Mulch Calculator
Work out how many bags or cubic yards of mulch a bed needs.
Last reviewed September 19, 2026
Result
Enter length and width to see the result.
https://calcgallery.com/landscaping/mulch-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Measure the bed, or enter the area directly.
- Set the depth. Around 75 mm, or 3 in, suppresses weeds without smothering roots.
- Choose the bag size you can buy, or read the cubic yards if you are ordering in bulk.
- Read the bag count.
How it is calculated
Mulch is a volume, which is the area multiplied by the depth:
Volume = area × depth
Bags = ceil(volume ÷ bag volume)Bags are rounded up, because you cannot buy most of one. That is why the bag count sometimes jumps by one for a small change in area.
Depth is the number to get right, and more is not better. Below about 50 mm, weeds come straight through. Above about 100 mm, water struggles to reach the soil and roots sit in a damp layer that does not drain. Three inches is the figure most growers land on.
Practical tips
Weed properly first. Mulch suppresses seeds; it does not kill established perennial weeds. Bindweed and couch grass will come straight through 3 in of bark and you will be pulling them out of a bed you can no longer hoe.
Water the bed before mulching, not after. Dry soil under fresh mulch stays dry for a surprisingly long time. Mulch sheds light rain rather than letting it through.
Keep it off the stems. A few inches of clearance around every trunk and crown. This is the single most common mulching mistake and it is the one that kills plants.
Bulk is cheaper but needs somewhere to go. A cubic yard is about the size of a washing machine and it arrives as a pile. If the only tipping spot is the drive you need that afternoon free.
Buy once, top up annually. Organic mulch breaks down into the soil, which is the point. Expect to add about half the depth again each year rather than starting over.
With your numbers
- Volume = 100 ft² × 3 in = 25 ft³
- Bags = that volume ÷ 2 ft³ per bag, rounded up to 13
Quick reference
Mulch settles as it weathers, so a bed laid to exactly 3 in reads thinner within a season. Nothing extra is added by default.
| Job | Bags to buy |
|---|---|
| 100 ft² bed, 3 in deep | 13 |
| 200 ft² bed, 3 in deep | 25 |
| 500 ft² bed, 3 in deep | 63 |
| 200 ft² bed, 2 in deep | 17 |
| 20 × 4 ft border, 3 in deep | 10 |
| 10 m² bed, 75 mm deep | 15 |
| 25 m² bed, 75 mm deep | 38 |
| 8 × 1.5 m border, 75 mm deep | 18 |
Common questions
- How many bags of mulch do I need for 100 square feet?
- 13 bags of 2 ft³ at a 3 in depth. At 3 ft³ per bag it is 9. Bulk is usually cheaper above about 10 bags, if you have somewhere for it to be tipped.
- How deep should mulch be?
- About 75 mm, or 3 in. Thinner does not suppress weeds. Much thicker starves the soil of air and holds water against stems, which is how mulch kills the plants it was meant to protect.
- When is bulk cheaper than bags?
- Usually somewhere around 10 to 15 bags, though it depends on delivery charges. A cubic yard is about 13 bags of 2 ft³, so if you are near a yard, price both.
- Should mulch touch the plant stems?
- No. Pull it back a few inches from trunks and stems. Mulch piled against bark holds moisture there and invites rot and pests. The volcano of mulch around a tree trunk is a common and damaging habit.
- How often does mulch need topping up?
- Organic mulches break down and need topping up roughly once a year, usually about half the original depth. That is a feature: it is feeding the soil as it goes.
- Does this work for bark, chips and compost?
- Yes. The maths is volume, so any loose material at a given depth works the same way. Only the bag sizes and how fast it breaks down differ.