How Much Soil to Fill a 25 Gallon Grow Bag? | The Exact Scoop

A standard 25-gallon grow bag needs 3.34 cubic feet (100 dry quarts) of soil to fill completely, but most growers aiming for a productive season should buy about 3.8 cubic feet to account for settling and proper headspace.

Stand a 25-gallon fabric grow bag next to a tomato cage, and the first question is always the same: how much soil does this thing actually swallow? The math is straightforward, but the buying decision depends on whether you fill to the brim or leave room for watering. Here’s what the calculators and experienced gardeners agree on, with the exact bag counts so you don’t overbuy or come up short.

Exact Volume and What That Means at the Store

The bag itself holds 3.342025 cubic feet (100 quarts) if filled to the 16.5-inch top rim. That converts cleanly to 25 gallons. In the real world, soil settles after the first few waterings and plants need headspace for water to pool without running off. Most seasoned growers fill to 3.0–3.2 cubic feet and leave 3–4 inches of open fabric at the top.

Here is what that volume translates to when you’re standing in the garden center aisle:

If you are ready to pick up a bag right now, our tested roundup of the best 25-gallon grow bags on the market walks through fabric thickness, handle quality, and drainage features that matter once the soil goes in.

How to Account for Settling and Soil Loss

Every grow bag loses 10–15 percent of its visible soil height within the first week. Water compacts the air gaps, and the mix settles into a denser, more root-friendly bed. The formula the soil calculators use: multiply your target volume by 1.15. For a 25-gallon bag, that brings the safe purchase total to about 3.8 cubic feet (114 quarts). Buy that amount and you will top off after settling without needing a second trip.

Skipping this buffer is the most common mistake. Filling to exactly 3.34 cubic feet leaves the bag looking half-empty once the tomatoes are two weeks old, and top-dressing a settled pot is harder than buying one extra bag at the start.

Two Rules That Save a Crop

Leave 3–4 inches of headspace. That gap is not wasted space — it catches water and keeps the soil from washing over the rim during heavy rain or deep soakings. Smart Pot’s own calculator and the independent Omni Calculator potting-soil tool both flag this as a non-negotiable step.

Never use garden soil. Fabric bags drain fast, and dense ground soil turns into a waterlogged brick that drowns roots. Stick with a potting mix designed for containers — one that includes perlite or vermiculite for aeration. If your mix feels heavy, add an extra handful of perlite per gallon of soil.

FAQs

Can I fold down the sides of the bag to use less soil?

Yes, and many growers do. Rolling the top of a 25-gallon bag down 4 inches reduces its active volume to roughly 18–20 gallons. This works well for shallow-rooted plants like peppers or lettuce but limits the room deep-rooted crops like tomatoes or squash need to spread.

Does the soil volume change if I add compost or perlite?

It does. Any amendment you mix in takes up space that would otherwise be base potting soil. Estimate the final mix at 80–85 percent potting soil and 15–20 percent amendments, then buy the base soil at roughly 3.0 cubic feet instead of the full 3.34. The amendments bring you back to a full bag.

How much does a fully filled 25-gallon grow bag weigh?

That load is safe on most decks and patios, but check the surface if you have aging wood or a balcony with a posted weight limit. Wet fabric bags also drip; place them on a tray or gravel bed to keep the area clean.