Plant Pot Volume Calculator: Liters, Gallons & Soil Needed
Ever stood in the garden aisle holding a bag of potting soil, wondering, "Is one bag enough?" You get home, start filling, and either run out halfway or end up with half-empty bags in the shed. The fix is simple: know your pot's volume before you buy. Enter your pot's inside measurements in the calculator below to get its volume in liters, US gallons, quarts and cubic feet. The guide underneath explains the formulas, so you can check any number yourself.
Handy benchmark: a small 4-inch pot holds only about 0.5 liters (roughly a pint). That small number surprises most people, and it is exactly why knowing pot volume saves money and frustration.
Result
- Full pot volume
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- Soil to fill (per pot)
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- Soil for all pots
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Results are geometric volumes of the inside of the pot. Bagged potting mix settles, so buying about 10% extra is sensible for large containers.
The Quick Answer: How Much Does a Pot Hold?
If you just want a fast number, here it is. Pot capacity grows quickly as the diameter increases, because volume depends on the radius squared. Double the width and you far more than double the soil needed. A 4-inch pot is about half a liter, but a 12-inch pot jumps to roughly 15–19 liters (4–5 gallons).
The chart below is your cheat sheet for the most common plant pot sizes. Keep it handy next time you shop.
The chart below is your cheat sheet for the most common plant pot sizes. Keep it handy next time you shop.
| Pot Diameter | US Gallons | Liters | Common Use |
|---|---|---|---|
| 4" | ~0.125 gal (pint) | ~0.5 L | Seedlings, herbs |
| 5–6" | ~0.5–1 gal | ~1.9–3.8 L | Small houseplants |
| 7–8" | ~1 gal | ~3.8 L | Medium houseplants |
| 8.5" | ~2 gal | ~7.6 L | Perennials |
| 10" | ~2.5–3 gal | ~9–11.5 L | Shrubs, larger perennials |
| 12" | ~4–5 gal | ~15–19 L | Medium shrubs |
| 14–16" | ~6–10 gal | ~23–38 L | Trees, large plants |
These figures are approximate because most pots taper inward at the base and you leave a little space below the rim for watering.
How Many Gallons Is Each Pot Size? (Inches to Gallons)
The table below converts common round pot diameters into approximate US gallons and litres, with each diameter in centimetres too. Values assume a typical, slightly tapered pot about as tall as it is wide. Straight-sided or extra-deep pots hold more, so use the calculator at the top of this page for your exact pot.
| Pot Diameter (in) | Diameter (cm) | Approx. Gallons | Approx. Litres |
|---|---|---|---|
| 6" | 15 cm | ~0.5–1 gal | ~2–3.8 L |
| 8" | 20 cm | ~1–1.5 gal | ~4–6 L |
| 10" | 25 cm | ~2.5–3 gal | ~9–11 L |
| 12" | 30 cm | ~4–5 gal | ~15–19 L |
| 14" | 36 cm | ~6–7 gal | ~23–26 L |
| 16" | 41 cm | ~9–10 gal | ~34–38 L |
| 18" | 46 cm | ~13–14 gal | ~50 L |
| 20" | 51 cm | ~18–20 gal | ~68–75 L |
| 22" | 56 cm | ~24 gal | ~90 L |
| 24" | 61 cm | ~30–32 gal | ~115 L |
Notice how the numbers climb fast. Going from a 12-inch to a 24-inch pot does not double the volume, it multiplies it many times over. That is the radius-squared effect at work, and it is why big planters drink so much soil.
Pot Sizes in Litres (Metric & cm)
Gardening in metric? Here are common metric pot sizes in litres, from a small 20cm pot to a large 50cm planter (including the popular 30cm and 35cm sizes).
| Metric Pot Size | Approx. Litres | Rough Gallons |
|---|---|---|
| 20 cm | ~3–4 L | ~1 gal |
| 22 cm | ~5 L | ~1.3 gal |
| 24 cm | ~6–7 L | ~1.7 gal |
| 26 cm | ~8–9 L | ~2.2 gal |
| 28 cm | ~11 L | ~2.9 gal |
| 30 cm | ~14–15 L | ~3.8 gal |
| 35 cm | ~22 L | ~5.8 gal |
| 50 cm | ~60 L | ~16 gal |
Quick Unit Benchmarks: For quick conversions the other way: 1 gallon is about 3.8 L, 5 gallon is about 18.9 L, and 10 gallon is about 37.9 L. Keep these handy when a bag lists one unit and your pot uses another.
How to Measure Your Plant Pot Correctly
Getting an accurate volume starts with measuring the right way. All you need is a tape measure and about 30 seconds. You mainly need two things: the diameter (half of it is the radius) and the height.
3 Easy Steps to Measure
1
Measure Top Diameter: Measure across the top, from inside rim to inside rim. Outside measurements include the pot wall and read slightly too big.
2
Measure Height (Depth): Measure straight down the inside, from the base to just below the rim, not the very top edge.
3
Measure Bottom Diameter (For Tapered Pots): For tapered pots, also measure the diameter at the bottom, since the pot is narrower there.
Why inside measurements matter: Terracotta and ceramic pots often have walls around half an inch (1 cm) thick, so outside measurements overstate how much soil the pot holds. With the inside diameter and height, use the formulas below or enter the numbers in the calculator at the top of this page.
How to Calculate Pot Volume (The Formulas)
Pots come in three basic shapes, and each pot volume formula is simple. Do not worry, you only need basic multiplication. (If you ever only have the circumference of a round pot, divide it by 3.1416 to get the diameter first.)
| Pot Shape | Formula | Variables |
|---|---|---|
| Round / Cylindrical | V = π × r² × h | r = radius (diameter ÷ 2), h = height |
| Rectangular / Square | V = l × w × h | l = length, w = width, h = height |
| Tapered / Frustum | V = (π × h ÷ 3) × (R² + Rr + r²) | R = top radius, r = bottom radius, h = height |
Volume Calculations by Shape
Round Pots: Most pots are round. Multiply pi (3.1416) by the radius squared, then by the height. Example: a 10-inch pot (radius 5) that is 9 inches tall gives 3.1416 × 25 × 9 = about 707 cubic inches, which is roughly 3.1 US gallons or 11.6 liters.
Rectangular and Square Planters: For window boxes and square planters, just multiply length by width by height. Example: A 12 × 8 × 8 inch box holds 768 cubic inches, about 3.3 gallons or 12.6 liters.
Tapered Pots (Why They Trip People Up): Treating a tapered pot like a straight cylinder overestimates the soil you need. Example: a pot 10 inches across the top, 8 inches across the bottom and 9 inches tall holds about 575 cubic inches (9.4 liters, 2.5 gallons) with the frustum formula, roughly 19% less than the 707 cubic inches a straight cylinder would suggest.
Converting Pot Sizes: Gallons, Liters and Cubic Feet
Soil is sold in different units around the world, so conversions matter. Here are the key numbers to multiply by.
| From | To | Multiply By |
|---|---|---|
| Cubic Inches | Liters | 0.016387 |
| Cubic Inches | US Gallons | 0.004329 |
| Cubic Feet | Gallons | 7.48052 |
| Liters | Quarts | 1.05669 |
Friendly Shortcuts: 1 US gallon is about 3.79 liters, and 1 cubic foot equals roughly 7.5 gallons or 28.3 liters. If you prefer to skip the arithmetic entirely, our gallon pot size finder converts between units instantly.
Cubic Feet per Gallon Pot
Buying soil or filling grow bags by the bag? Bags are often labeled in cubic feet, so this cubic feet to gallons table answers questions like "how many cubic feet in a 5 gallon pot" right up to 30 gallons.
| Pot Size (Gallons) | Cubic Feet | Litres |
|---|---|---|
| 5 gallon | 0.67 cu ft | 18.9 L |
| 7 gallon | 0.94 cu ft | 26.5 L |
| 10 gallon | 1.34 cu ft | 37.9 L |
| 15 gallon | 2.01 cu ft | 56.8 L |
| 20 gallon | 2.67 cu ft | 75.7 L |
| 25 gallon | 3.34 cu ft | 94.6 L |
| 30 gallon | 4.01 cu ft | 113.6 L |
Instant Unit Conversions: Skip the manual math when buying soil! Use our interactive gallon pot size finder to convert between cubic feet, gallons, and liters automatically.
Nursery Pot Sizes Explained (#1, #3, #5)
This one confuses almost everyone. A "1-gallon" nursery pot does not actually hold a full gallon of soil.
Key fact: A #1 "trade gallon" nursery pot holds only about 0.7 to 0.76 real gallons, roughly 3 quarts or 2.8 liters, not a full 3.8-liter gallon. The name is an old industry tradition, not a true measurement.
Key fact: A #1 "trade gallon" nursery pot holds only about 0.7 to 0.76 real gallons, roughly 3 quarts or 2.8 liters, not a full 3.8-liter gallon. The name is an old industry tradition, not a true measurement.
Nursery "trade gallon" sizes are container classes (#1, #3, #5 and so on) described in the American Standard for Nursery Stock. Each class covers a range of volumes, and actual capacity varies by pot manufacturer, so treat the figures below as typical rather than exact.
Nursery Pot Chart: Typical Capacities
#1 nursery pot: about 0.7 gallon (2.8 L)
#3 nursery pot: about 2.3 gallons (8.7 L)
#5 nursery pot: about 3.6 gallons (13.6 L)
#7 nursery pot: about 6.5 gallons (24.6 L)
#10 nursery pot: about 8 gallons (30.3 L)
Bottom Line: So when a recommendation says "plant it in a 5-gallon pot," you can usually trust the trade size on the label, just know the real soil volume runs a bit lower.
How Much Potting Soil Do I Need?
Once you know your pot's volume, buying soil is easy. Match your total pot volume to the bag size, which is usually labeled in liters or cubic feet. Remember two things: leave about an inch (2–3 cm) below the rim for watering, and if your pot tapers, expect to need roughly 10–20% less than a straight-cylinder estimate. Potting mix also settles after the first few waterings, so a little extra for topping up is useful.
Soil Needed for Popular Container Sizes
14-inch pot: about 23–26 L (roughly 0.8–0.9 cu ft).
16-inch pot: about 34–38 L (roughly 1.2–1.3 cu ft).
18-inch pot: about 50 L (roughly 1.8 cu ft).
22-inch pot: about 90 L (roughly 3.2 cu ft).
The material matters too. A terracotta or ceramic pot dries faster than a plastic pot, while fabric grow bags and raised planters drain freely and need watching in heat. Whatever the container, good drainage and a quality potting mix—think compost, peat moss or coco coir, plus perlite or vermiculite—keeps roots healthy.
Explore Specialty Soil Guides & Tools: If you grow specialty plants, our orchid potting mix calculator and soil type guide help you pick the right blend, not just the right amount.
Best Pot Size for Common Plants
Volume is not just about soil bags. The right pot size keeps roots happy and plants thriving, and the best pot size for vegetables is often bigger than people expect. Here is a quick guide.
Recommended Pot Sizes by Plant Type
Herbs and seedlings: 4–6 inch pots (about 0.5–3 L) are plenty to start.
Most houseplants: 6–10 inch pots suit their root balls nicely.
Vegetables like tomatoes: Go big, at least a 5-gallon (19 L) pot. If your container tomatoes struggle in summer, see our guide on why tomato leaves curl in heat.
Tall flowers: Many prefer the open ground, but dwarf types grow well in 16-inch-plus pots. Our hollyhock growing guide explains which varieties suit containers.
Pro Tip: When in doubt, size up. A slightly larger pot holds more moisture and gives roots room to grow.
Choosing the container itself: Pot material affects drainage and how fast soil dries. Our guide to types of flower pots compares the options, and the indoor plant care calculator helps with houseplant-specific needs.
A Quick Home and Garden Safety Note
Container gardening often means grow lights, seed-heating mats, and extension cords running across the potting bench or windowsill. Those little electrical helpers are handy, but keep them away from water, damp soil, and clutter to avoid a fire risk.
Indoor Safety Check: While you are setting up indoor growing gear, it is a good moment to press the test button on your smoke alarms. It takes two minutes.
Smoke alarm placement: Our smoke detector calculator helps you work out how many alarms your home needs and where they go. Happy potting, and stay safe.
Frequently Asked Questions
How many liters is a 4-inch pot?
A 4-inch pot holds about 0.5 liters, roughly one pint. It is ideal for seedlings, small herbs, and starter plants before they move to a bigger home.
How many gallons is a 12-inch pot?
A typical 12-inch pot holds about 4 to 5 US gallons (15 to 19 liters), depending on its height and taper. That suits medium shrubs and larger flowering plants.
Is a 6-inch pot 1 gallon?
Not quite. A 6-inch pot usually holds about 0.5 to 1 gallon (1.9 to 3.8 liters), depending on its height and taper. A true 1-gallon size is closer to a 7-inch pot.
How do I calculate the volume of a round pot?
Use V = π × r² × h. Take the radius (half the top diameter), square it, multiply by pi (3.1416), then multiply by the height. Convert cubic inches to liters by multiplying by 0.016387.
Why does a 1-gallon nursery pot hold less than a gallon?
Nursery pots use "trade gallon" container classes rather than exact liquid measures. A #1 pot typically holds about 0.7 US gallons (roughly 2.8 liters), though exact capacity varies by manufacturer.
How much potting soil do I need for a 16-inch planter?
A typical 16-inch planter needs about 34 to 38 liters (roughly 1.2 to 1.3 cubic feet) of potting mix. Buy a little extra to top up as the mix settles.
How do I measure a pot correctly?
Measure the diameter across the top from inside rim to inside rim, then measure the inside height down to just below the rim. For tapered pots, also measure the narrower bottom diameter.
Ready to measure? Jump back to the calculator for your exact pot, or plan a bed layout with our plant spacing calculator.
Final Thoughts
Knowing your pot's volume turns guesswork into confidence. Remember the simple anchors: a 4-inch pot holds about half a liter, a 12-inch pot holds roughly 4–5 gallons, and nursery "gallons" are really trade sizes that hold a bit less. Measure inside rim to inside rim, use the right formula for your pot's shape, and buy soil to match.
Confident Gardening: Do that, and you will never overbuy or run short again. Your plants get the perfect amount of root space, your wallet stays happy, and your garden thrives. Now go measure that mystery pot on the patio—you have got this!
How these numbers were calculated
- Volumes use standard geometry: cylinder (π × r² × h), frustum for tapered pots, and length × width × height for boxes.
- Unit conversions: 1 US gallon = 231 cubic inches; 1 cubic inch = 16.387 cm³ (NIST unit conversions).
- Nursery container classes: American Standard for Nursery Stock (AmericanHort).
- Chart values are rounded estimates for typical pots. Pots vary in shape, so measure yours for an exact figure. Spotted an error? Contact us.
Related guides: More help with containers and soil from Prime Home And Garden.
