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The small-space growing journal

Harvest in Pots

MEASURE FIRST. BUY WITH A PLAN.

How much potting mix
will your pots need?

Estimate mix for tapered pots, round pots or rectangular planters. Compare liters, US gallons, cubic feet and whole bags, then take the result with you.

Measure your containers

Read the mix-bag label

Use the labeled volume, not the bag’s weight. The count rounds up after combining all identical pots.

Enter your measurements, then calculate your mix estimate.

No account required. Measurements are processed on this page and are not sent to our server. Your estimates and shopping list clear on reload; download them before leaving.

Combine different pots into one shopping list

Calculate one size above, add it below, then change the measurements for your next pot or group. You can combine up to 20 groups. The calculator rounds up bags after adding all the volumes, so you do not buy a separate rounded-up bag count for every pot.

Calculate a pot group above to enable Add calculated group.

    Bag A
    Add a calculated container group to start your shopping list.

    The list stays only in this open page. Reloading or leaving clears it; download it first. No account, saved profile or server upload is used.

    Why the cheapest price per liter may cost more today

    Suppose your combined pots need 60 liters and you have 10 liters of suitable mix, leaving 50 liters to buy. A 40-liter bag at an illustrative $12 costs $0.30 per liter, but two bags cost $24 and leave 30 liters. A 25-liter bag at an illustrative $8 costs $0.32 per liter, but two bags cost $16 with no calculated surplus. The smaller bag costs $8 less for this purchase, even though its price per liter is higher.

    These are invented shopping inputs to explain the arithmetic, not current prices or a product recommendation. Leftover mix can be useful for a later project; compare storage, delivery and the suitability of each mix before choosing. No settling allowance is added automatically.

    Measure the space the mix will occupy

    1. Measure inside the pot. Outside dimensions include walls and decorative rims.
    2. Choose the intended mix surface below the watering rim. Measure depth from the inside bottom to that surface.
    3. For a tapered pot, measure across the bottom and across the intended fill line. Use the round option only when the sides are straight.
    4. For a rectangular planter, record inside length, width and fill depth. The quantity field repeats one shape. Add each calculated group to the shopping list to combine different sizes.
    Three measurements for a tapered potTop diameter is measured across the intended fill line, bottom diameter across the inside base, and depth vertically between them. Leave room above the fill line for watering.Diameter at the fill lineInside bottom diameterFilldepthPotting mixSchematic, not to scale
    Keep watering headroom above the measured fill line. Match the diagram to the inside of your actual pot.

    Check the estimate with two examples

    A straight rectangular planter measuring 24 × 12 × 10 inches holds 2,880 cubic inches: about 1.67 cubic feet or 47.19 liters. One 2-cubic-foot bag covers that empty volume, with some mix left over.

    A tapered round pot measuring 14 inches across at the fill line, 10 inches at the bottom, and 12 inches deep holds about 22.45 liters. Treating it as a 14-inch-wide cylinder would estimate about 30.27 liters. Measuring the taper can make a meaningful difference before you buy.

    What the calculator does

    The rectangle uses length × width × depth. A straight round pot uses π × diameter² × depth ÷ 4. A tapered round pot uses π × depth × (top diameter² + top diameter × bottom diameter + bottom diameter²) ÷ 12. Keep all dimensions in the same unit.

    Cubic centimeters ÷ 1,000 gives liters; cubic inches ÷ 1,728 gives cubic feet. One US liquid gallon is 3.785411784 liters, and one cubic foot is 28.316846592 liters. These are volume conversions, not a promise that a nursery pot’s trade-size name equals its measured capacity.

    What to adjust yourself

    The result assumes empty, regular shapes filled to your measured line. Existing root balls, reusable mix, internal reservoirs and ledges reduce the space for new mix. Bulging fabric and irregular shapes do not follow these geometries exactly. Settling and bag-fill variation also affect the final amount. No extra safety margin or gravel layer is automatically added.

    Round up only after comparing the total with your bag label. If you already have mix, use the calculated volume as a reference and subtract only an amount you can actually measure. This tool estimates mix volume; it cannot assess balcony load limits or the weight of saturated pots.

    Next: compare potting mixes, check the crop’s space needs, or check drainage before filling.

    Updated October 28, 2026: Added a mixed-container shopping list, measured existing-mix deduction, whole-bag cost comparison and downloadable shopping list.

    Updated October 3, 2026: Added tapered pots, identical-container totals, custom bag volumes, downloadable estimates and visible calculation methods.

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