Fertilizer Calculator

Fertilizer application is based on the desired nutrient rate and the fertilizer’s NPK ratio.

NPK ratio represents the percentage (by weight) of:

• N = Nitrogen (promotes leaf growth)
• P = Phosphorus (promotes roots and flowers)
• K = Potassium (overall plant health)

Imperial (lbs per 1,000 sq ft):

Fertilizer (lbs) = (Desired N rate per 1,000 sq ft / N percentage) × (Area sq ft / 1,000)

For example, to apply 1 lb of nitrogen per 1,000 sq ft with a 10-10-10 fertilizer: Fertilizer = (1 / 0.10) × (Area / 1,000) = 10 lbs per 1,000 sq ft

Metric (kg per 100 m²):

Fertilizer (kg) = (Desired N rate per 100 m² / N percentage) × (Area m² / 100)

For example, to apply 0.5 kg of nitrogen per 100 m² with a 10-10-10 fertilizer: Fertilizer = (0.5 / 0.10) × (Area / 100) = 5 kg per 100 m²

Common lawn fertilizer rates:

• Spring: 0.5–1 lb N per 1,000 sq ft (0.25–0.5 kg N per 100 m²)
• Summer: 0.5 lb N per 1,000 sq ft (0.25 kg N per 100 m²)
• Fall: 1–1.5 lb N per 1,000 sq ft (0.5–0.75 kg N per 100 m²)

One pound of nitrogen per 1,000 sq ft works out at 4.9 grams per square metre, which is why the metric column above reads 0.5 kg per 100 m². If a source quotes you tens of grams per square metre for a lawn feed, check it. That is roughly ten times a normal rate and it will scorch turf brown within a week.

Common NPK ratios:

• 10-10-10: Balanced, all-purpose
• 20-5-10: High nitrogen for lawns
• 5-10-10: High P and K for flowers/fruit

How much is too much

Never put down more than 1 lb of nitrogen per 1,000 sq ft (0.5 kg per 100 m²) in a single application, whatever the annual total you are aiming at. Above that the grass takes up more nitrogen than it can build into tissue, the excess draws water out of the roots, and you get the pale straw-coloured streaks that follow a spreader’s path. Split a heavier annual programme into applications six to eight weeks apart instead.

Slow-release products are the exception. A coated or organic nitrogen source releases over weeks rather than days, so labels often permit a heavier single pass. Follow the bag.

Watering it in

Granular fertilizer sitting dry on grass blades does nothing except risk a burn. Water it in with about half an inch of irrigation immediately after spreading, or spread it when rain is due within a day. Do not apply to wet foliage, since the granules stick to the leaves rather than falling to the soil.

Just a lawn, in imperial?

The lawn fertilizer calculator does the same sum with fewer boxes, in pounds per 1,000 square feet only, and carries the annual nitrogen budget for each grass type: fine fescue wants 1 to 2 lbs a year, Kentucky bluegrass 2 to 4, Bermuda 3 to 5. That is the number to divide into feeds before you come back here for the quantity.
The formula is identical on both, so the answers match. 5,000 sq ft of a 28% nitrogen feed at 1 lb N per 1,000 comes to 17.9 lbs either way. Use this page when you want metric, a bag count, or you are feeding something other than grass.