Potting Soil Calculator

Measure the inside length, width, and depth for straight planters, or the inside radius and depth for round pots, then divide the total cubic feet by the bag size and round up to the next full bag. Guessing often leaves you short on mix or stuck with an extra bag and a half-filled container. This guide shows how to measure straight-sided, round, and tapered pots, convert inches to liters or cubic feet, and buy the right number of bags.

Press Calculate to see the math with your numbers.

What the calculator tells you

A potting soil calculator turns container measurements into a soil volume, then into bag counts you can shop with. It works for common container shapes, including rectangular planters, round pots, and tapered flower pots. The result is an estimate of full-container fill, so partial fill depth and settling still matter.

That estimate helps you plan purchases in quarts, liters, cubic feet, or bag counts. Potting mix is sold by volume, typically in quarts, and retail bag sizes vary by supplier. A calculator keeps you from buying too little or paying for more bags than you need.

What the answer means for shopping

For a straight-sided pot, the volume is fixed by the inside length, width, and depth. For a round pot, the radius and depth set the volume. For a tapered pot, the shape changes from top to base, so the calculator has to use a frustum formula instead of cylinder math.

Use the output as a buying estimate, not a perfect fill guarantee. Soil settles after watering and handling, and many containers are filled below the rim.

Measure the inside of the container

Measure inside the walls, not the outside shell. Outside measurements include plastic, ceramic, or terracotta thickness that does not hold soil, so they overstate how much mix you need. If the pot has a lip, ignore it and measure the usable interior.

Use the container’s actual fill depth. If you plan to stop 1 inch below the rim, subtract that from the inside depth before you calculate.

How to measure straight-sided and round containers

1. Measure the inside length, width, and depth for a rectangular planter.
2. Measure the inside diameter for a round pot, then divide by 2 to get the radius.
3. Measure the inside depth from the soil line to the bottom.
4. Convert units if needed before you calculate. Use the same unit for every dimension.

For irregular or tapered pots, measure the top inside diameter, the base inside diameter, and the inside depth. That keeps the math aligned with the container’s changing shape.

Partial fill depth changes the math

Fill depth drives total volume. A pot filled to 8 inches needs far more mix than the same pot filled to 6 inches. If the container will be planted with a top layer of mulch or left below the rim for watering, use that shallower depth in the calculation.

Suppose a 12-inch round pot has an inside depth of 10 inches, but you only want 8 inches of soil. Use 8 inches in the formula, not 10. That change alone can reduce the bag count.

Which formula should you use?

Choose the formula by shape. Use length × width × depth for a rectangular planter, π × radius² × depth for a round pot, and the truncated-cone formula for a tapered flower pot. Straight sides use cylinder math; sloped sides do not.

That split keeps the calculation clean. If the pot is straight from top to bottom, treat it like a box or cylinder. If the sides narrow toward the base, treat it like a frustum.

Rectangular planter formula

For a rectangular planter, use inside length × inside width × inside depth. This works for troughs, window boxes, and raised-bed-style planters that have straight sides. The volume comes out in cubic inches if your measurements are in inches.

Example: a planter with an inside length of 24 inches, width of 12 inches, and depth of 10 inches holds 2,880 cubic inches. That equals about 49.9 quarts, or about 1.67 cubic feet.

Round pot formula

For a round pot, use π × R² × depth, where radius is half the inside diameter. If the sides are straight, the shape reduces to cylinder math. Use the inside diameter, not the rim diameter on the outside edge.

Example: a 12-inch round pot with a 12-inch inside diameter and 10-inch depth has a radius of 6 inches. The volume is about 1,131 cubic inches, or about 20 quarts.

Tapered flower pot formula

For a tapered flower pot, use the frustum, or truncated-cone, volume equation. You need the top inside diameter, the base inside diameter, and the inside depth. This is the right choice for many terracotta and plastic nursery pots.

For a frustum, the formula is V = h × π ÷ 12 × (D_top² + D_top × D_base + D_base²), using the same unit for every dimension. If you use inches, the result is cubic inches.

How do you turn volume into bag counts?

Convert the calculated volume into quarts, liters, or cubic feet, then divide by the bag size and round up. A standard quart is a volume unit, and 1 cubic foot equals 1,728 cubic inches. If a bag size is listed as 8 quarts, 1.5 cubic feet, or 2 cubic feet, use that exact bag volume in the math.

Round up to complete the fill. If your total comes out to 1.3 bags, buy 2. That is the practical answer for container gardening, because a half-bag on the shelf does not help when you are halfway through a repotting job.

Convert between common soil units

Use 0.016387064 liters per cubic inch when you need liters. Use 57.75 cubic inches per US quart, or 1,728 cubic inches per cubic foot, when you want bag counts or a quick cross-check. The numbers line up cleanly enough for shopping, and they are easier to compare than guessing by eye.

Say your calculation gives 864 cubic inches. That is 14.96 quarts, about 13.6 liters, or 0.5 cubic feet. In practice, that means two 8-quart bags, one 1.5-cubic-foot bag, or one 2-cubic-foot bag, with extra left over.

How settling changes the buy count

Add a settling allowance before you shop. A practical buffer is 10%, which covers compaction after watering, settling during handling, and small fill adjustments at the top of the pot. That buffer is especially useful for fresh transplants and loose potting mix.

Plant Calculator includes a 10% settling allowance option, and that is a sensible buying margin here too. If the bare calculation says you need 40 quarts, plan for 44 quarts before you round bag counts.

Potting soil calculator table

This table gives inside dimensions, calculated volume, and rounded-up bag counts for three common retail sizes. The 8-quart, 1.5-cubic-foot, and 2-cubic-foot columns are shopping counts, not exact fill promises. The last column adds a 10% settling buffer to the volume.

The table shows why bag size matters. The same window box needs seven 8-quart bags, two 1.5-cubic-foot bags, or one 2-cubic-foot bag. For larger containers, bigger bags often cut cost and waste.

How much extra soil should you buy?

Buy extra when the pot will be topped off after watering, when the soil is loose and fluffy, or when the container has a deep profile that settles fast. A 10% buffer is a practical starting point. For a large planter or a full-day repotting project, 15% is safer if you want to avoid stopping for another bag.

Partial fills need less soil, but they still need a buffer if the surface will be corrected later. A common first-timer mistake is buying the exact calculated amount for a tall container, then finding the mix drops below the rim after the first watering. That is why rounding up matters.

When to round up more aggressively

Round up harder if the container is tapered, the mix is very airy, or the planter will be filled in stages. A nursery pot that narrows sharply toward the base can hide more volume than it first appears. A raised bed with corners can also hold more than a quick eyeball estimate suggests.

Picture a gardener filling three 12-inch terracotta pots. If the calculation says 23.5 quarts per pot, the total is 70.5 quarts before settling. With a 10% buffer, the shopping target jumps to about 77.6 quarts, which means ten 8-quart bags, or two 1.5-cubic-foot bags plus one 8-quart bag.

Frequently asked questions

How do I calculate how much potting soil I need?

Measure the inside dimensions, choose the right shape formula, and calculate the volume in cubic inches or cubic feet. Then convert to quarts or liters and divide by the bag size. Round up, because bag counts are shopping counts, not exact fill counts.

How is potting soil measured?

Potting mix is usually sold by volume, typically in quarts, though many bags also list cubic feet or liters. Use the bag label as the final shopping unit. For conversion, 1 cubic foot is 1,728 cubic inches, and 1 US quart is 57.75 cubic inches.

Should I use inside or outside measurements for a potting soil calculator?

Use inside measurements. Outside dimensions include wall thickness, lips, and curved edges that do not hold soil. If you use the outside size, the result will be too high and you may buy more mix than the pot can actually hold.

How much potting mix do I need for a rectangular planter?

Use length × width × depth, all measured inside the planter. This works for troughs, window boxes, and raised-bed-style planters with straight sides. If the fill stops below the rim, subtract that gap from the depth before calculating.

How much potting soil do I need for a round pot?

Use π × radius² × depth, with radius equal to half the inside diameter. This is the right formula for straight-sided round pots. If the pot tapers toward the bottom, use the frustum formula instead, or the result will be too high.

How much extra potting mix should I buy for settling?

A 10% settling buffer is a practical amount for most container jobs. It covers compaction from watering, handling, and top-offs after the first soak. If the mix is very loose or the container is tall and tapered, increasing the buffer slightly is reasonable.