

Synthetic mulches

Synthetic mulches include plastic sheeting, rubber, dyed products and more. Unlike organic mulches they are not biodegradable, so they are more convenient and durable and need replacing far less often. Because they do not break down, or do so extremely slowly, they are less sustainable and give the soil no nutrients or organic matter. In some cases they can leach harmful chemicals into the surrounding area. They still have their uses, offering heat and moisture retention much like organic mulches while being easy to find and maintain.

Rubber mulch. Made from recycled rubber, it insulates the soil from heat and improves moisture retention, since rubber does not absorb water and lets it pass straight through. It suppresses weeds because seeds dry out before they reach the soil. As a synthetic it adds no nutrients, can contaminate the soil in some situations, and is a genuine hazard if it ignites: it releases toxic fumes and is unusually hard to extinguish.

Plastic mulch. Plastic sheeting prevents compaction, increases heat and water retention and blocks weeds, and it lasts because it does not decompose. That permanence is also its main drawback, since it has to be replaced periodically at a cost with environmental consequences. It can get too hot for some plants in warm climates, and by preventing evaporation entirely it can leave soil waterlogged and drown roots. Worth thinking through before committing a bed to it.

Dyed mulch. A dyed mulch is an organic mulch coloured with synthetic additives for appearance, so it keeps the advantage of decomposing and feeding the soil while being available in various colours. When these first appeared they were suspected of containing toxic substances, heavy metals and other contaminants; studies since have argued for their safety. If you plan to use dyed mulch it is still worth finding out how it was coloured before spreading it, particularly around edible crops.

These are only a few examples. Many other mulches exist, both organic and synthetic, including straw, pine needles, cocoa hulls, gravel and landscape fabric.

Applying it well

Keep mulch away from the base of trunks and stems. Piling it against the bark, often called volcano mulching, traps moisture against the trunk and invites rot and pests. Leave a clear collar of a few centimetres around each plant.

Timing matters too. Applied in late spring once the soil has warmed, mulch holds that warmth. Applied too early over cold ground, it insulates the cold in and slows the bed down. Autumn mulching works differently again, protecting roots through winter frosts rather than warming anything.

Using this calculator

The first calculator takes a total area, which suits a bed you have already measured or a figure from a plan. The second takes lengths and widths, adding up to twelve separate rectangles, which suits a garden made of several beds measured individually. Both give the answer in litres, cubic metres, cubic yards and cubic feet at once, since suppliers quote in all four.

The optional price field handles loose material by volume as well as bagged, so entering a price per 2 cubic foot bag or per 60 litre bag gives the total for the job rather than a unit price you then have to multiply out.