How Much Soil is in a Bag? | Volume, Not Weight, Is the Key

A standard bag of soil is sold by volume, not weight, with the most common large bag being 1.5 cubic feet — enough to fill roughly 1.1 cubic feet of a raised bed after settling.

Standing in the garden center aisle staring at bags labeled “40 lb” and “2 cu ft” is confusing. You’re not alone: the single biggest mistake home gardeners make is assuming a heavy bag means a full bag. Soil density varies wildly between brands and types — a bag of moist topsoil weighs more than a bag of fluffy potting mix, even when they hold the same volume. The fix is simple: ignore the weight number and read the cubic-foot figure printed on the front of the bag. Once you know that number, calculating how many bags your project needs takes exactly two steps.

Standard Bag Sizes and Their Volume

The market has settled on a handful of standard bag sizes, and most retailers carry the same options. Here is the current range you’ll see in 2025 and 2026.

One cubic foot of dry soil equals 25.7 dry quarts or about 7.5 gallons. A 1.5 cubic foot bag holds roughly 38.5 dry quarts — enough to fill one standard 10-gallon grow bag completely, or two 5-gallon grow bags with a little left over.

What 1 Cubic Yard Looks Like in Bags

Cubic yards are the standard unit for planning, but bags are what you haul home. Here is how the math breaks down for one cubic yard (27 cubic feet):

• 1 cu ft bags: 27 bags per yard
• 1.5 cu ft bags: 18 bags per yard
• 2 cu ft bags: 14 bags per yard
• 0.75 cu ft (40 lb) bags: 36 bags per yard

If you’re filling a 4-foot-by-8-foot raised bed that is 12 inches deep, you need about 1.2 cubic yards of soil — roughly 22 standard 1.5-cubic-foot bags. For the best deals on bulk bags, check our roundup of the top-rated big bags of soil for large projects.

How To Calculate Exactly How Many Bags You Need

The math works for any shape — rectangular bed, round pot, or square planter. Grab a tape measure and a calculator.

1. Measure in feet. Length, width, and depth. If your depth is in inches, divide by 12 (12 inches = 1 foot; 6 inches = 0.5 feet).
2. Multiply L × W × D. That gives you total cubic feet needed.
3. Divide by the bag size. Total cubic feet ÷ bag’s cubic feet = number of bags. Example: 32 cubic feet needed ÷ 1.5 cubic feet per bag = 21.3 bags. Round up to 22.
4. Add 10–15%. Soil settles after the first watering, especially in containers. Add 10% to your bag count so you don’t have to top off in two weeks.

Worked example: a 4 × 8 bed at 12 inches deep. Depth = 12 in ÷ 12 = 1 ft. Volume = 4 × 8 × 1 = 32 cubic feet. 32 ÷ 27 = about 1.2 cubic yards. For 1.5 cu ft bags: 32 ÷ 1.5 = 21.3, round to 22 bags.

Do Bags Of Topsoil And Potting Mix Weigh The Same?

No — weight is untrustworthy. A “40 lb” bag of dense topsoil might fill only 0.75 cubic feet, while a “40 lb” bag of light potting mix could fill 1.5 cubic feet. The Eartheasy soil calculator warns that topsoil density varies significantly by brand, so the cubic-foot number printed on the bag is the only reliable figure.

Also, topsoil is meant for in-ground use — it’s too heavy and poorly aerated for containers. Use a potting or container mix for raised beds and grow bags, and topsoil only for filling in-ground beds.

Common Mistakes That Waste Money

• Ignoring settling. Soil compresses by 10–15% after watering. Buy extra or you’ll be back at the store next weekend.
• Mixing topsoil into containers. Topsoil lacks the perlite and organic matter that container plants need for drainage.
• Underestimating bulk. A standard 4×8 bed (10 inches deep) needs about 18 large 1.5-cubic-foot bags — that’s a carload, not a trunk load.
• Assuming bag sizes are universal. Costco often sells 1.5 cu ft bags; Home Depot carries 1.5 and 2 cu ft. Check the label, not the store aisle.

Grow Bag Soil Needs at a Glance

A 5-gallon grow bag needs about 0.668 cubic feet of soil — exactly one standard 0.75-cubic-foot bag with just under a quart left over. A 10-gallon grow bag uses roughly 1.34 cubic feet, which is just under one 1.5-cubic-foot bag.

Starter Checklist: Buying Soil Bags Without Errors

This is the quick do-and-don’t list to keep at hand on shopping day:

• Read the cubic-foot number on the bag, not the pound or quart number.
• Calculate your bed’s volume in cubic feet before you leave home. Most major hardware stores have a soil calculator in their app — Home Depot and Lowe’s both offer one.
• Add 10–15% for settling. Round up, not down.
• Buy potting mix or container mix for raised beds and pots, not topsoil.
• If the project is bigger than 1.5 cubic yards, call a bulk supplier. 27 bags per cubic yard is a lot of trips.