Enter the rate from the bag and the pots or bed it goes on. The answer is how much to put on each pot and how much for the whole job, for compost, worm castings, a dry fertilizer or a granular pesticide.
Bag Rate
Pots or Bed
Amount
Top Dressing Steps
Find the rate on the label and enter it as printed: 1 tbsp per gallon of soil.
Use level scoops: 5 tbsp per pot. Keep these cups for the garden only.
Spread it evenly over the whole surface, out to the edge of the pot or bed.
Brush any granules off the leaves and keep a small gap around the stem.
Water it in so it settles into the soil instead of sitting on top.
Bags give rates in different ways: per square foot of surface, per gallon or cubic foot of soil, per plant, or as a layer to spread. Pick the one the bag uses and enter its numbers as printed. A rate per gallon of soil uses the pot size, so a 5 gallon pot gets five times the one gallon amount.
A rate per square foot, or a layer, depends on the pot's surface, not its volume. Measure across the inside of the top rim. Two pots of the same volume can have very different tops, so the tool asks instead of guessing.
A rate per cubic foot of soil is a mixing rate. On a bed it needs the depth the amendment gets worked into. For a product left on the surface, a rate per square foot or a layer fits better.
The answer stays in the bag's own kind of unit. A rate in cups comes out in spoons, cups or gallons, and a rate in pounds comes out in ounces or pounds. A cup of bone meal and a cup of compost weigh very different amounts, so the tool does not convert between the two.
Compost is usually spread as a layer. Batches vary a lot in salt, moisture and how finished they are, so the amount here is a starting point. Compost that still smells sour or feels warm is not finished and can harm roots in a pot.
The math works the same for granular insecticides, fungicides and weed killers. For these the label is the law: use the rate the label gives for that site and pest, never more, and wear the gear it lists. Some are labeled for lawns or beds only, not pots. Some container labels give a rate per inch of pot width, which is one of the rate options above.
With a lab report in hand, the Soil Test Calculator works out what a bed needs. For a new potting mix, the Soilless Mix Calculator sizes the amendments for the batch.