How much mulch do you need? A calculator, the right depth and when to mulch

To work out how much mulch you need, multiply the bed’s area in square feet by the depth in inches and divide by 324. That gives cubic yards. For most Ontario beds, aim for 2–3 inches deep, and expect one yard to cover about 108 sq ft at 3 inches.

Most people buy mulch by eye instead: a few bags, then a few more, then one last trip to the store. The math takes two minutes with a tape measure. The harder part is the depth and the timing. Too thin and weeds come straight through. Too thick, or piled against stems, and it does more harm than good.

Mulch installation is one of the jobs we do around Markham as part of our garden bed, planting and mulching work. It’s also an easy one to do yourself once you have the numbers below.

How deep should mulch be?

Two to three inches (5–7.5 cm) suits most garden beds, and four inches (10 cm) is the upper limit. University of Tennessee Extension says a 2–4 inch layer cuts down weed growth and holds moisture in the soil, and that the layer should be no more than four inches deep.

The University of Wisconsin–Madison Extension says to apply most organic mulches about 3 inches deep.

What you’re mulching changes the number a little:

Penn State Extension says to use less on poorly drained soil, and Iowa State Extension suggests 2–3 inches on heavy soils. If the bed already has mulch, you’re only topping it back up to 2–3 inches, often an inch or less.

How much mulch do I need?

Multiply the bed’s area in square feet by the depth in inches, then divide by 324. The answer is cubic yards.

That’s the formula Texas AgriLife Extension gives. It works because a cubic yard is 27 cubic feet, and 27 × 12 = 324.

To get the area, measure the length and width of each bed in feet and multiply. For curved or odd-shaped beds, split them into rough rectangles and add them up. A few square feet either way won’t matter.

For example, an L-shaped bed made of a 20 by 4 ft strip and an 8 by 4 ft return is 80 + 32 = 112 sq ft. At 3 inches deep, that’s 112 × 3 ÷ 324, or about 1.04 cubic yards.

Here’s how far one cubic yard goes at common depths:

The 108 sq ft figure at 3 inches matches what University of Tennessee Extension publishes. One yard holds about 13.5 bags of 2 cu ft, so for bagged mulch, check the size printed on the bag and divide.

Three worked examples

1. A front bed along the house, 20 ft by 4 ft, mulched 3 inches deep: 80 × 3 ÷ 324 = 0.74 cubic yards. That’s 20 cu ft, or 10 bags of 2 cu ft.
2. Beds around a backyard totalling 300 sq ft, new and bare, 3 inches deep: 300 × 3 ÷ 324 = 2.8 cubic yards. Order 3 yards in bulk rather than 38 bags.
3. The same 300 sq ft the next spring, topped up by 1 inch: 300 × 1 ÷ 324 = 0.93 cubic yards, roughly 13 bags.

When is the best time to mulch in Ontario?

Mulch in spring once the soil has warmed and the extra water has drained away, or in fall once the soil has started to cool. The Morton Arboretum gives that timing for trees and shrubs. In fall, it says to wait until frosts are coming regularly, since a single frost doesn’t mean the soil has cooled.

The Wisconsin extension warns that organic mulch put down too early in spring keeps the soil cold longer and slows plant growth.

Around Markham and the rest of the GTA, that means not rushing out the week the snow goes. Beds are often still cold and soggy then. In spring, wait until:

• the soil is workable and crumbles in your hand instead of smearing
• the perennials have started to come up, so you can see where they are and don’t bury them

If you want free mulch and can haul it yourself, the City of Markham runs a self-serve mulch giveaway around Earth Month. In 2026 it was on May 2 and 3 at Milliken Mills Park, and residents brought their own bags.

Check the city’s website in spring for the next dates. Our Markham area page has the city’s yard waste pickup dates too.

Fall works well too, and it’s a good time to top up after a fall cleanup. For a winter layer over perennials, the Wisconsin extension suggests waiting until after several freezes.

Common mulching mistakes

• Mulch piled against tree trunks and shrub stems, keeping the bark wet all season.
• A fresh 3 inches spread over old mulch every spring, until the bed is 6 or 8 inches deep and the plants struggle.
• Mulch spread over weeds that weren’t pulled first. Cornell’s weed management guide says to clear weeds before mulching, because weeds can still push up through the layer.
• Mulch heaped against the house so it touches wood siding or door and window frames. The University of Kentucky’s termite guide notes that mulch holds moisture and says it should never touch them.
• Fine, shredded mulch spread thick and left to pack down. The Morton Arboretum notes that fine-textured mulches tend to pack down and hold moisture, and Penn State Extension suggests breaking up matted layers by hand or with a rake.
• Wood chips dug into the soil when planting. Oregon State University Extension says mixing chips into soil makes them more likely to tie up nitrogen. Left on top as mulch, they break down slowly and get worked in by worms over time.

Is mulch installation worth paying for?

For a few small beds, usually not. It’s an easy Saturday morning job, and buying bags means no pile on the driveway.

Where it gets heavy is volume. Three yards of bulk mulch is a lot of wheelbarrow trips, and it goes much faster if the beds are weeded and edged first.

It’s usually worth getting help when:

• the beds are overgrown and need weeding and a new edge before any mulch goes down
• there’s old mulch to pull back from trees and the house
• you’re redoing several beds at once

Our guide on low-maintenance garden beds covers how to set beds up so they need less work each year. The spring cleanup checklist shows where mulching fits in the season.

If you’d like us to take a look, call or text (437) 684-2535, or ask for a free quote on our garden beds, planting and mulching page.