Fertilizer Calculator
Enter garden dimensions and the fertilizer application rate to estimate exactly how much to buy.
Also handy: Raised-Bed Soil Calculator, Mulch Calculator
Calculate fertilizer needed
Enter the garden length and width, then add the fertilizer application rate. This calculator estimates the total fertilizer required.
4.00 lb (1.81 kg)
Based on a 20 ft by 10 ft area and a rate of 2 lb per 100 sq ft.
How to use this calculator
Enter the area dimensions in feet, then enter the fertilizer rate per 100 square feet. The calculator estimates the total amount needed for the full area.
Tips for success
- Always follow the product label. Application rates vary by crop, soil type, and fertilizer formulation. The label overrides any rule of thumb.
- Less is more. Too much fertilizer burns roots, leaches into groundwater, and promotes leafy growth at the expense of flowers and fruit.
- Split applications work better. Half now, half in 4–6 weeks gives more uniform growth than a single heavy dose.
- Test your soil first. Soil testing every 2–3 years tells you what the bed actually needs. You may not need all three macronutrients.
- Water after granular applications. Watering thoroughly washes fertilizer off foliage and activates the nutrients without burning roots.
Common mistakes
- Guessing the application rate. Using a rough guess instead of the product label rate leads to under- or over-application.
- Using the wrong unit. Pounds vs. kilograms vs. ounces per 1000 sq ft vs. per 100 sq ft are all different. Read carefully.
- Over-applying to small beds. A small bed concentrates any over-application, and root burn shows up fast in tight spaces.
- Ignoring crop needs. Tomatoes and corn need more nitrogen than root vegetables. Match the formula to the crop.
- Not measuring the bed accurately. An extra foot of error on a large bed can mean a full pound of misapplied fertilizer.
FAQ
Does this replace the fertilizer label?
No. Always follow the product label and crop recommendations first.
Can I use this for organic fertilizer?
Yes, as long as you know the recommended application rate.
Can I use this for raised beds?
Yes. It works for raised beds, garden plots, and any rectangular growing area.
When should I apply fertilizer to my garden?
For vegetables, apply a balanced starter fertilizer at planting, then a second application when plants begin active growth (about 4 to 6 weeks later). Heavy feeders like tomatoes, corn, and squash benefit from a third application when fruit sets. Avoid fertilizing in the 2 weeks before harvest. For lawns, fall is the single most important feeding, followed by early spring. Never fertilize drought-stressed plants, water first, fertilize after recovery.
What happens if I apply too much fertilizer?
Over-application burns roots and leaves (visible as brown edges on foliage), stunts growth, and flushes excess nitrogen into groundwater. Synthetic fertilizers are the most common cause because they release all at once. If you over-apply, water deeply to flush excess salts below the root zone. Over-fertilizing also encourages leafy growth at the expense of flowers and fruit, a tomato plant fed too much nitrogen grows huge but produces little fruit.