Calculators Fertiliser dose
How much fertiliser does my lawn need?
Put in the nitrogen percentage off the bag and your lawn's size. This works in grams of actual nitrogen, which is why you can swap between a 25% blend and a 12% blend without doubling your dose by accident.
Not sure? Work it out here.
Odd shape? Add it up here.
The first number in the N-P-K analysis. If the label gives a range like 22 to 27, use the higher one.
Every granular fertiliser has a different amount of nitrogen in it. PURPLE is about 10%, Baileys 3.1.1 is nearly 17%, a high analysis blend can be over 25%. Spread all three at the same rate and you have fed your lawn wildly different amounts.
So the sum starts from the other end. Grass can only use so much nitrogen in a year. Take that figure, divide it by how often you feed, then work backwards through the N percentage to get a weight of product.
The longer version
Where the yearly figure comes from
Kikuyu and buffalo want around 2.0kg of nitrogen per 100m² per year. Couch is hungrier at about 2.5kg. Zoysia is slower at 1.8kg. The cool season grasses want less again.
Ranges on the label
Some bags quote 22 to 27 percent. Use the top of the range. The dose comes out slightly smaller and the yearly tally slightly larger, so both errors push away from overfeeding.
Liquids
Liquid analyses are weight per volume, so the arithmetic is the same as long as the units line up. Kelp, humates and soil wetters carry no nitrogen and do not count against the budget at all.
What this does not know
It has no idea what you have already put down this year. Feed heavily in spring and an even split across the rest will take you over the cap.
The app does this and keeps score
This works out one dose in isolation. The lawn app holds your lawn's details so you never re-enter them, and it tracks the nitrogen you have already put down this year so later doses shrink instead of pushing you over the cap. Free, no account, works offline.