Plant Spacing & Layout

Grow Bag Size Calculator

Pick what you want to grow and this calculator tells you the minimum and ideal grow bag size in gallons, how much potting mix it takes to fill it, and which common bag sizes will work.

How to use the grow bag size calculator

Choose what you want to grow from the dropdown and press Find My Bag Size. The calculator instantly shows the minimum grow bag size that will work, the ideal size for the best harvest, and how much potting mix it takes to fill that ideal bag in both quarts and cubic feet. It also rates the common bag sizes you will find in stores so you can see at a glance whether the bag in your cart is too small, just enough, or ideal.

Why grow bag size matters

A grow bag is basically a portable root zone, and the volume of soil it holds sets the ceiling for how big and productive a plant can become. A bigger bag stores more water and nutrients and gives roots room to spread, so plants are less stressed, need watering less often, and usually yield more. A bag that is too small restricts the roots, dries out quickly, and runs short of nutrients, which leads to wilting, blossom-end rot on tomatoes, and stunted growth. Matching the bag to the plant is the single easiest way to set a container garden up for success.

Minimum versus ideal size

For every plant we list two numbers. The minimum is the smallest bag the plant will tolerate — it can produce a crop, but you will need to water more often, sometimes twice a day in summer, and feed more regularly because there is less soil to buffer moisture and nutrients. The ideal size gives the roots enough room and soil volume to stay evenly moist and well fed, which means healthier plants, bigger harvests, and far less daily fuss. If you have the space and the potting mix, the ideal size is almost always worth it.

How much soil will you need?

Grow bags are measured by volume in gallons, and potting mix is sold by the quart or cubic foot, so it helps to convert between them. As a rule of thumb, one gallon of bag volume holds about four quarts of potting mix, or roughly 0.134 cubic feet. That means a 5-gallon bag takes about 20 quarts, a 10-gallon bag about 40 quarts, and a 20-gallon bag about 80 quarts. The calculator does this math for the ideal bag automatically so you know how much mix to buy before you head to the garden center.

Watering and care tips for grow bags

  • Fabric bags breathe, so they dry out about twice as fast as plastic pots — check the top inch of soil daily in hot weather.
  • Use a quality potting mix, not garden soil, which compacts and drains poorly in containers.
  • Feed regularly; frequent watering flushes nutrients out, so a steady liquid feed or slow-release fertilizer keeps plants productive.
  • Raise bags on feet or pavers for drainage and airflow, and stand them where they get the right amount of sun for the crop.
  • Give heavy feeders like tomatoes their own bag, and reserve wide, shallow bags for grouping lettuce, herbs, and other shallow-rooted plants.

Frequently Asked Questions

What size grow bag do I need?

It depends on the plant. Leafy greens and herbs are happy in 1 to 3 gallon bags, most fruiting vegetables like tomatoes, peppers, and cucumbers want 5 to 10 gallons, and large plants such as potatoes, blueberries, and dwarf fruit trees need 15 gallons or more. The calculator lists a minimum size that will work and an ideal size that gives the best results for each plant.

Are bigger grow bags always better?

Generally yes, up to a point. A larger bag holds more soil, which means more room for roots, more available nutrients, and more moisture, so the plant is healthier and you water less often. The main trade-offs are cost, the amount of potting mix required, and weight once filled. For most vegetables the ideal size in this calculator is a good balance of plant health and practicality.

How much potting soil does a grow bag hold?

Roughly 4 quarts of potting mix per gallon of bag volume, or about 0.134 cubic feet per gallon. So a 5-gallon bag takes about 20 quarts, a 10-gallon bag about 40 quarts, and a 20-gallon bag about 80 quarts. The calculator shows the exact quarts and cubic feet needed to fill the ideal bag for your chosen plant.

Why do grow bags dry out so fast?

Fabric grow bags are porous, so air reaches the roots from all sides. That air pruning is great for root health but it also lets water evaporate from the sides as well as the top, so bags dry out roughly twice as fast as solid plastic pots. In hot weather, check the soil daily and water whenever the top inch feels dry.

Can I grow more than one plant per grow bag?

For shallow-rooted, compact crops like lettuce, spinach, radishes, and many herbs you can fit several plants in one wider bag. Larger fruiting plants such as tomatoes, peppers, blueberries, and dwarf trees should get their own bag so they are not competing for water and nutrients. When in doubt, give heavy feeders their own space.

How deep should a grow bag be for carrots and other root crops?

Root crops need depth more than width. Carrots should have a bag at least 12 inches deep so the roots can grow long and straight, and potatoes do best in tall bags so you can add soil ("hill" them) as they grow. A 5 to 10 gallon bag that is taller than it is wide is ideal for these crops.

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GardenCalc Editorial Team

Horticulture Writers & Master Gardeners

Our calculators and guides are written and fact-checked by gardeners with hands-on experience in vegetable production, soil management, and home landscaping.