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Worm Castings Calculator

Enter your garden area and application rate to find the worm castings needed.

How it is calculated

The tool multiplies the bed area by the application rate: pounds = area × rate. Enter the area in square feet and the rate in pounds of castings per square foot, so the product comes out in pounds. It then converts to kilograms with kg = pounds ÷ 2.20462 and counts bags of 40 lb with bags = ceil(pounds ÷ 40).

The bag count is always rounded up to a whole bag, because you cannot buy part of one, so you will usually end up with a little left over; even a tiny amount needs one bag. Pounds are shown to one decimal and kilograms to two. Area and rate must both be greater than zero, and the total cannot exceed one trillion pounds; a zero, negative or empty field gives no result. The tool does not know your soil or crop, so the rate is your choice.

Worked example

A vegetable bed measures 100 square feet and you plan to apply 0.5 lb of castings per square foot. Weight = 100 × 0.5 = 50.0 lb. In kilograms that is 50 ÷ 2.20462 = 22.68 kg. For bags, 50 ÷ 40 = 1.25, which rounds up to 2 bags (40 lb). You would have about 30 lb left over for top-dressing or potting mix. If the bed were 80 square feet at the same rate, you would need exactly 40 lb, one bag; at 81 square feet the 40.5 lb already needs a second bag.

Try this example →

Common mistakes

  • Entering the area in square metres while the rate is per square foot. One square metre is about 10.76 square feet, so the weight comes out roughly ten times too small.
  • Typing the rate as a total for the whole bed instead of per square foot. That multiplies the amount by the area again and gives an absurd weight.
  • Buying exactly the pound figure. The result is rounded up to full 40 lb bags, and bags of other sizes need your own division.
  • Using the area of the whole garden including paths. Only the planted or amended bed area should be entered.
  • Expecting the bag count to follow the rounded pound figure. Bags are counted from the exact weight, so a bed that needs just over 40 lb already needs two bags.

About this calculator

Worm castings — the nutrient-rich excrement produced by vermicomposting worms — are a slow-release organic soil amendment prized for improving soil structure, water retention and microbial activity without the burn risk of synthetic fertilizer. Because they’re applied by weight relative to bed area, typical application rates are expressed as pounds per square foot of garden bed.

This calculator takes your garden bed’s area in square feet and a chosen application rate in pounds per square foot to estimate how many pounds (and kilograms) of castings you need, plus how many standard 40 lb bags to buy. It’s a practical planning step for organic and home gardeners amending vegetable beds or raised beds, and for anyone running a vermicomposting operation figuring out how much of their harvested castings a given planting area will actually use.

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Comments (1)

  • Editorial team

    The tool turns a rate in pounds per square foot into a shopping list. A 200 sq ft bed at 0.5 lb per sq ft needs 100.0 lb, or 45.36 kg, which rounds up to 3 bags of 40 lb because 2.5 bags cannot be bought. Leftovers keep in a covered, slightly damp container. Do you top-dress beds or mix castings into potting soil, and what rate works for you?

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