Mulch Calculator (Cubic Yards & Bags)

The formula

• Volume in cubic feet = length × width × (depth in inches ÷ 12)
• Cubic yards = cubic feet ÷ 27
• 2 cu ft bags = cubic yards × 27 ÷ 2, rounded up
• Bulk cost = cubic yards × price per cubic yard
• One cubic yard covers about 100 sq ft at 3 in deep

A worked example

A 20 ft by 10 ft bed at 3 in deep holds 50 cubic feet, 1.85 cubic yards. That’s 25 of the standard 2 cu ft bags, or one small bulk delivery. At $45 a cubic yard, the bulk mulch costs $83.33.

More about the Mulch Calculator

How deep should it actually be

Three inches is the sweet spot for mulch: enough to suppress weeds and hold moisture, not so much that it suffocates roots. Thinner than two inches and weeds push straight through; deeper than four and you risk starving the soil of air.

For top ups on existing beds, two inches is plenty. Measure what’s already there first, most established beds need only a refresh, not a full three inches, and the calculator will happily price either.

Bags vs bulk: the real math

One cubic yard equals 27 cubic feet, that’s 13.5 of the standard 2 cu ft bags. A 20 × 10 ft bed at 3 in deep holds 50 cubic feet, or 1.85 cubic yards: 25 bags, or one small bulk delivery. At $45 a yard, bulk costs about $83.

The rule of thumb: bulk wins for beds over roughly 100 sq ft if you can get a delivery in, a yard of bulk often costs what 6 to 8 bags cost. Below that, bags are cheaper once you count delivery fees, and far easier to handle.

Measuring beds that are not rectangles

For roughly rectangular beds, measure the longest length and widest width, slight overestimation beats running short. For circles, measure the diameter and treat it as a square of that size; the extra the calculation adds is your safety margin.

For winding borders, break them into sections, calculate each, and add them up. And always round the bag count up: half a bag left over becomes next spring’s top up, but half a bag short is another trip.

Keep it off the stems

The most common mulching mistake is piling it against trunks and stems, ‘volcano mulching’. Bark held against a trunk stays damp, rots the bark, and invites disease and insects. Pull mulch back 3 to 4 inches from every trunk and stem, every time.

The same goes for perennials: mulch tucked against crowns causes rot in wet weather. Think donut, not volcano, a ring of mulch with a clear gap at the centre. It looks tidier too.

When to refresh and what to buy

Top up once a year, ideally in spring after the soil warms. A 1 to 2 inch refresh holds the 3 inch target without redoing the bed. If the old layer has matted into a waterproof sheet, rake it loose first so water can get through.

For weed suppression, shredded bark beats nuggets, it knits together and stays put on slopes. For vegetable beds, compost or leaf mulch feeds the soil as it breaks down. Match the material to the job, not just the price tag.

Mulch and water bills

A proper 3 inch mulch layer cuts watering needs dramatically, less evaporation, cooler soil, fewer weeds stealing moisture. In dry regions, the mulch often pays for itself in one summer of reduced watering.

Water before mulching, not after: lay mulch onto moist soil so it traps the moisture in. Mulching bone dry soil just preserves the drought underneath. A deep watering the evening before mulching day is the pro move.

When NOT to mulch

Don’t pile mulch against tree trunks or plant stems, the infamous ‘mulch volcano’ traps moisture against bark and invites rot and pests. Keep a 3 to 4 inch clear ring around trunks; mulch the root zone, not the bark.

Skip fresh wood chips in vegetable beds unless you add nitrogen: decomposing wood temporarily locks up soil nitrogen, yellowing your plants. Aged chips or straw are safer for annuals.

And don’t mulch before the soil warms in spring if you grow heat lovers (tomatoes, peppers), mulch insulates, which delays warming. Wait until soil temperatures are reliably up, then mulch to lock the warmth in.

An L shaped bed measured in two rectangles

Irregular beds defeat guesswork, but they surrender to one trick: split the shape into rectangles and add the pieces. Suppose an L shaped bed consists of a main run 12 feet by 4 feet and a return leg 4 feet by 4 feet. The areas are 48 and 16 square feet, giving 64 square feet in total. At a 3 inch depth, which is 0.25 feet, the volume is 64 multiplied by 0.25, exactly 16 cubic feet.

Convert that volume both ways before you decide how to buy. Sixteen cubic feet divided by 27 is about 0.59 cubic yards, which many bulk suppliers will round to a half or three quarter yard depending on their minimums. Divided by the 2 cubic feet in a standard bag, it is exactly 8 bags, with no rounding drama at all. For a bed this size, bags often win on simplicity even when bulk wins slightly on price.

Inches, feet and cubic yards without conversion slips

Almost every mulch miscalculation is a unit slip, not a measuring slip. Depth arrives in inches but area lives in square feet, so the depth must be divided by 12 before the two can meet. Three inches is 0.25 feet, not 0.3 feet, and that small rounding on a big bed quietly buys you a fifth more material than you need. Do the division honestly, keep one decimal place through the working, and round only at the very end.

The second slip is treating a cubic yard as if it were 9 or 12 cubic feet because the word yard suggests threes. A cubic yard is 3 feet by 3 feet by 3 feet, which is 27 cubic feet, and every bag count depends on that 27. If a result ever tells you a modest bed needs dozens of yards, a unit has gone wrong somewhere, and reworking the conversions is faster than arguing with the delivery driver.

Rounding the order is different from rounding the bed

Measure precisely, then round only the purchase, and round it in the direction your buying method forces. Bag buyers round up to the next whole bag, because half a bag does not exist at the till, but the leftover half bag in the shed is genuinely useful for topping up thin spots later in the season. Bulk buyers face a coarser choice, since suppliers typically sell in half or quarter yard steps, so the rounding decision moves real money.

Compaction and settling justify a small deliberate allowance, not a shrug. Fresh fluffy mulch settles into itself over the first weeks, and beds measured at a true 3 inches can look noticeably thinner by midsummer without a single handful having left. If you want the settled depth to stay near your target, say so in the calculation, add your allowance as a visible line, and you will never confuse a planned extra with a measuring error again.

How to use the Mulch Calculator

1. Measure the bed length and width in feet at the widest points.
2. Enter the mulch depth in inches, 3 in is standard for weed suppression, 2 in for top ups.
3. Add your local price per cubic yard if you want a cost estimate.
4. Press Calculate: use the bag count for small beds and the cubic yard figure when ordering bulk.