Calculators Fertiliser dose
How much fertiliser does my lawn need?
Less than the bag says, usually. Put in the nitrogen percentage off the label and your lawn's size, and this works out what your grass can actually use, so you are not paying to mow more often.
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.
Guide only. The rate on the label is the one that counts, and for herbicides and pesticides it is a legal requirement, not a suggestion. Read the label on the container in your hand every time.
Bag rates are generous. They are written by people who sell fertiliser, and following them usually means overfeeding. That does not get you a better lawn, it gets you a faster one: more mowing, more clippings, more thatch, and a dethatch you did not need.
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 on the label to get a weight of product. You buy less and you mow less.
The longer version
Why the percentage matters
Every granular fertiliser carries a different amount of nitrogen. 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, which is why the sum works in nitrogen rather than product weight.
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.
It also counts what you have already used
This page works out one dose, once, and forgets it. The app already knows your lawn's size and grass type, so it turns any label rate into what actually goes in your sprayer or spreader. Log it once and it tells you when it is due again.
Free. No account. No Pro tier. Nothing is locked and nothing is paywalled, and your lawn data never leaves your phone.