GROWING TOOLS
NPK Fertilizer Calculator
The bag's three numbers are the percent by weight of nitrogen, phosphate, and potash. They do not say how many scoops to use. Match the crop and the space, then stay at or under the lowest rate printed on that bag. A soil test replaces this comparison. Never pour a lawn or bed rate into a small pot.
Enter nitrogen, phosphate, and potash, then the bed area or the pot diameter, and the crop. The plan says use the label once, split a high-nitrogen bag, use half an indoor rate, or skip. The label and a soil test still win.
How this dose is capped
Clemson Extension's fertilizer-label note says the three numbers are percents by weight: nitrogen, phosphate as P2O5, and potash as K2O. A 10-10-10 is a balanced fertilizer. Clemson's vegetable fact sheet gives a preplant example of 10 pounds of 10-10-10 per 1,000 square feet, which is 1 pound per 100 square feet, only when a soil test calls for nitrogen, phosphorus, and potassium. The same sheet says a pint is roughly a pound for most fertilizers. Sandy soil and a crop that was not in that table need a different amount, which is why this tool will not print a pounds-per-acre prescription. Illinois Extension tells houseplant growers to follow the label, never mix a stronger solution, and to skip fertilizer in winter unless the plant is under lights and still growing. Its container note says to fertilize moist mix, and that half-strength is one way to feed with every watering. A bed rate dumped into a small pot is the usual way to burn roots.
Example dose plan
Use the label once. Clemson's preplant example is 1 pound of 10-10-10 per 100 square feet when a soil test calls for nitrogen, phosphorus, and potassium. Scaled by nitrogen only, that is about 1 pound of this product on 100 square feet. Treat that as a ceiling to check against the bag, not as a prescription, and not as proof the soil needs phosphorus. Use the lowest garden rate printed on this bag if it is at or under that ceiling. A higher middle number is not a reason to use a bigger scoop. Do not add a second product the same week.
The example is 10-10-10 on 100 square feet of leafy crops. Press the button to use your bag, space, and crop. The label and a soil test still win.
Published · Updated
Which plan is this?
| Plan | When | What you do |
|---|---|---|
| Use the label once | A bed of leafy crops or flowers, and the bag is under 16 percent nitrogen | The lowest rate on the bag, at or under the nitrogen ceiling. Not a second product the same week. |
| Split a high-nitrogen bag | Nitrogen is 16 percent or higher, on a bed or on a pot wider than 8 inches | Two lighter feeds, weeks apart, still under the label. |
| Half the label | A houseplant, a pot 8 inches or narrower, or fruiting vegetables on a bed | Half the indoor or container rate, or the lower rate on the bag. Never a lawn rate in the pot. |
| Skip | Succulents and cacti | A cactus rate on the label, once, in active growth, or do not feed. |
Lime versus sulfur is a different decision: soil pH calculator. The layer a garden sample actually represents is the soil profile. Compost percent is organic matter, not a scoop of this bag. How soon to water after you feed is the watering calculator. A bed you are filling, rather than feeding, is the raised-bed soil calculator.
Common questions
Will this tell me exactly how many pounds to buy?+
No. Clemson’s 10-10-10 example applies when a soil test asks for nitrogen, phosphorus, and potassium. This page scales that example by nitrogen so you can see a ceiling. The printed label, and the soil test, still win.
Can I use a lawn fertilizer in a pot?+
No. A bed or lawn rate in a small volume of mix is a common way to burn roots. Houseplants stay at half the indoor rate on the label, and only while they are growing.
What if the middle number is high?+
A higher phosphate number does not mean a bigger scoop. If the soil test already shows enough phosphorus, adding more is waste. Use the lowest rate on the bag or skip it.
Does organic matter replace this bag?+
No. Compost and organic matter are a different measurement. Use the organic-matter guide for the percent target and the soil pH calculator for lime versus sulfur.
Sources and notes
- Clemson Extension: fertilizing vegetables
- Clemson Extension: reading a fertilizer label
- Illinois Extension: houseplant care
- Illinois Extension: fertilizing container gardens
The plan is a ceiling to compare with the bag. It is not a soil-test report and not a shopping list. Sources checked November 7, 2026. Editorial responsibility.