Free Fertilizer Optimizer

🌱 FREE GARDEN TOOL

Fertilizer Optimizer

Work out exactly how much fertilizer to spread on a bed or in containers — in pounds, kilograms, cups and grams — from your plant’s nitrogen need and your product’s N-P-K.

✓ Free & instant ✓ Imperial & metric ✓ Beds and containers

Fertilizer Calculator

Units

Bed length (ft)

Bed width (ft)

What are you feeding?

Product nitrogen (N%)

Product phosphorus (P%)

Product potassium (K%)

Please check your numbers — length, width and nitrogen must be above zero.

Your fertilizer amount

Product to apply—Spread evenly over the bed.

Bed area—Length times width.

Nitrogen needed—For the plants you are feeding.

In cups and grams—About 1 cup of granular fertilizer weighs 0.5 lb (227 g).

Phosphorus and potassium applied—From the product you chose.

Your fertilizer amount will appear here.

How the fertilizer optimizer works

Nitrogen is the nutrient plants use most, so the rate is set by nitrogen. We work out your bed area, multiply by the nitrogen your plants need, then divide by the nitrogen percentage in your product. The result is the weight of product to spread — not the weight of nitrogen.

For example, a 10-10-10 fertilizer is 10% nitrogen, so to give a bed 1 lb of nitrogen you need 10 lb of product. Spread it evenly, rake it lightly into the soil, and water it in.

Worked example

A 1,000 sq ft vegetable bed (25 ft by 40 ft) needs 1 lb of nitrogen per 1,000 sq ft. With a 10-10-10 product, you need 10 lb of fertilizer — that is 4.54 kg, about 20 cups, or about 4,536 g. It also applies 1.0 lb of phosphorus (P2O5) and 1.0 lb of potassium (K2O).

Nitrogen needs at a glance

Plant	Nitrogen needed	Grams per m²
Vegetables	1.0 lb per 1,000 sq ft	4.9 g
Tomatoes	1.5 lb per 1,000 sq ft	7.3 g
Lawns	1.0 lb per 1,000 sq ft	4.9 g
Leafy greens	0.75 lb per 1,000 sq ft	3.7 g
Flowers	0.5 lb per 1,000 sq ft	2.4 g
Herbs	0.25 lb per 1,000 sq ft	1.2 g

Common fertilizing mistakes

Guessing the amount. Too little does nothing; too much burns roots and washes into waterways. Measure your bed and read the product label.
Applying to dry soil. Water the bed first, or apply before rain, so granules dissolve and reach the roots instead of sitting on dry soil.
Fertilizing dormant or stressed plants. Do not feed plants in winter dormancy, in a heatwave, or while they are wilted — it causes more harm than good.
Using bed rates for containers. Pots hold far less soil and dry out fast. For containers, use about half the bed rate and feed little and often.
Skipping a soil test. If your soil already has plenty of phosphorus or potassium, a balanced fertilizer just adds more. A cheap soil test tells you what is actually missing.

Pro tip: Feed the soil, not just the plant. A layer of compost adds slow-release nutrients and helps fertilizer work better. Our Compost Estimator works out how much compost your beds need.

Frequently asked questions

They are the percentages of nitrogen (N), phosphorus (P2O5) and potassium (K2O) in the bag, in that order. A 10-10-10 fertilizer is 10% nitrogen, 10% phosphorus and 10% potassium by weight. Nitrogen drives leafy growth, phosphorus supports roots and flowers, and potassium helps overall plant health and disease resistance.

More than the calculator suggests is too much. Over-fertilizing burns leaf edges and roots, makes plants grow soft and leggy, and the excess washes away in rain. If you are unsure, apply half the amount now and the other half in two to three weeks.

Yes, if you can. A basic soil test costs little and tells you which nutrients your soil already has. Many gardens have plenty of phosphorus and potassium but lack nitrogen, so a nitrogen-only feed may be all you need.

No — use about half the bed rate for containers, and feed little and often rather than all at once, because pots drain quickly and nutrients wash out. A liquid feed every two weeks works well for container vegetables.

Apply fertilizer when plants are actively growing in spring and early summer, on moist soil, and water it in. Avoid fertilizing in the heat of the day, before heavy rain, or late in autumn when growth is slowing down.